tag:theconversation.com,2011:/us/topics/jet-stream-14987/articlesJet stream – The Conversation2024-01-25T13:17:10Ztag:theconversation.com,2011:article/2219562024-01-25T13:17:10Z2024-01-25T13:17:10ZDiagnosing ‘warming winter syndrome’ as summerlike heat sweeps into central and eastern US<figure><img src="https://images.theconversation.com/files/578072/original/file-20240226-22-sj8mc9.jpg?ixlib=rb-1.1.0&rect=0%2C43%2C7333%2C4830&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Chicago topped 70 degrees on Feb. 26, 2024. That's not normal.</span> <span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/WarmWeatherChicago/e74246d1976f4048a494c6cb1ce2c0dd/photo">AP Photo/Charles Rex Arbogast</a></span></figcaption></figure><p>One of the most robust measures of Earth’s changing climate is that winter is warming more quickly than other seasons. The cascade of changes it brings, including ice storms and rain in regions that were once reliably below freezing, are symptoms of what I call “warming winter syndrome.”</p>
<p>Wintertime warming represents the global accumulation of heat. During winter, direct heat from the Sun is weak, but storms and <a href="https://theconversation.com/extreme-cold-still-happens-in-a-warming-world-in-fact-climate-instability-may-be-disrupting-the-polar-vortex-221276">shifts in the jet stream bring warm air</a> up from more southern latitudes into the northern U.S. and Canada. As <a href="https://climate.copernicus.eu/copernicus-2023-hottest-year-record">global temperatures</a> and <a href="https://climatereanalyzer.org/clim/sst_daily/">the oceans warm</a>, that stored heat has an influence on both temperature and precipitation. </p>
<p>The U.S. has been feeling this warming in the winter of 2023-24, the <a href="https://www.ncei.noaa.gov/news/national-climate-202402">warmest on record</a> for the Lower 48 states. </p>
<p>Snowfall has been <a href="https://twitter.com/NWSEastern/status/1757548891602358455">below average</a> in much of the country. On the Great Lakes, the ice cover has been at <a href="https://www.climate.gov/news-features/event-tracker/ice-coverage-nearly-nonexistent-across-great-lakes-historical-peak">record lows</a>. Late February saw a wave of <a href="https://twitter.com/NWS/status/1762158623793233989">summerlike temperatures</a> spread up into the central and eastern U.S., along with <a href="https://www.accuweather.com/en/severe-weather/severe-storms-with-nocturnal-tornado-risk-to-blitz-over-a-dozen-states/1625378">dangerous thunderstorms</a> and wildfires, including <a href="https://theconversation.com/texas-fires-with-over-1-million-acres-of-grassland-burned-cattle-ranchers-face-struggles-ahead-to-find-and-feed-their-herds-224840">Texas’ largest on record</a>. And forecasters expected <a href="https://graphical.weather.gov/sectors/conusLoop.php#tabs">another above-average warm spell in early March</a>.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/571278/original/file-20240124-21-v3590w.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/571278/original/file-20240124-21-v3590w.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/571278/original/file-20240124-21-v3590w.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/571278/original/file-20240124-21-v3590w.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/571278/original/file-20240124-21-v3590w.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/571278/original/file-20240124-21-v3590w.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/571278/original/file-20240124-21-v3590w.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The average lowest annual temperature, which affects where certain plants can grow, has shifted over the past half-century, reflecting the changing freezing line across the U.S.</span>
<span class="attribution"><a class="source" href="https://www.climatecentral.org/climate-matters/shifting-planting-zones-2023">Climate Central</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The longer warming trend is evident in changes to growing seasons, reflected in recent updates to plant hardiness zones printed on the <a href="https://planthardiness.ars.usda.gov/">back of seed packages</a>. These maps show the northward and, sometimes, westward movement of freezing temperatures in eastern North America. </p>
<h2>Ice storms and wet snow</h2>
<p>I <a href="https://scholar.google.com/citations?user=viGxwOwAAAAJ&hl=en">study the impact of global warming</a> and have documented changes to the climate and weather over the decades.</p>
<p>On average, <a href="https://nca2023.globalchange.gov/chapter/2/">freezing temperatures are moving</a> northward and, along the Atlantic coast, toward the interior of the continent. For individual storms, the transition to freezing temperatures even in the dead of winter can now be as far north as Lake Superior and southern Canada in places where, 50 years ago, it was reliably below freezing from early December through February.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/571354/original/file-20240125-23-x46q84.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two bar charts and a bell curve show the shifting average temperatures to more Januaries above freezing in recent decades." src="https://images.theconversation.com/files/571354/original/file-20240125-23-x46q84.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/571354/original/file-20240125-23-x46q84.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=290&fit=crop&dpr=1 600w, https://images.theconversation.com/files/571354/original/file-20240125-23-x46q84.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=290&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/571354/original/file-20240125-23-x46q84.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=290&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/571354/original/file-20240125-23-x46q84.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=365&fit=crop&dpr=1 754w, https://images.theconversation.com/files/571354/original/file-20240125-23-x46q84.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=365&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/571354/original/file-20240125-23-x46q84.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=365&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">In northwest Wisconsin, along Lake Superior, there were no Januarys in the 1951-1980 time frame in which the average high temperature was even close to exceeded freezing. That has changed in recent years.</span>
<span class="attribution"><a class="source" href="https://www.ncei.noaa.gov/access/monitoring/dyk/us-climate-divisions#grdd_">Omar Gates/GLISA</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>When temperatures are close to the freezing point, water can be rain, snow or ice. Regions on the colder side, which historically would have been below freezing and snowy, are seeing an <a href="https://glisa.umich.edu/freezing-rain/">increase in ice storms</a>. </p>
<p>The character of snow also changes near the freezing line. When the temperature is well below freezing, the snow is dry and fluffy. Near freezing, snow has big, wet, heavy flakes that turn roads into slush and stick on tree branches and bring down power lines.</p>
<p>Because the climate in which snowstorms are forming is warmer due to global accumulation of heat, and wetter because of more evaporation and warmer air that can hold more moisture, individual snowstorms can also result in <a href="https://www.washingtonpost.com/weather/2019/11/26/with-climate-change-washington-may-have-entered-era-more-blockbuster-snowstorms-less-snow-overall/">more intense snowfalls</a>. However, as temperatures get warmer in the future, the scales will tilt toward rain, and the <a href="https://glisa.umich.edu/resources-tools/climate-impacts/lake-effect-snow-in-the-great-lakes-region/">total amount of snow</a> will decrease.</p>
<p><iframe id="mRX0t" class="tc-infographic-datawrapper" src="https://datawrapper.dwcdn.net/mRX0t/1/" height="400px" width="100%" style="border: none" frameborder="0"></iframe></p>
<p>Indeed, on the warmer side of the freezing line, winter rain is already <a href="https://glisa.umich.edu/resources-tools/climate-impacts/lake-effect-snow-in-the-great-lakes-region/">becoming the dominate type of precipitation</a>, a trend that is expected to continue. With the warmer oceans as a major source of moisture, the already wet eastern U.S. can expect <a href="https://nca2023.globalchange.gov/chapter/2/">more winter precipitation over the next 30 years</a>. Looking to the future, soggy wet winters are more likely.</p>
<h2>Disaster and water planning gets harder</h2>
<p>For communities, planning for water supplies and extreme weather gets more complicated in a rapidly changing climate. Planners can’t count on the weather 30 years in the future being the same as weather today. It’s changing too quickly.</p>
<p>In many places, snow will not persist as late into spring. In regions like California and the Rockies that rely on the snowpack for water through the year, those supplies will <a href="https://nca2023.globalchange.gov/chapter/front-matter/">become less reliable</a>.</p>
<p>Rain falling on snowpack can also speed up melting, trigger flooding and change the flows of creeks and rivers. This shows up in changing <a href="https://theconversation.com/climate-change-is-driving-rapid-shifts-between-high-and-low-water-levels-on-the-great-lakes-118095">runoff patterns in the Great Lakes</a>, and it led to <a href="https://weather.com/news/weather/video/rain-melting-snow-tidal-water-bring-flooding-to-east-coast">flooding on the East Coast</a> in January 2024.</p>
<p>For road planners, the rate of freeze-thaw cycles that can damage roads will increase during winters in many regions unaccustomed to such quick shifts.</p>
<figure class="align-center ">
<img alt="A satellite image shows open water on the western shores the Great Lakes and storms forming to dump snow on the eastern shores." src="https://images.theconversation.com/files/571288/original/file-20240124-29-okndmd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/571288/original/file-20240124-29-okndmd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=577&fit=crop&dpr=1 600w, https://images.theconversation.com/files/571288/original/file-20240124-29-okndmd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=577&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/571288/original/file-20240124-29-okndmd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=577&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/571288/original/file-20240124-29-okndmd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=725&fit=crop&dpr=1 754w, https://images.theconversation.com/files/571288/original/file-20240124-29-okndmd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=725&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/571288/original/file-20240124-29-okndmd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=725&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A lake-effect snowstorm in 2020 shows how cold, dry air passing over the Great Lakes picks up moisture and heat, becoming snow on the other side.</span>
<span class="attribution"><a class="source" href="https://en.m.wikipedia.org/wiki/File:Lake_Effect_Snow_on_Earth.jpg">NASA</a></span>
</figcaption>
</figure>
<p>An especially interesting effect happens in the Great Lakes. Already, the <a href="https://research.noaa.gov/2024/01/18/why-low-ice-coverage-on-the-great-lakes-matters/">Great Lakes do not freeze as early</a> or as completely as in the past. This has large effects on the famous lake-effect precipitation zones.</p>
<p>With the lakes not frozen, more water evaporates into the atmosphere. In places where the wintertime air temperature is still below freezing, <a href="https://glisa.umich.edu/resources-tools/climate-impacts/lake-effect-snow-in-the-great-lakes-region/">lake-effect snow is increasing</a>. The Buffalo, New York, region saw <a href="https://theconversation.com/what-causes-lake-effect-snow-like-buffalos-extreme-storms-194953">6 feet of snow</a> from one lake-effect storm in 2022. As the air temperature flirts with the freezing line, these events are more likely to be rain and ice than snow.</p>
<p>These changes <a href="https://theconversation.com/extreme-cold-still-happens-in-a-warming-world-in-fact-climate-instability-may-be-disrupting-the-polar-vortex-221276">don’t mean cold is gone for good</a>. There will be occasions when Arctic air dips down into the U.S. This <a href="https://www.wilx.com/2024/01/24/fog-rain-wednesday-todays-headlines/">can cause flash freezing</a> and fog when warm wet air surges back over the frozen surface.</p>
<h2>Enormous consequences for economies</h2>
<p>What we are experiencing in warming winter syndrome is a consistent and robust set of symptoms on a fevered planet. </p>
<p>Novembers and Decembers will be milder; Februarys and Marches will be more like spring. Wintry weather will become more concentrated around January. There will be unfamiliar variability with snow, ice and rain. Some people may say these changes are great; there is less snow to shovel and heating bills are down.</p>
<figure class="align-center ">
<img alt="People walk with umbrellas in freezing rain in New York in January 2024." src="https://images.theconversation.com/files/571457/original/file-20240125-21-uuw03m.jpeg?ixlib=rb-1.1.0&rect=0%2C2%2C1917%2C1270&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/571457/original/file-20240125-21-uuw03m.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/571457/original/file-20240125-21-uuw03m.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/571457/original/file-20240125-21-uuw03m.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/571457/original/file-20240125-21-uuw03m.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/571457/original/file-20240125-21-uuw03m.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/571457/original/file-20240125-21-uuw03m.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Rain or snow? As global temperatures rise, cities accustomed to snowy winters will see more rain and ice storms during the winter months.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/people-walk-through-light-snow-in-manhattan-as-new-york-news-photo/1945522082">Spencer Platt/Getty Images</a></span>
</figcaption>
</figure>
<p>But on the other side, <a href="https://www.freep.com/in-depth/news/local/michigan/2021/01/03/michigan-winter-festivals-climate-change/3954384001/">whole economies are set up for wintertime</a>, many <a href="https://www.climatehubs.usda.gov/sites/default/files/Chill%20Hours%20Ag%20FS%20_%20120620.pdf">crops rely on cool winter temperatures</a>, and many farmers rely on freezing weather to control pests. Anytime there are changes to temperature and water, the <a href="https://www.wbez.org/stories/less-ice-on-the-great-lakes-due-to-warmer-winters/af180e73-78ac-4c64-acd5-d7118c46f89c">conditions in which plants and animals thrive are altered</a>. </p>
<p>These changes, which affect <a href="https://theconversation.com/how-climate-change-threatens-the-winter-olympics-future-even-snowmaking-has-limits-for-saving-the-games-177040">outdoor sports and recreation</a>, <a href="https://glisa.umich.edu/resources-tools/climate-impacts/fish-wildlife/">commercial fisheries</a> and agriculture, <a href="https://nca2023.globalchange.gov/chapter/11">have enormous consequences</a> not only to the ecosystems but also to our relationship to them. In some instances, traditions will be lost, such as ice fishing. Overall, people just about everywhere will have to adapt.</p>
<p><em>This article, originally published Jan. 25, 2024, has been updated with above-average heat across much of the central and eastern U.S. in late February and forecast in early March.</em></p><img src="https://counter.theconversation.com/content/221956/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Richard B. (Ricky) Rood receives funding from the National Oceanographic and Atmospheric Administration (NOAA).</span></em></p>As the climate changes and weather warms, the freezing line is shifting, bringing rain to many regions more accustomed to snow.Richard B. (Ricky) Rood, Professor Emeritus of Climate and Space Sciences and Engineering, University of MichiganLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2212762024-01-17T18:53:24Z2024-01-17T18:53:24ZExtreme cold still happens in a warming world – in fact climate instability may be disrupting the polar vortex<figure><img src="https://images.theconversation.com/files/569735/original/file-20240117-17-9me2cx.jpg?ixlib=rb-1.1.0&rect=0%2C43%2C5762%2C3519&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A blizzard with brutally cold temperatures hit Iowa and neighboring states on Jan. 12, 2024.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/with-three-days-until-the-iowa-caucus-iowans-plow-and-news-photo/1920707716">Melina Mara/The Washington Post via Getty Images</a></span></figcaption></figure><p>Extremely cold Arctic air and severe winter weather swept southward into much of the U.S. in mid-January 2024, <a href="https://www.theguardian.com/us-news/2024/jan/15/us-weather-arctic-blast-extreme-cold">breaking daily low temperature records</a> from Montana to Texas. Tens of millions of people were affected by <a href="https://www.nytimes.com/interactive/2024/us/freezing-temperatures-cold-weather-map.html">dangerously cold temperatures</a>, and heavy lake-effect snow and snow squalls have had severe effects across the Great Lakes and Northeast regions.</p>
<p>These severe cold events occur when the <a href="https://svs.gsfc.nasa.gov/3864">polar jet stream</a> – the familiar jet stream of winter that runs along the boundary between Arctic and more temperate air – dips deeply southward, bringing the cold Arctic air to regions that don’t often experience it. </p>
<figure class="align-center ">
<img alt="A globe showing most of the US covered in below-normal temperatures" src="https://images.theconversation.com/files/569882/original/file-20240117-17-7w73t0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/569882/original/file-20240117-17-7w73t0.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=571&fit=crop&dpr=1 600w, https://images.theconversation.com/files/569882/original/file-20240117-17-7w73t0.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=571&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/569882/original/file-20240117-17-7w73t0.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=571&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/569882/original/file-20240117-17-7w73t0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=718&fit=crop&dpr=1 754w, https://images.theconversation.com/files/569882/original/file-20240117-17-7w73t0.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=718&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/569882/original/file-20240117-17-7w73t0.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=718&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Surface temperatures at 7 a.m. EST on Jan. 16, 2024. Temperatures below freezing are in blue; those above freezing are in red. The jet stream is indicated by the light blue line with arrows.</span>
<span class="attribution"><span class="source">Mathew Barlow/UMass Lowell</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>An interesting aspect of these events is that they often occur in association with changes to another river of air even higher above the jet stream: the <a href="https://www.climate.gov/news-features/understanding-climate/understanding-arctic-polar-vortex">stratospheric polar vortex</a>, a great stream of air moving around the North Pole in the middle of the stratosphere. </p>
<p>When this stratospheric vortex becomes <a href="https://www.weather.gov/bis/sudden_stratospheric_warming_events">disrupted or stretched</a>, it can distort the jet stream as well, pushing it southward in some areas and causing cold air outbreaks. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/569628/original/file-20240116-27-naovil.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two globes, one showing a stable polar vortex and the other a disrupted version that brings brutal cold to the South." src="https://images.theconversation.com/files/569628/original/file-20240116-27-naovil.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/569628/original/file-20240116-27-naovil.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=324&fit=crop&dpr=1 600w, https://images.theconversation.com/files/569628/original/file-20240116-27-naovil.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=324&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/569628/original/file-20240116-27-naovil.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=324&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/569628/original/file-20240116-27-naovil.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=407&fit=crop&dpr=1 754w, https://images.theconversation.com/files/569628/original/file-20240116-27-naovil.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=407&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/569628/original/file-20240116-27-naovil.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=407&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Arctic polar vortex is a strong band of winds in the stratosphere, 10-30 miles above the surface. When this band of winds, normally ringing the North Pole, weakens, it can split. The polar jet stream can mirror this upheaval, becoming weaker or wavy. At the surface, cold air is pushed southward in some locations.</span>
<span class="attribution"><a class="source" href="https://www.climate.gov/news-features/understanding-climate/understanding-arctic-polar-vortex">NOAA</a></span>
</figcaption>
</figure>
<p>The January 2024 Arctic cold blast fit into this pattern, with the polar vortex stretched so far over the U.S. in the lower stratosphere that it had nearly split in two. There are multiple causes that may have led to this stretching, but it is likely related to <a href="https://doi.org/10.1175/BAMS-D-16-0259.1">high-latitude weather</a> in the prior two weeks.</p>
<figure class="align-center ">
<img alt="A globe showing most of the US covered in below-normal temperatures and two wavy lines following a similar track." src="https://images.theconversation.com/files/569884/original/file-20240117-27-8pkxk5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/569884/original/file-20240117-27-8pkxk5.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=571&fit=crop&dpr=1 600w, https://images.theconversation.com/files/569884/original/file-20240117-27-8pkxk5.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=571&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/569884/original/file-20240117-27-8pkxk5.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=571&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/569884/original/file-20240117-27-8pkxk5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=718&fit=crop&dpr=1 754w, https://images.theconversation.com/files/569884/original/file-20240117-27-8pkxk5.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=718&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/569884/original/file-20240117-27-8pkxk5.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=718&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Surface temperatures and the jet stream at 7 a.m. EST on Jan. 16, 2024, with the stratospheric polar vortex also shown as the dark blue line.</span>
<span class="attribution"><span class="source">Mathew Barlow/UMass Lowell</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<figure class="align-center ">
<img alt="A polar view of the stratosphere showing two cold blobs over the US and Europe." src="https://images.theconversation.com/files/569903/original/file-20240117-27-sr3rpe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/569903/original/file-20240117-27-sr3rpe.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=599&fit=crop&dpr=1 600w, https://images.theconversation.com/files/569903/original/file-20240117-27-sr3rpe.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=599&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/569903/original/file-20240117-27-sr3rpe.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=599&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/569903/original/file-20240117-27-sr3rpe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=753&fit=crop&dpr=1 754w, https://images.theconversation.com/files/569903/original/file-20240117-27-sr3rpe.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=753&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/569903/original/file-20240117-27-sr3rpe.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=753&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A polar view of the winds in the lower stratosphere at 7 a.m. EST on Jan. 16, 2024. The winds shown are approximately 10 miles above the surface, in the lower stratosphere.</span>
<span class="attribution"><span class="source">Mathew Barlow/UMass Lowell</span></span>
</figcaption>
</figure>
<h2>No, cold doesn’t contradict global warming</h2>
<p>After Earth just experienced its <a href="https://www.noaa.gov/news/2023-was-worlds-warmest-year-on-record-by-far">hottest year on record</a>, it may seem surprising to set so many cold records. But does this cold snap contradict human-caused global warming? As an <a href="https://scholar.google.com/citations?user=qWV-WIQAAAAJ&hl=en">atmospheric and climate scientist</a>, I can tell you, absolutely and unequivocally, it does not. </p>
<p>No single weather event can prove or disprove global warming. Many studies have shown that the <a href="https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-11/">number of extreme cold events is clearly decreasing</a> with global warming, as predicted and understood from physical reasoning.</p>
<p>Whether global warming may, contrary to expectations, be playing some supporting role in the intensity of these events is an open question. Some research suggests it does.</p>
<p>The February 2021 cold wave that <a href="https://energy.utexas.edu/sites/default/files/UTAustin%20%282021%29%20EventsFebruary2021TexasBlackout%2020210714.pdf">severely disrupted the Texas electric grid</a> was also <a href="https://theconversation.com/how-arctic-warming-can-trigger-extreme-cold-waves-like-the-texas-freeze-a-new-study-makes-the-connection-166550">associated with a stretched stratospheric polar vortex</a>. My colleagues and I have provided <a href="https://doi.org/10.1126/science.abi9167">evidence suggesting that Arctic changes</a> associated with global warming have increased the likelihood of such vortex disruptions. The effects of the enhanced high latitude warming known as <a href="https://doi.org/10.1038/s43247-022-00498-3">Arctic amplification</a> on regional snow cover and sea ice may enhance the weather patterns that, in turn, result in a stretched polar vortex.</p>
<p>More recently, we have shown that for large areas of the U.S., Europe and Northeast Asia, while the number of these severe cold events is clearly decreasing – as expected with global warming – it <a href="https://doi.org/10.1038/s43247-023-01008-9">does not appear that their intensity</a> is correspondingly decreasing, despite the rapid warming in their Arctic source regions.</p>
<p>So, while the world can expect fewer of these severe cold events in the future, many regions need to remain prepared for exceptional cold when it does occur. A better understanding of the pathways of influence between Arctic surface conditions, the stratospheric polar vortex and mid-latitude winter weather would improve our ability to anticipate these events and their severity.</p><img src="https://counter.theconversation.com/content/221276/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Mathew Barlow has received funding from the US National Science Foundation and the US National Oceanic and Atmospheric Administration (NOAA) to study weather and climate extremes.</span></em></p>The world can expect fewer severe cold events as average temperatures rise, but people still need to be prepared for wintery blasts.Mathew Barlow, Professor of Climate Science, UMass LowellLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2206722024-01-09T19:17:25Z2024-01-09T19:17:25ZA heatwave in Antarctica totally blew the minds of scientists. They set out to decipher it – and here are the results<figure><img src="https://images.theconversation.com/files/568365/original/file-20240109-23-ijfvqy.jpeg?ixlib=rb-1.1.0&rect=0%2C0%2C3941%2C970&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source"> DM Bergstrom</span>, <span class="license">Author provided</span></span></figcaption></figure><p>Climate scientists don’t like surprises. It means our deep understanding of how the climate works isn’t quite as complete as we need. But unfortunately, as climate change worsens, surprises and unprecedented events keep happening.</p>
<p>In March 2022, Antarctica experienced an extraordinary heatwave. Large swathes of East Antarctica experienced temperatures up to 40°C (72°F) above normal, shattering temperature records. It was the <a href="https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GL104910">most intense</a> heatwave ever recorded anywhere in the world.</p>
<p>So shocking and rare was the event, it blew the minds of the Antarctic climate science community. A major global research project was launched to unravel the reasons behind it and the damage it caused. A team of 54 researchers, including me, delved into the intricacies of the phenomenon. The team was led by Swiss climatologist Jonathan Wille, and involved experts from 14 countries. The collaboration resulted in two <a href="https://journals.ametsoc.org/view/journals/clim/37/3/JCLI-D-23-0175.1.xml#:%7E:text=At%20the%20peak%20of%20the,possible%20under%20future%20climate%20projections.">groundbreaking papers</a> published today.</p>
<p>The results are alarming. But they provide scientists a deeper understanding of the links between the tropics and Antarctica – and give the global community a chance to prepare for what a warmer world may bring.</p>
<h2>Head-hurting complexity</h2>
<p>The papers tell a complex story that began half a world away from Antarctica. Under <a href="http://www.bom.gov.au/climate/updates/articles/a020.shtml">La Niña conditions</a>, tropical heat near Indonesia poured into the skies above the Indian Ocean. At the same time, repeated weather troughs pulsing eastwards were generating from southern Africa. These factors combined into a late, Indian Ocean tropical cyclone season.</p>
<p>Between late February and late March 2022, 12 tropical storms had brewed. Five storms revved up to become tropical cyclones, and heat and moisture from some of these cyclones mashed together. A meandering jet stream picked up this air and swiftly transported it vast distances across the planet to Antarctica.</p>
<p>Below Australia, this jet stream also contributed to blocking the eastward passage of a high pressure system. When the tropical air collided with this so-called “blocking high”, it caused the most intense atmospheric river ever observed over East Antarctica. This propelled the tropical heat and moisture southward into the heart of the Antarctic continent. </p>
<hr>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/like-rivers-in-the-sky-the-weather-system-bringing-floods-to-queensland-will-become-more-likely-under-climate-change-176711">Like rivers in the sky: the weather system bringing floods to Queensland will become more likely under climate change</a>
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<h2>Luck was on Antarctica’s side</h2>
<p>The event caused the vulnerable Conger Ice Shelf to <a href="https://theconversation.com/conger-ice-shelf-has-collapsed-what-you-need-to-know-according-to-experts-180077">finally collapse</a>. But the impacts were otherwise not as bad as they could have been. That’s because the heatwave struck in March, the month when Antarctica transitions to its dark, extremely cold winter. If a future heatwave arrives in summer – which is more likely under climate change – the results could be catastrophic.</p>
<p>Despite the heatwave, most inland temperatures stayed below zero. The spike included a new all-time temperature high of -9.4°C (15.1°F) on March 18 near Antarctica’s Concordia Research Station. To understand the immensity of this, consider that the previous March maximum temperature at this location was -27.6°C (-17.68°F). At the heatwave’s peak, 3.3 million square kilometres in East Antarctica – an area about the size of India – was affected by the heatwave.</p>
<p>The impacts included widespread rain and surface melt along coastal areas. But inland, the tropical moisture fell as snow – lots and lots of snow. Interestingly, the weight of the snow offset ice loss in Antarctica for the year. This delivered a temporary reprieve from Antarctica’s contribution to global sea-level rise.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/568364/original/file-20240109-25-o9q0sw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An ice shelf before (left) and after (right) a collapse." src="https://images.theconversation.com/files/568364/original/file-20240109-25-o9q0sw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/568364/original/file-20240109-25-o9q0sw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=381&fit=crop&dpr=1 600w, https://images.theconversation.com/files/568364/original/file-20240109-25-o9q0sw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=381&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/568364/original/file-20240109-25-o9q0sw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=381&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/568364/original/file-20240109-25-o9q0sw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=478&fit=crop&dpr=1 754w, https://images.theconversation.com/files/568364/original/file-20240109-25-o9q0sw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=478&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/568364/original/file-20240109-25-o9q0sw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=478&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">These images, acquired by the Copernicus Sentinel-2 satellites on January 30 2022 (left) and March 21 2022 (right), show the Conger ice shelf before and after the collapse, which was triggered by a shocking heatwave.</span>
<span class="attribution"><a class="source" href="https://www.copernicus.eu/en/media/image-day-gallery/collapse-conger-ice-shelf">European Union, Copernicus Sentinel-2 satellite imagery</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Learning from the results</h2>
<p>So what are the lessons here? Let’s begin with the nice bit. The study was made possible by international collaboration across Antarctica’s scientific community, including the open sharing of datasets. This collaboration is a touchstone of the Antarctic Treaty. It serves as a testament to the significance of peaceful international cooperation and should be celebrated. </p>
<p>Less heartwarmingly, the extraordinary heatwave shows how compounding weather events in the tropics can affect the vast Antarctic ice sheet. The heatwave further reduced the extent of sea ice, which was already at record lows. This loss of sea ice was exacerbated this <a href="https://theconversation.com/devastatingly-low-antarctic-sea-ice-may-be-the-new-abnormal-study-warns-212376">year</a> resulting in the lowest summer and winter sea ice ever recorded. It shows how disturbances in one year can compound in later years.</p>
<p>The event also demonstrated how tropical heat can trigger the collapse of unstable ice shelves. Floating ice shelves don’t contribute to global sea-level rise, but they acts as dams to the <a href="https://theconversation.com/antarctic-tipping-points-the-irreversible-changes-to-come-if-we-fail-to-keep-warming-below-2-207410">ice sheets behind them</a>, which do contribute.</p>
<p>This research calculated that such temperature anomalies occur in Antarctica about once a century, but concluded that under climate change, they will occur more frequently. </p>
<p>The findings enable the global community to improve its planning for various scenarios. For example, if a heatwave of similar magnitude hit in summer, how much ice melt would there be? If an atmospheric river hit the <a href="https://theconversation.com/antarcticas-doomsday-glacier-how-its-collapse-could-trigger-global-floods-and-swallow-islands-173940">Doomsday glacier</a> in the West Antarctic, what rate of sea level rise would that trigger? And how can governments across the world <a href="https://theconversation.com/who-moves-and-who-pays-managed-retreat-is-hard-but-lessons-from-the-past-can-guide-us-196038">prepare coastal communities</a> for sea level rise greater than currently calculated?</p>
<p>This research contributes another piece to the complex jigsaw puzzle of climate change. And reminds us all, that delays to action on climate change will raise the price we pay.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/record-smashing-heatwaves-are-hitting-antarctica-and-the-arctic-simultaneously-heres-whats-driving-them-and-how-theyll-impact-wildlife-179659">Record-smashing heatwaves are hitting Antarctica and the Arctic simultaneously. Here’s what’s driving them, and how they’ll impact wildlife</a>
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</em>
</p>
<hr>
<p><em>This article has been amended to correct an error in converting a 40°C temperature difference from Celsius to Fahrenheit.</em></p><img src="https://counter.theconversation.com/content/220672/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dana M Bergstrom past position was at the Australian Antarctic Division. She is affiliated with the Pure Antarctic Foundation, a groups of scientists and artists interesting in communication science and knowledge to the broader community.</span></em></p>A heatwave in 2022 redefined scientific expectations of the Antarctic climate. Now the global community must prepare for what a warmer world may bring.Dana M Bergstrom, Honorary Senior Fellow, University of WollongongLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2133042023-09-15T14:21:55Z2023-09-15T14:21:55ZHow weather ‘blocks’ have triggered more extreme heatwaves and floods across Europe<p>On several occasions this summer, Europe’s weather seemed to get itself stuck, leading to prolonged heatwaves and floods. In the UK, a long hot and dry spell throughout May and June gave way to a similarly persistent cool and wet period. </p>
<p>In September, Europe saw widespread flooding in southern Europe while the UK basked in its longest ever September heatwave. These were all the result of “<a href="https://x.com/TimWoollings/status/1668898975636676610?s=20">blocked” weather patterns</a>.</p>
<p>So what is blocking – and are events like this here to stay?</p>
<p>A blocking event is a disruption to the usual weather patterns of Earth’s middle latitudes. The normal state for Europe’s climate is to be led by the Atlantic, with weather systems forming over the ocean and sweeping eastward. Individual systems might bring sun or rain but in either case they are transient – here today and gone tomorrow. </p>
<p>This movement is closely linked to the <a href="https://theconversation.com/a-battle-for-the-jet-stream-is-raging-above-our-heads-125906">jet stream</a>: a fast-moving current of air which encircles the globe. This not only steers the weather systems but also drives the prevailing westerly winds that help keep Europe relatively cool in summer and mild in winter. </p>
<p>But when a block occurs, this prevailing maritime influence is lost. Blocking events are often responsible for the hottest days of summer but also the coldest days of winter. </p>
<p>Weather systems effectively ride the jet stream and distort it as they go. On weather maps we see this as a meandering of the jet, veering alternately north and south as it snakes its way east. </p>
<p>During blocking events, these meanders get larger until eventually the jet breaks up into swirling eddies. With the jet stream disrupted, weather patterns stick around, often for a week or longer. Some places get scorched while others are inundated, day after day. </p>
<p>Take the recent block, for example. The jet snaked south, then north, then south again, tracing out a giant Greek letter omega (Ω) over Europe. At the centre of the pattern was a large northward meander of the jet which kept warm air from further south stationary over France and the UK, gifting the latter <a href="https://www.theguardian.com/uk-news/2023/sep/10/seven-days-of-30c-heat-in-september-is-new-uk-record-but-storms-on-the-way">seven days of 30°C heat</a>.</p>
<p>Flanking this on either side were southward meanders of colder air, which helped to anchor intense stationary cyclones. This resulted in huge amounts of rain falling across the Mediterranean, and hence floods: in Spain on one side of the omega, and in Greece and Libya on the other. </p>
<p>The latter were particularly affected as <a href="https://public.wmo.int/en/media/news/storm-daniel-leads-extreme-rain-and-floods-mediterranean-heavy-loss-of-life-libya">Storm Daniel</a>, in the eastern part of the omega, intensified and developed signs of a “medicane”, or Mediterranean hurricane. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/548134/original/file-20230913-23-d0ia4s.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A weather map of north-west Europe showing the omega pattern." src="https://images.theconversation.com/files/548134/original/file-20230913-23-d0ia4s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/548134/original/file-20230913-23-d0ia4s.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/548134/original/file-20230913-23-d0ia4s.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/548134/original/file-20230913-23-d0ia4s.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/548134/original/file-20230913-23-d0ia4s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/548134/original/file-20230913-23-d0ia4s.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/548134/original/file-20230913-23-d0ia4s.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=377&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">September’s ‘omega block’ concentrated hot, dry weather over the UK and floods over Southern Europe and North Africa. Map shown for September 7 2023.</span>
<span class="attribution"><a class="source" href="https://www.ecmwf.int/">European Centre for Medium-Range Weather Forecasts (ECMWF)</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Blocking occurs once or twice every season, on average. But rising temperatures due to climate change have yielded more intense heatwaves in the warm parts of the block. </p>
<p>Since warmer air holds more moisture, rainfall is also usually more <a href="https://www.carbonbrief.org/guest-post-how-hourly-rainfall-extremes-are-changing-in-a-warming-climate/">intense</a>. If Europe had had exactly the same omega pattern 50 years ago, many of the impacts would have been weaker.</p>
<h2>Are blocking events getting more common?</h2>
<p>Some meteorologists argue that the jet stream is getting <a href="https://doi.org/10.1038/s41467-018-05256-8">weaker</a> and more prone to disruptions such as this. However, the trends are inconclusive and so these remain <a href="https://www.carbonbrief.org/jet-stream-is-climate-change-causing-more-blocking-weather-events/">theories</a>. </p>
<p>The fact that scientists disagree on this issue highlights how uncertain some of the effects of climate change still are. Humankind is clearly messing with a highly complex system that we do not fully understand. </p>
<p>Climate models <a href="https://doi.org/10.1007/s40641-018-0108-z">indicate</a> that blocking might become rarer as the world warms and the jet stream shifts a little further north, on average. But this is still unclear and if it does happen, it’s likely to be a pretty small change. </p>
<p>Blocking isn’t going away any time soon and many impacts, such as heatwaves and flooding, are only likely to get worse with higher air temperatures. In fact, it’s quite likely that more severe events than those of this summer could strike in today’s climate. </p>
<p>Since blocking is relatively rare and sporadic, scientists don’t have a good sample of events on which to base their estimates of risk. It’s certainly possible that the blocks this summer could have lasted even longer than they did, or struck at a worse time.</p>
<p>Had they occurred at the peak of the annual cycle, rather than the start and end of the summer as they did, the heat would have been more intense. The UK’s first 40°C temperature was observed in July 2022 during a mercifully short heatwave. A persistent block at that time of year would be fierce. </p>
<p>And the jet stream itself adds another twist to the tale. The jet has tended to <a href="https://doi.org/10.1002/joc.8095">shift south</a> over Europe in summers since 2000, in a pattern that favours relatively cool and wet conditions in north-west Europe. This appears to be a feature of natural variations in Earth’s climate, at least in part, but our understanding of this remains poor. </p>
<p>If or when the jet shifts back north, it would bring higher average summer temperatures as well as the risk of blocking-induced heatwaves. We had an early taste of this combination in <a href="https://doi.org/10.1029/2019GL084601">2018</a>, the UK’s warmest summer on record. European summer heat has been getting <a href="https://theconversation.com/hottest-days-are-warming-twice-as-fast-as-average-summer-temperature-in-north-west-europe-new-research-205500">worse</a> due to climate change, but we should be preparing for even worse in the near future.</p>
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<figure class="align-right ">
<img alt="Imagine weekly climate newsletter" src="https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
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<p class="fine-print"><em><span>Tim Woollings receives funding from the UK Natural Environment Research Council. For more on the jet stream, blocking and climate change, see Tim’s popular science book Jet Stream: A Journey Though our Changing Climate.</span></em></p>An ‘omega block’ helped Storm Daniel wreak devastation in Libya.Tim Woollings, Professor in Physical Climate Science, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2003642023-02-23T17:47:02Z2023-02-23T17:47:02ZWhy the UK has only had one named storm so far this winter – an expert explains<p><a href="https://www.metoffice.gov.uk/about-us/press-office/news/weather-and-climate/2023/storm-otto-named-by-danish-met-service#:%7E:text=A%20low%2Dpressure%20system%20which,75mph%20to%20some%20northern%20areas.">Storm Otto</a>, which was named by the Danish Meteorological Institute, hit Scotland and north-east England last Friday (February 17 2023) with wind gusts of over 80mph, disrupting power to 61,000 homes. </p>
<p>Otto was the first named storm of the UK’s current winter storm season and the first to hit the country’s shores since <a href="https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/weather/learn-about/uk-past-events/interesting/2022/2022_01_storms_dudley_eunice_franklin_r1.pdf">storms Dudley, Eunice and Franklin</a> last February. Over the course of a week, these three storms barrelled in from the North Atlantic causing wind and flood damage worth over <a href="https://www.perils.org/losses">€3.7 billion (£3.2 billion) in insured losses</a> across Europe. </p>
<p>The UK has seen only a few notable instances of stormy weather so far this winter. For example, heavy rainfall in the first few weeks of January led to <a href="https://www.itv.com/news/westcountry/2023-01-19/pictures-show-scale-of-flooding-on-somerset-levels">flooding on the Somerset Levels</a>. But this storm was not intense enough to be named. This happens only when a <a href="https://www.metoffice.gov.uk/weather/warnings-and-advice/uk-storm-centre/index">storm has the potential</a> to be severe enough to cause an amber or red warning. </p>
<p>But storms are a common feature of winters in the UK. Since the current naming scheme started in 2016, between five and ten named storms have hit the UK each winter. So what’s been going on with the weather this year and why did the UK wait such a long time between named storms? </p>
<h2>Variable UK weather</h2>
<p>Chance can always play a role, particularly in the case of UK weather. </p>
<p>A narrow band of strong winds in the upper atmosphere, known as the jet stream, steers storms that originate over the North Atlantic towards Europe and the UK. But the jet stream itself is naturally very variable and can shift in position and strength. This can cause the UK’s weather to vary a lot from year to year. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Lg91eowtfbw?wmode=transparent&start=4" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">What is the jet stream and how does it affect our weather?</span></figcaption>
</figure>
<p>The UK has therefore gone long periods without large storms before. For example, the <a href="https://digital.nmla.metoffice.gov.uk/">winter of 1985-86</a> was relatively quiet and was marked by storms at either end of the season with only one strong storm in January.</p>
<h2>La Niña conditions</h2>
<p>A global weather phenomenon called La Niña is also likely to have contributed to this year’s weather. La Niña is one phase of the <a href="https://www.climate.gov/news-features/blogs/enso/what-el-ni%C3%B1o%E2%80%93southern-oscillation-enso-nutshell">El Niño Southern Oscillation</a> and is characterised by relatively cool sea surface temperatures in tropical areas of the Pacific. This winter, sea surface temperatures in the tropical Pacific Ocean have been <a href="https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/">nearly 1°C cooler</a> than average. </p>
<p>La Niña can influence the weather experienced in the northern hemisphere. Cooler sea surface temperatures in the Pacific shift the position of rainfall in the tropics. These changes then propagate into the mid-latitudes, almost like ripples on a pond, and influence the position of the jet stream over the North Atlantic.</p>
<p>The impact of La Niña on weather in the North Atlantic is different in early and late winter. </p>
<p>Early in the season, La Niña tends to <a href="https://journals.ametsoc.org/view/journals/clim/31/11/jcli-d-17-0716.1.xml">shift the jet stream to the south</a>, steering storms that would normally hit the UK towards southern Europe. This may partly explain the bouts of stormy weather that brought flooding to Portugal, Spain and Italy in November and December of 2022. On December 7, <a href="https://www.efas.eu/en/news/floods-portugal-and-spain-december-2022">flash flooding</a> swept through the streets of Lisbon, the capital of Portugal, after 82.3mm of rain fell in 24 hours.</p>
<h2>Sudden stratospheric warming</h2>
<p>Later in the winter season, La Niña tends to shift the jet stream back towards the north and should bring stormier weather to the UK. During a typical winter, cooling air causes a vortex of westerly winds to form in the stratosphere, 10-50km above the Arctic. </p>
<p>The <a href="https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/polar-vortex#:%7E:text=What%20is%20the%20Polar%20Vortex,about%20it%20for%20many%20years.">stratospheric polar vortex</a>, as it is called, remains remarkably stable most winters. But some years, the polar vortex slows and breaks up suddenly, causing the stratospheric air over the Arctic to warm rapidly. Called a sudden stratospheric warming, such an event has been occurring since <a href="https://blog.metoffice.gov.uk/2023/02/07/are-we-expecting-a-sudden-stratospheric-warming/">mid-February</a>.</p>
<p>These <a href="https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/wind/sudden-stratospheric-warming">sudden stratospheric warmings</a>, and the rapid slow down of the polar vortex, causes the North Atlantic jet stream below to slow down and meander. In some cases, sudden stratospheric warming can create a large area of high pressure over the North Atlantic and Scandinavia, bringing a spell of dry weather to northern Europe. This happened following the last <a href="https://www.rmets.org/metmatters/polar-vortex-sudden-stratospheric-warmings-and-beast-east">sudden stratospheric warming</a> event in January 2021. </p>
<p>But the effect of a sudden stratospheric warming on the UK’s weather can vary. The European blocking associated with sudden stratospheric warming can sometimes bring in freezing air from Europe, increasing the risk of snow. This was the case in late February 2018, where a sudden stratospheric warming drew winds from the Eurasian continent, causing storms and severe snowfall to affect much of the UK – known as the <a href="https://www.rmets.org/metmatters/beast-east-bites-uk">“beast from the east”</a>.</p>
<p>Although the UK might expect some colder weather in the next few weeks, there is no indication in current weather forecasts that this year’s sudden stratospheric warming will lead to the extreme cold weather seen in 2018.</p>
<p>This winter has been less stormy than usual. Yet, the UK relies on rain from North Atlantic storms to refill its rivers, reservoirs and aquifers. This is particularly important this year given last year’s exceptionally hot and dry summer that lead to drought conditions in much of the UK. </p>
<p>Despite only having one named storm, <a href="https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1136621/Monthly_water_situation_report_for_England_January_2023.pdf">rainfall across most of the UK</a> has fortunately been around or above average since October. This has helped to replenish water resources. But even then, <a href="https://www.gov.uk/government/news/one-hot-dry-spell-away-from-drought-returning-this-summer-national-drought-group-warns">East Anglia and Cornwall</a> remain in drought. So to top up the UK’s water resources and banish the spectre of a 2023 drought, a few more moderate storms over the next few months would be very welcome.</p>
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<figure class="align-right ">
<img alt="Imagine weekly climate newsletter" src="https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><strong><em>Don’t have time to read about climate change as much as you’d like?</em></strong>
<br><em><a href="https://theconversation.com/uk/newsletters/imagine-57?utm_source=TCUK&utm_medium=linkback&utm_campaign=Imagine&utm_content=DontHaveTimeTop">Get a weekly roundup in your inbox instead.</a> Every Wednesday, The Conversation’s environment editor writes Imagine, a short email that goes a little deeper into just one climate issue. <a href="https://theconversation.com/uk/newsletters/imagine-57?utm_source=TCUK&utm_medium=linkback&utm_campaign=Imagine&utm_content=DontHaveTimeBottom">Join the 10,000+ readers who’ve subscribed so far.</a></em></p>
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<p class="fine-print"><em><span>Len Shaffrey receives funding from the Natural Environment Research Council and the European Commission's Horizon Europe funding scheme.</span></em></p>An expert explains why the UK’s winter has been relatively calm.Len Shaffrey, Professor of Climate Science, National Centre for Atmospheric Science, University of ReadingLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1855692022-06-22T12:35:23Z2022-06-22T12:35:23ZWhat is a heat dome? An atmospheric scientist explains the weather phenomenon baking Texas and the Southwest<figure><img src="https://images.theconversation.com/files/534155/original/file-20230626-27-2k6wiq.png?ixlib=rb-1.1.0&rect=0%2C143%2C811%2C564&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A heat dome began sizzling Texas and its neighbors in mid-June 2023, with warm nights providing little relief.</span> <span class="attribution"><a class="source" href="https://graphical.mdl.nws.noaa.gov/sectors/sectorDay.php?view=public&sector=conus&element=T">National Weather Service</a></span></figcaption></figure><p>A heat dome occurs when a persistent region of high pressure traps heat over an area. The heat dome can stretch over several states and linger for days to weeks, leaving the people, crops and animals below to suffer through stagnant, hot air that can feel like an oven.</p>
<p>Typically, heat domes are tied to the behavior of <a href="https://www.youtube.com/watch?v=o203JXAnSA0">the jet stream</a>, a band of fast winds high in the atmosphere that generally runs west to east. </p>
<p>Normally, the jet stream has a <a href="https://www.weather.gov/jetstream/jet">wavelike pattern, meandering</a> north and then south and then north again. When these meanders in the jet stream become bigger, they move slower and can become stationary. That’s when heat domes can occur. </p>
<figure class="align-center ">
<img alt="Map of U.S. with a bubble over the Midwest showing arrows moving, with the ridge air sinking" src="https://images.theconversation.com/files/470123/original/file-20220621-23-no5s1r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/470123/original/file-20220621-23-no5s1r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=329&fit=crop&dpr=1 600w, https://images.theconversation.com/files/470123/original/file-20220621-23-no5s1r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=329&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/470123/original/file-20220621-23-no5s1r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=329&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/470123/original/file-20220621-23-no5s1r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=413&fit=crop&dpr=1 754w, https://images.theconversation.com/files/470123/original/file-20220621-23-no5s1r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=413&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/470123/original/file-20220621-23-no5s1r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=413&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Heat domes involve high-pressure areas that trap and heat up the air below.</span>
<span class="attribution"><a class="source" href="https://oceanservice.noaa.gov/facts/heat-dome.html">NOAA</a></span>
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<p>When the jet stream swings far to the north, air piles up and sinks. The <a href="https://glossary.ametsoc.org/wiki/Heat_dome">air warms as it sinks</a>, and the sinking air also keeps skies clear since it lowers humidity. That allows the sun to create hotter and hotter conditions near the ground. </p>
<p>If the air near the ground passes over mountains and descends, it can <a href="https://blog.weather.us/downsloping/">warm even more</a>. This downslope warming played a large role in the extremely hot temperatures in the Pacific Northwest during a <a href="https://en.wikipedia.org/wiki/2021_Western_North_America_heat_wave">heat dome event in 2021</a>, when Washington set a state record with 120 degrees Fahrenheit (49 Celsius), and temperatures reached 121 F in British Columbia in Canada, surpassing the previous Canadian record by 8 degrees F (4 C).</p>
<h2>The human impact</h2>
<p>Heat domes normally persist for several days in any one location, but they can last longer. They can also move, influencing neighboring areas over a week or two. The heat dome that started in Texas and Mexico in <a href="https://origin.wpc.ncep.noaa.gov/discussions/hpcdiscussions.php?disc=pmdspd">June 2023</a> spread into the Southwest in July, with little relief in sight.</p>
<p>On rare occasions, the heat dome can be more persistent. That happened in the <a href="https://www.hsdl.org/c/tl/1980-us-heat-wave/">southern Plains in 1980</a>, when as many as 10,000 people died during weeks of high summer heat. It also happened over much of the United States <a href="https://doi.org/10.1038/s41558-020-0771-7">during the Dust Bowl years</a> of the 1930s.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1677793035487444992"}"></div></p>
<p>A heat dome can have serious impacts on people, because the stagnant weather pattern that allows it to exist usually results in weak winds and an increase in humidity. Both factors make the heat feel worse – <a href="https://theconversation.com/knowing-how-heat-and-humidity-affect-your-body-can-help-you-stay-safe-during-heat-waves-163700">and become more dangerous</a> – because the human body is not cooled as much by sweating.</p>
<p>The <a href="https://www.weather.gov/ama/heatindex">heat index</a>, a combination of heat and humidity, is often used to convey this danger by indicating what the temperature will feel like to most people. The high humidity also reduces the amount of cooling at night. Warm nights can leave people without air conditioners unable to cool off, which increases the risk of heat illnesses and deaths. With global warming, <a href="https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature">temperatures are already higher</a>, too.</p>
<p>One of the worst recent examples of the impacts from a heat dome with high temperatures and humidity in the U.S. occurred <a href="https://www.weather.gov/lot/1995_heatwave_anniversary">in the summer of 1995</a>, when an estimated <a href="https://www.chicagomag.com/Chicago-Magazine/July-2015/1995-Chicago-heat-wave/">739 people died in the Chicago area</a> over five days.</p>
<p><em>This article was updated July 10, 2023, with the heat dome in the Southwest.</em></p><img src="https://counter.theconversation.com/content/185569/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>William Gallus receives funding from the National Science Foundation. </span></em></p>Heat domes are a dangerous part of summer weather.William Gallus, Professor of Atmospheric Science, Iowa State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1846492022-06-20T04:53:25Z2022-06-20T04:53:25ZChanges in the jet stream are steering autumn rain away from southeast Australia<figure><img src="https://images.theconversation.com/files/469657/original/file-20220620-26-83rfjs.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2543%2C1419&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>You wouldn’t know it from the torrential rains that have inundated large parts of New South Wales and Queensland this year, but average late-autumn rainfall over southeast Australia has declined significantly since the 1990s. </p>
<p>Less rain in these areas is <a href="https://bom.gov.au/state-of-the-climate/">an expected consequence of global warming</a>. In both the northern and southern hemispheres, the paths of the weather systems that bring rain in the middle latitudes have been moving away from the equator and towards the poles. </p>
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Read more:
<a href="https://theconversation.com/australias-dry-june-is-a-sign-of-whats-to-come-80469">Australia's dry June is a sign of what's to come</a>
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<p>We studied in detail the drop in rainfall during April and May in southeast Australia, and found it is just one consequence of far-reaching <a href="https://www.mdpi.com/2225-1154/10/6/84/htm">changes in the behaviour of high-altitude winds</a> over Australia.</p>
<h2>Jet streams</h2>
<p>These high-altitude winds are called jet streams: narrow bands of rapidly flowing air that typically occur at altitudes around the cruising height of commercial passenger aircraft. In April and May, the westerly jet stream over southeast Australia normally splits into a northern branch (called the subtropical jet) and a southern branch (called the polar-front jet). </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/469641/original/file-20220620-26-h60763.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Four maps showing conditions in the jet stream over southern Australia and New Zealand at different times of year." src="https://images.theconversation.com/files/469641/original/file-20220620-26-h60763.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/469641/original/file-20220620-26-h60763.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/469641/original/file-20220620-26-h60763.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/469641/original/file-20220620-26-h60763.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/469641/original/file-20220620-26-h60763.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=505&fit=crop&dpr=1 754w, https://images.theconversation.com/files/469641/original/file-20220620-26-h60763.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=505&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/469641/original/file-20220620-26-h60763.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=505&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The jet stream over southern Australian latitudes splits in two over autumn and winter.</span>
<span class="attribution"><a class="source" href="https://www.mdpi.com/2225-1154/10/6/84/htm">Speer, Leslie & Hartigan, Climate (2022)</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Since the mid 1990s, the location of this split has moved and the speeds of the winds involved have also changed. We found these changes, which are related to global warming, are responsible for a decrease in the number of low-pressure systems bringing rain to southeast Australia.</p>
<p>The maps you might see on weather apps or TV forecasts usually show what’s going on at ground level: high- and low-pressure systems, cold fronts, and so on. However, these ground-level systems are largely driven by the jet streams and related atmospheric processes.</p>
<h2>Humidity and rotation</h2>
<p>As well as the changes in the jet stream, there are two other important changes reducing rainfall in the early cool season. </p>
<p>First, the air over parts of inland southeast Australia has become significantly drier since the 1990s. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/469651/original/file-20220620-12-2owuja.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A map of Australia and New Zealand showing how humidity has changed since the 1990s." src="https://images.theconversation.com/files/469651/original/file-20220620-12-2owuja.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/469651/original/file-20220620-12-2owuja.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=403&fit=crop&dpr=1 600w, https://images.theconversation.com/files/469651/original/file-20220620-12-2owuja.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=403&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/469651/original/file-20220620-12-2owuja.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=403&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/469651/original/file-20220620-12-2owuja.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=506&fit=crop&dpr=1 754w, https://images.theconversation.com/files/469651/original/file-20220620-12-2owuja.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=506&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/469651/original/file-20220620-12-2owuja.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=506&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Humidity over parts of southeast Australia has declined significantly since the 1990s.</span>
<span class="attribution"><a class="source" href="https://www.mdpi.com/2225-1154/10/6/84/htm">Speer, Leslie & Hartigan, Climate (2022)</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>And second, areas of strongly rotating air have moved further east and south, over the Tasman Sea. </p>
<p>Both humidity and air rotation are important contributors to the development of low-pressure areas that bring rain. As a result of these changes, there has been a significant decrease in late autumn rainfall in southeast Australia.</p>
<h2>The bigger picture</h2>
<p>Much of the variation in rainfall from year to year depends on the phase of El Niño–Southern Oscillation (ENSO), a large-scale climate phenomenon over the Pacific Ocean. When this is in one phase, called La Niña, eastern Australia experiences lower temperatures and higher rainfall – in the opposite phase, El Niño, it’s the other way around.</p>
<p>In the absence of La Niña years, and particularly groups of repeat La Niña events such as those of 2010–12 and 2020–22, we have seen extreme droughts following dry summers and dry late-autumn periods.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/droughts-and-flooding-rains-it-takes-three-oceans-to-explain-australias-wild-21st-century-weather-56264">Droughts and flooding rains: it takes three oceans to explain Australia's wild 21st-century weather</a>
</strong>
</em>
</p>
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<p>Changes in the atmospheric circulation, especially in the jet stream location and strength, have contributed to the multiple-year droughts we have seen since the 1990s, namely, the periods 1997–2007 and 2017–2019. For example, rain-bearing “<a href="https://doi.org/10.3390/cli9030044">east coast lows</a>” are often forming further south, and there are <a href="https://doi.org/10.1002/qj.4201">fewer cut-off low pressure systems</a> over inland southeast Australia. </p>
<h2>Unfavourable winds</h2>
<p>The effects of drought at ground level are easy to see. </p>
<p>The drought periods since 1997 have killed huge numbers of river fish, reduced the viability of broad acre and pastoral farming and other economic industries, and reduced river flows and sustainable access to water in many areas. In a future warming climate, these drought periods are expected to continue. </p>
<p>However, changes to the jet stream also have less obvious effects at higher altitudes. In particular, these changes have implications for air transport.</p>
<p>Changes in speed, location and structure of jet streams will mean planes will use more fuel on many routes, including in Australia. Less favourable winds, and an increase in sudden “<a href="https://en.wikipedia.org/wiki/Clear-air_turbulence">clear-air turbulence</a>”, will <a href="https://doi.org/10.1175/BAMS-D-19-0239.1">increase aviation fuel consumption</a>.</p><img src="https://counter.theconversation.com/content/184649/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Global warming is changing the high-altitude autumn winds over southeast Australia, which means less rain and trouble for air travel.Milton Speer, Visiting Fellow, School of Mathematical and Physical Sciences, University of Technology SydneyLance M Leslie, Professor, School of Mathematical And Physical Sciences, University of Technology SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1774682022-02-18T15:22:11Z2022-02-18T15:22:11ZWhy Storm Eunice was so severe – and will violent wind storms become more common?<figure><img src="https://images.theconversation.com/files/447308/original/file-20220218-49159-3yu3uc.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C5760%2C3837&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Porthcawl Lighthouse in South Wales is buffeted by waves during Storm Eunice's approach.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/waves-start-increase-storm-eunice-gathers-2124985670">Leighton Collins/Shutterstock</a></span></figcaption></figure><p>The UK Met Office has issued two <a href="https://www.independent.co.uk/climate-change/news/storm-eunice-met-office-warning-b2017349.html">red weather warnings</a> in as many months for strong winds. These are the highest threat levels meteorologists can announce, and are the first wind-only red warnings to be issued since 2016’s <a href="https://www.metoffice.gov.uk/weather/warnings-and-advice/uk-storm-centre/storm-gertrude">Storm Gertrude</a>. </p>
<p>So what’s behind the UK’s recent spate of dangerous wind storms? And are these events likely to become more common in future?</p>
<p>Storm Arwen in late November 2021 caused devastation across Scotland, northern England and parts of Wales. Winds of 100mph <a href="https://www.bbc.co.uk/news/uk-59435965">killed three people</a>, <a href="https://www.independent.co.uk/news/uk/home-news/storm-arwen-forests-trees-damage-b1975781.html">ripped up trees</a>, and left <a href="https://news.sky.com/story/storm-arwen-power-restored-to-97-of-homes-but-30-000-face-sixth-day-without-electricity-12483799">9,000 people without power</a> for over a week in freezing temperatures. </p>
<p>The destruction caused by Arwen is still apparent in some areas, and the clean-up from Storm Dudley – which battered eastern England on Wednesday February 16 – is <a href="https://www.theguardian.com/uk-news/2022/feb/17/thousands-left-without-power-as-storm-dudley-wreak-havoc-across-uk">underway</a> at the time of writing.</p>
<p>Now the UK faces Storm Eunice, and its gusts of up to <a href="https://news.sky.com/story/storm-eunice-britons-urged-to-stay-at-home-as-rare-red-weather-warning-issued-12544848">122 miles per hour</a>. Eunice bears a striking similarity to the <a href="https://www.telegraph.co.uk/men/thinking-man/weatherman-michael-fish-missing-great-storm-1987-saw-happened/">“Great Storm” of 1987</a>, which unleashed hurricane-force winds and claimed 22 lives across Britain and France in October of that year. Both are predicted to contain a “<a href="https://news.sky.com/story/what-is-a-sting-jet-and-how-could-it-make-storm-eunice-deadly-12544077">sting jet</a>”: a small, narrow airstream that can form inside a storm and produce intense winds over an area smaller than 100 km.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1494635055955775490"}"></div></p>
<p>Sting jets, which were first discovered in 2003, and likely occurred during <a href="https://theconversation.com/sting-jet-the-mysterious-cause-of-the-1987-great-storms-worst-winds-85620">the Great Storm</a> and Storm Arwen, can last anywhere between one and 12 hours. They are difficult to forecast and relatively rare, but make storms more dangerous.</p>
<p>Sting jets occur in a certain type of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/extratropical-cyclone">extratropical cyclone</a> – a rotating wind system that forms outside of the tropics. These airstreams form around 5km above the Earth’s surface then descend on the southwest side of a cyclone, close to its centre, accelerating as they do and bringing fast-moving air from high in the atmosphere with them. When they form, they can produce much higher wind speeds on the ground than might otherwise be forecast by studying pressure gradients in the storm’s core alone.</p>
<p>Meteorologists are still working to understand sting jets, but they are likely to have a significant influence on the UK’s weather in a warming climate.</p>
<h2>Windier winters ahead?</h2>
<p>In 1987, the models used for weather forecasts were incapable of representing sting jets, but improvements mean that forecasters predicted Storm Eunice before it had even begun to form in the Atlantic.</p>
<p>Over the past decade, our team at Newcastle University has worked closely with colleagues at the UK Met Office to develop new <a href="https://www.youtube.com/watch?v=m19QM7QHPdU">high-resolution climate models</a> that can simulate sting jets, as well as hail and lightning, to illuminate how extreme weather events might change in a warming climate.</p>
<p>We already know that, as the world warms, <a href="https://www.dw.com/en/german-floods-climate-change/a-58959677#:%7E:text=Burning%2520fossil%2520fuels%2520made%2520the,rapid%2520attribution%2520study%2520has%2520found.&text=The%2520floods%252C%2520which%2520killed%2520more,between%2520July%252012%2520and%252015.">downpours are intensifying</a>. The simple reason is that warmer air can hold more moisture. The UK saw the <a href="https://rmets.onlinelibrary.wiley.com/doi/10.1002/asl.1033">wettest day on record</a> in 2020, already estimated to be 2.5 times more likely because of greenhouse gas emissions. </p>
<p>Our research team’s new high-resolution climate models predict <a href="https://journals.ametsoc.org/view/journals/clim/33/17/jcliD200089.xml">bigger increases in winter rainfall</a> than standard <a href="https://www.gfdl.noaa.gov/climate-modeling/#:%7E:text=of%2520data%2520storage.-,What%2520is%2520a%2520Global%2520Climate%2520Model%253F,to%2520long%252Dterm%2520climate%2520prediction.">global climate models</a> due to a large increase in rainfall from thunderstorms during winter.</p>
<p>We are less certain about how the pattern of extreme wind storms, like Eunice, will change, as the relevant processes are much more complicated. The UK’s recent cluster of winter wind storms is related to a particularly strong <a href="https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/polar-vortex">polar vortex</a> creating low pressure in the Arctic, and a faster jet stream – a core of very strong wind high in the atmosphere that can extend across the Atlantic – bringing stormier and very wet weather <a href="https://www.theguardian.com/news/2020/jan/22/how-the-polar-vortex-influences-britain-weather">to the UK</a>. </p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/Lg91eowtfbw?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
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<p>A stronger jet stream makes storms more powerful and its orientation roughly determines the track of the storm and where it affects. Some aspects of climate change strengthen the jet stream, leading to <a href="https://www.nature.com/articles/s41561-017-0001-8">more UK wind storms</a>. Other aspects, like the higher rate of warming over the poles compared with the equator, may <a href="https://www.nature.com/articles/s41467-022-28283-y">weaken it</a> and the westerly flow of wind towards the UK.</p>
<p>Our high-resolution models predict <a href="https://link.springer.com/article/10.1007/s00382-021-06011-4">more intense wind storms</a> over the UK as climate change accelerates, with much of this increase coming from storms that develop sting jets. </p>
<p>Projections from global climate models are <a href="https://link.springer.com/article/10.1007/s40641-019-00146-7">uncertain</a> and suggest only small increases in <a href="https://wcd.copernicus.org/preprints/wcd-2021-75/wcd-2021-75.pdf">the number of extreme cyclones</a>. But these models fail to represent sting jets and poorly simulate the processes that cause <a href="https://journals.ametsoc.org/view/journals/clim/33/15/JCLI-D-19-0928.1.xml">storms to build</a>. As a result, these models probably underestimate future changes in storm intensity.</p>
<p>We think that using high-resolution climate models, which can represent important processes like sting jets, alongside information from global models on how large-scale conditions might change, could give a more accurate picture. But the UK isn’t doing enough to prepare for the <a href="https://www.theccc.org.uk/publication/independent-assessment-of-uk-climate-risk/">increasingly severe extreme weather</a> already predicted.</p>
<p>Humanity has a choice in how much warmer the world gets based on the rate at which we reduce greenhouse gas emissions. While more research will confirm if more extreme wind storms will hit the UK in the future, we are certain that winter storms will produce stronger downpours and more rain and flooding when they do occur.</p>
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<img alt="Imagine weekly climate newsletter" src="https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/434988/original/file-20211201-21-13avx6y.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p class="fine-print"><em><span>Hayley J. Fowler receives funding from the Natural Environment Research Council. She is a member of the BEIS Science Expert Group and President of the British Hydrological Society. </span></em></p><p class="fine-print"><em><span>Colin Manning receives funding from the Natural Environment Research Council. He also receives support from the UK Met Office as a visiting scientist.</span></em></p>Sting jets are poorly understood, but could have a big influence on Britain’s future winter storms.Hayley J. Fowler, Professor of Climate Change Impacts, Newcastle UniversityColin Manning, Postdoctoral Research Associate in Climate Science, Newcastle UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1758252022-01-27T19:46:48Z2022-01-27T19:46:48ZWhat is a bomb cyclone? An atmospheric scientist explains<figure><img src="https://images.theconversation.com/files/443016/original/file-20220127-22-1gefps5.jpeg?ixlib=rb-1.1.0&rect=6%2C0%2C1042%2C946&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A bomb cyclone over the U.S. East Coast on Jan. 4, 2017.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/east-coast-bomb-cyclone-seen-by-noaas-goes-16-satellite">NOAA/CIRA</a></span></figcaption></figure><p>A bomb cyclone is a large, intense midlatitude storm that has low pressure at its center, weather fronts and an array of associated weather, from blizzards to severe thunderstorms to heavy precipitation. It becomes a bomb when its central pressure decreases very quickly – by at least 24 millibars in 24 hours. Two famed meteorologists, <a href="https://news.mit.edu/2006/obit-sanders">Fred Sanders</a> and <a href="https://www.mcgill.ca/meteo/facultystaff/gyakum">John Gyakum</a>, gave this pattern its name in a <a href="https://doi.org/10.1175/1520-0493(1980)108%3C1589:SDCOT%3E2.0.CO;2">1980 study</a>. </p>
<p>When a cyclone “bombs,” or undergoes bombogenesis, this tells us that it has access to the optimal ingredients for strengthening, such as high amounts of heat, moisture and rising air. Most cyclones don’t intensify rapidly in this way. Bomb cyclones put forecasters on high alert, because they can produce significant harmful impacts.</p>
<p>The U.S. Eastern Seaboard is one of the <a href="https://doi.org/10.1175/1520-0493(2002)130%3C2188:ECDITS%3E2.0.CO;2">regions where bombogenesis is most common</a>. That’s because storms in the <a href="https://www.merriam-webster.com/dictionary/midlatitudes">midlatitudes</a> – a temperate zone north of the tropics that includes the entire continental U.S. – draw their energy from large temperature contrasts. Along the U.S. East Coast during winter, there’s a naturally potent thermal contrast between the cool land and the warm <a href="https://scijinks.gov/gulf-stream/#">Gulf Stream current</a>. </p>
<p>Over the warmer ocean, heat and moisture are abundant. But as cool continental air moves overhead and creates a large difference in temperature, the lower atmosphere becomes unstable and buoyant. Air rises, cools and condenses, forming clouds and precipitation.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/ZfvfIUhJKg0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">UK meteorologist Alex Deakin explains how unstable air causes cumulus clouds to form.</span></figcaption>
</figure>
<p>Intense cyclones also require favorable conditions above the surface. Particularly strong upper-level winds, also known as “jet streaks,” and <a href="https://www.weather.gov/jetstream/longshort">high-amplitude waves</a> embedded within storm tracks can help force air to rise. </p>
<p>When a strong jet streak overlies a developing low-pressure system, it creates a feedback pattern that makes warm air rise at an increasing rate. This allows the pressure to drop rapidly at the center of the system. As the pressure drops, winds strengthen around the storm. Essentially, the atmosphere is trying to even out pressure differences between the center of the system and the area around it.</p>
<p>Weather forecasters are predicting that the northeastern U.S. will be affected by a <a href="https://www.wpc.ncep.noaa.gov/discussions/hpcdiscussions.php?disc=pmdspd">potent winter storm on Jan. 28-30, 2022</a>. Forecast models are calling for a swath of snow from coastal North Carolina northward to Maine.</p>
<p>While precise locations and amounts of snowfall are still uncertain, parts of coastal New England appear most at risk of receiving <a href="https://www.wpc.ncep.noaa.gov/wwd/wssi/wssi.php">8-12 inches or more of heavy accumulating snow</a>. Coupled with winds forecast to be over 50 miles per hour along the coast, the storm is likely to produce blizzard conditions, storm surge, coastal flooding, wind damage and beach erosion. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/trvbHGTDHVg?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Bomb cyclones are sometimes called ‘winter hurricanes,’ but they are a different type of storm.</span></figcaption>
</figure>
<p>This storm’s life is expected to begin offshore of the southeast U.S. as a weak low-pressure system. Just 24 hours later, global models predict that its central pressure will drop by 35-50 millibars. </p>
<p>If this storm develops as forecasts predict, aided by winds blowing at over 150 miles per hour in the upper atmosphere, very warm sea surface temperatures just offshore (2-4 degrees Fahrenheit warmer than average), and a highly unstable atmosphere, it will have the critical ingredients for a bomb cyclone. </p>
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<p class="fine-print"><em><span>Esther Mullens receives funding from the National Science Foundation. </span></em></p>The key ingredients for a storm to undergo bombogenesis are an unstable atmosphere, temperature differences and high-speed winds in the upper atmosphere.Esther Mullens, Assistant Professor of Geography, University of FloridaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1739042021-12-16T06:36:01Z2021-12-16T06:36:01ZHurricane-force wind gusts in Colorado, dust storms in Kansas, tornadoes in Iowa in December – here’s what fueled a day of extreme storms<figure><img src="https://images.theconversation.com/files/437954/original/file-20211216-23-w5ytwd.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1559%2C1059&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A satellite view on the night of Dec. 15, 2021, at the same time tornadoes were reported in Iowa.</span> <span class="attribution"><a class="source" href="https://www.star.nesdis.noaa.gov/GOES/conus.php?sat=G16">NOAA</a></span></figcaption></figure><p><em>Extremely powerful winds swept across a large part of the U.S. on Dec. 15, 2021, <a href="https://twitter.com/NWSSPC/status/1471333229537873924">hitting several states with hurricane-force gusts</a>. Record temperatures helped generate <a href="https://www.desmoinesregister.com/story/weather/2021/12/15/iowa-high-wind-tornado-map-weather-damage-power-outage-updates/8901263002/">tornadoes in Iowa</a>, winds spread <a href="https://twitter.com/NWSWichita/status/1471248332886470657">grass fires</a> and <a href="https://twitter.com/KWCH12/status/1471203794641997826">dust clouds</a> in Kansas, and wind damage was reported from <a href="https://www.thedenverchannel.com/weather/weather-news/see-the-highest-wind-gusts-recorded-during-colorados-wednesday-wind-storm">Colorado</a> through the Midwest. The National Weather Service described it as a “historical weather day” with a “<a href="https://twitter.com/NWS/status/1471117913767743495">never-before-seen storm outlook</a>.” A meteorologist with the service later <a href="https://www.radioiowa.com/2021/12/17/wednesdays-storm-is-derecho-part-two-for-iowa/">said it qualified as a “serial derecho”</a> – the powerful storm had winds over 80 mph but more spread out than the intense <a href="https://theconversation.com/what-is-a-derecho-an-atmospheric-scientist-explains-these-rare-but-dangerous-storm-systems-140319">derecho that hit Iowa in 2020</a>. At least <a href="https://apnews.com/article/tornadoes-iowa-nebraska-storms-kansas-ee9378aae1cb74911e0f9842a01ccaa1">five people died</a> in the storm, the Associated Press reported.</em></p>
<p><em>We asked <a href="https://ge-at.iastate.edu/directory/william-gallus/">atmospheric scientist William Gallus</a>, whose office at Iowa State University was at the heart of the storms, to explain what caused the extreme weather and why it was so unusual.</em></p>
<h2>What happened in the atmosphere to trigger such extreme weather over such a wide area?</h2>
<p>We were seeing very strong winds because of a very powerful disturbance in the <a href="https://www.weather.gov/jetstream/jet">jet stream</a>. That disturbance helped to create a very intense <a href="https://scijinks.gov/high-and-low-pressure-systems/">low-pressure system</a>, which creates strong winds and storms. But the low pressure wasn’t what made this event unusual.</p>
<p>It was unprecedented because an incredible amount of warm air got <a href="https://twitter.com/BMcNoldy/status/1471151049016688640/photo/1">pulled up from the south</a> ahead of the storm.</p>
<p>Here in Iowa, <a href="https://twitter.com/NWSWPC/status/1471307948945285123">temperatures were the hottest</a> they’ve ever been in December, with <a href="https://twitter.com/JerryWVTM13/status/1471221081000267783">temperatures in the mid 70s</a> on Dec. 15, and a very unusual amount of humidity came up with those temperatures. That’s why we were seeing <a href="https://twitter.com/NWSDesMoines/status/1471268524849078280">tornado warnings</a> <a href="https://twitter.com/NWSDesMoines/status/1471265167610572800">across the region</a> – and reports of <a href="https://www.desmoinesregister.com/story/weather/2021/12/15/iowa-high-wind-tornado-map-weather-damage-power-outage-updates/8901263002/">tornado damage</a>. </p>
<p>Tornadoes are <a href="https://twitter.com/capitalweather/status/1471281957569011713">extremely rare in Iowa in December</a>. Minnesota, which had never had a tornado in December, also had tornado warnings and <a href="https://twitter.com/NWSTwinCities/status/1471319877390508036">a possible sighting</a>.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1471258306903687169"}"></div></p>
<p>The wind speeds with this particular system were about as strong as we’ve seen. But it was all of the other weather parameters coming together in December that put this storm system off the scale.</p>
<p>With the warm moist air, we also had thunderstorms, and thunderstorms tend to make the winds even stronger. If you went up 1,000 feet in the sky, you would find it’s much <a href="https://news.stanford.edu/news/2009/june24/high-altitude-winds-062309.html">windier up there</a>. When you have thunderstorms, the rain helps create a current of wind that goes downward, which we call a downdraft. If you have this downdraft, it tends to carry the really strong winds down to the ground. Thunderstorms in the conditions we were seeing could bring winds that could easily get over 80 mph.</p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/what-is-a-derecho-an-atmospheric-scientist-explains-these-rare-but-dangerous-storm-systems-140319">What is a derecho? An atmospheric scientist explains these rare but dangerous storm systems</a>
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<h2>Colorado saw wind gusts over 100 mph. What happened there?</h2>
<p>In Colorado, the mountains also help accelerate the wind. </p>
<p>The winds have to rise over the Rocky Mountains. If you get a <a href="https://www.weather.gov/media/lzk/inversion101.pdf">temperature inversion</a>, where the temperature actually starts to go up rather than down as you get higher in the atmosphere just above the top of the mountains, it can act like a lid that traps the momentum of the wind going over the mountains. The wind can’t really spread out, so instead it rushes downward once you’re on the east side of the mountains.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1471158494787108866"}"></div></p>
<p>With anything going downward, gravity accelerates it, just like if you drop a ball from the top of a skyscraper. The <a href="https://manoa.hawaii.edu/exploringourfluidearth/physical/atmospheric-effects/wind-formation">same thing happens to these winds</a>. As they flow down the east face of the Rockies, they accelerate. </p>
<p>When you’re on the leeward side of a mountain range, like Denver and Boulder, winds in those areas can <a href="https://twitter.com/NWSWPC/status/1471226860499525635">get really strong</a> as they’re descending.</p>
<h2>What role does the jet stream play in a storm like this?</h2>
<p>When we get a low-pressure system, it’s because of wiggles that are happening in the <a href="https://www.weather.gov/jetstream/jet">jet steam</a>. We call them troughs in meteorology.</p>
<figure class="align-right ">
<img alt="Image of the jet stream, looking like a wave" src="https://images.theconversation.com/files/437930/original/file-20211216-25-u8aefo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/437930/original/file-20211216-25-u8aefo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/437930/original/file-20211216-25-u8aefo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/437930/original/file-20211216-25-u8aefo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/437930/original/file-20211216-25-u8aefo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/437930/original/file-20211216-25-u8aefo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/437930/original/file-20211216-25-u8aefo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">An visualization of the jet stream.</span>
<span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/3864">Trent L. Schindler/NASA Scientific Visualization Studio</a></span>
</figcaption>
</figure>
<p>If you look at a map showing the jet stream, the jet stream <a href="https://svs.gsfc.nasa.gov/3864">looks like a roller coaster</a> – it oscillates up and down, from north to south. Any time you’re out ahead of one of these troughs, where the jet stream bends down toward the south and then back toward the north, the air must rise out ahead of it, and this results in a low-pressure system. The winds that blow around it can become very strong. </p>
<p>In this case, there was an especially sharp trough in the jet stream, almost in a “V” shape, that intensified the effect. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/o203JXAnSA0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">What is the jet stream?</span></figcaption>
</figure>
<h2>Is there a connection between this storm and the deadly tornadoes that hit Kentucky and other states on Dec. 10-11?</h2>
<p>It’s hard to say if there was one trigger somewhere on the planet that managed to create these two different ripples in the jet stream.</p>
<p>What’s interesting is that there is <a href="https://www.climate.gov/news-features/blogs/enso/december-2021-la-ni%C3%B1a-update-visual-aids">La Niña going on in the Pacific Ocean</a>. When we have La Niña conditions, we often find that the far northern part of the United States ends up colder than normal and the south ends up warmer, so you have this bigger contrast in temperatures than normal and it often leads to a stronger jet stream.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/tornadoes-and-climate-change-what-a-warming-world-means-for-deadly-twisters-and-the-type-of-storms-that-spawn-them-173645">Tornadoes and climate change: What a warming world means for deadly twisters and the type of storms that spawn them</a>
</strong>
</em>
</p>
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<p>[<em>Over 140,000 readers rely on The Conversation’s newsletters to understand the world.</em> <a href="https://memberservices.theconversation.com/newsletters/?source=inline-140ksignup">Sign up today</a>.]</p>
<p><em>This article was updated with a National Weather Service meteorologist describing the storm as a “serial derecho.”</em></p><img src="https://counter.theconversation.com/content/173904/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>William Gallus does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Forecasters described it as a ‘historical weather day.’ An atmospheric scientist who was at the heart of the storms explains what happened.William Gallus, Professor of Atmospheric Science, Iowa State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1642992021-08-16T12:09:09Z2021-08-16T12:09:09ZHow a volcano and flaming red sunsets led an amateur scientist in Hawaii to discover jet streams<figure><img src="https://images.theconversation.com/files/414701/original/file-20210804-13508-4zcdw5.jpg?ixlib=rb-1.1.0&rect=17%2C14%2C1001%2C491&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The eruption of Krakatoa in 1883 sent volcanic dust and gases circling the Earth, creating spectacular sunsets captured by artists.</span> <span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa#/media/File:Houghton_71-1250_-_Krakatoa,_twilight_and_afterglow.jpg">William Ashcroft via Houghton Library/Harvard University</a></span></figcaption></figure><p>On the evening of Sept. 5, 1883, people in Honolulu witnessed a spectacular sunset followed by a period of extended twilight described as a “<a href="https://evols.library.manoa.hawaii.edu/bitstream/10524/14502/1/1883092201.pdf#page=4">singular lurid after sunset glow</a>.” There were no signs of anything else out of the ordinary, but these exceptional twilight glows returned each morning and evening over the following weeks.</p>
<p>Among the mystified Honolulu citizens was 56-year-old <a href="https://archive.org/details/reminiscencesofo00bish/page/4/mode/2up">Rev. Sereno Edwards Bishop</a>, who in his varied career in Hawaii had been a chaplain, school principal and surveyor, and who had a keen interest in science. Over the subsequent weeks and months, the exceptional twilight glows occurred around the whole globe. Remarkably, as scientists first grappled with understanding the origin of the twilight glows, Bishop’s efforts would lead to the first convincing explanation. </p>
<figure class="align-right ">
<img alt="Profile photo of Bishop with a beard and glasses and wearing a suit" src="https://images.theconversation.com/files/411159/original/file-20210714-25-yq4tfb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/411159/original/file-20210714-25-yq4tfb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=718&fit=crop&dpr=1 600w, https://images.theconversation.com/files/411159/original/file-20210714-25-yq4tfb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=718&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/411159/original/file-20210714-25-yq4tfb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=718&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/411159/original/file-20210714-25-yq4tfb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=903&fit=crop&dpr=1 754w, https://images.theconversation.com/files/411159/original/file-20210714-25-yq4tfb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=903&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/411159/original/file-20210714-25-yq4tfb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=903&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Rev. Sereno Edwards Bishop (1827–1909)</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Sereno_Edwards_Bishop#/media/File:Sereno_E._Bishop.jpg">Wikipedia</a></span>
</figcaption>
</figure>
<p>His discoveries led to scientific investigations of the winds high above the ground and ultimately yielded information that today is used to <a href="https://link.springer.com/article/10.1007/s00382-008-0379-5">forecast weather over extended periods</a>.</p>
<p>I am a <a href="http://iprc.soest.hawaii.edu/users/kph/">meteorologist</a> in Hawaii who helped revive appreciation of <a href="https://www.tandfonline.com/doi/full/10.1080/07055900.2011.639736">Bishop’s seminal contribution</a> to the scientific exploration of the upper atmosphere.</p>
<h2>A volcanic eruption half a world away</h2>
<p>Today we know that the 1883 glows were caused by the sun below the visible horizon illuminating a mist of <a href="https://skyandtelescope.org/observing/volcanoes-turn-twilights-purple/">small liquid droplets in the atmosphere</a> high above the ground.</p>
<p>The mist was made of sulfuric acid droplets that were formed by reactions of the massive amounts of sulfur dioxide gas produced by the explosive eruption of Mount Krakatoa close to the equator in Indonesia on Aug. 27, 1883. The eruption sent the droplets high into the atmosphere, where the winds transported them around the world. They spread gradually, and it was November before people in <a href="http://www.simonwinchester.com/krakatoa">London</a> began to notice the glow.</p>
<p>Much later, scientists observed <a href="http://www.dewbow.co.uk/glows/sunset5.html">similar effects</a> after the June 1991 <a href="https://pubs.usgs.gov/pinatubo/self/">eruption of Mount Pinatubo in the Philippines</a>. The material Pinatubo injected into the upper atmosphere could be followed in detail with satellite observations, and their connection with spectacular sunsets and twilight glows was <a href="https://pubs.usgs.gov/pinatubo/self/">clearly established</a>.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/414683/original/file-20210804-27-qb2zwz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/414683/original/file-20210804-27-qb2zwz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=863&fit=crop&dpr=1 600w, https://images.theconversation.com/files/414683/original/file-20210804-27-qb2zwz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=863&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/414683/original/file-20210804-27-qb2zwz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=863&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/414683/original/file-20210804-27-qb2zwz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1085&fit=crop&dpr=1 754w, https://images.theconversation.com/files/414683/original/file-20210804-27-qb2zwz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1085&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/414683/original/file-20210804-27-qb2zwz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1085&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Sketches of twilight and afterglow on one evening in 1883 in London following the Krakatoa eruption.</span>
<span class="attribution"><span class="source">William Ashcroft via Houghton Library/Harvard University</span></span>
</figcaption>
</figure>
<p>In 1883, Bishop had no idea that there had been a volcanic eruption until the San Francisco newspapers arrived. Very quickly, he formulated a hypothesis that he published as <a href="https://evols.library.manoa.hawaii.edu/bitstream/10524/14502/1/1883092201.pdf#page=4">a letter</a> in his <a href="https://evols.library.manoa.hawaii.edu/bitstream/10524/14502/1/1883092201.pdf">local newspaper</a>.</p>
<p>“I am disposed to conjecture that some very light element among the vapors of the Java eruptions has continued at a very great height in the atmosphere, and has been borne … across the Pacific into this region,” Bishop wrote.</p>
<p>He realized that he could connect the eruption to the glowing skies most credibly by gathering reports of the first appearance of the glows elsewhere and tracking the initial spread of the “vapor” from Krakatoa. Bishop continued his letter: “I earnestly invite, in behalf of science, all shipmasters and mates to publish what they may have observed at sea.”</p>
<p>Bishop assembled a dozen such reports over the first three weeks after the eruption and was able to show that the “vapor” that produced the glows had moved westward from Krakatoa, along the equator to reach Honolulu 10 days later. This implied that there was a wind high in the atmosphere blowing steadily with an extreme speed that, at ground level, is seen only in hurricanes.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/414682/original/file-20210804-13-1iv4fpx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/414682/original/file-20210804-13-1iv4fpx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=255&fit=crop&dpr=1 600w, https://images.theconversation.com/files/414682/original/file-20210804-13-1iv4fpx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=255&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/414682/original/file-20210804-13-1iv4fpx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=255&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/414682/original/file-20210804-13-1iv4fpx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=321&fit=crop&dpr=1 754w, https://images.theconversation.com/files/414682/original/file-20210804-13-1iv4fpx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=321&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/414682/original/file-20210804-13-1iv4fpx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=321&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Tracking the red sunsets following the Krakatoa eruption. The stars mark the initial reports and dates of seeing the exceptional twilight colors in 1883.</span>
</figcaption>
</figure>
<p>Bishop <a href="https://ia800604.us.archive.org/25/items/afw8023.0001.001.umich.edu/afw8023.0001.001.umich.edu.pdf#page=122">published his observations</a> in <a href="https://onlinebooks.library.upenn.edu/webbin/serial?id=hawaiianmonthly">The Hawaiian Monthly</a>, concluding that there was “a vast stream of smoke due west with great precision along a narrow equatorial belt with an enormous velocity, around the globe.”</p>
<h2>The equatorial jet stream</h2>
<p>Bishop called the motion of the volcanic aerosol a “smoke stream.” In fact, the equatorial winds transporting the aerosol were the first discovery of what meteorologists now call a jet stream. </p>
<p>A half-century would pass before the experiences of pilots flying at heights of several miles revealed the <a href="http://scihi.org/wiley-post-jetstream/">existence of the extratropical jet streams</a> lower down in the atmosphere that are now familiar from TV newscasts. Jet streams are strong, typically narrow bands of wind. The more familiar lower atmospheric jet streams move weather systems in the middle latitudes from west to east. By contrast, Bishop’s jet stream circles the equator at high altitudes and actually can blow from east to west.</p>
<p>Bishop’s work opened further exploration of the equatorial jet stream that culminated in the <a href="https://agupubs.onlinelibrary.wiley.com/doi/10.1029/JZ066i003p00813">1961 discovery</a> that the equatorial jet stream varied from strong east winds to strong west winds roughly every other year. This so-called <a href="https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1999RG000073">Quasi-biennial Oscillation</a> has been shown to <a href="https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2009JD011737">connect with weather near the ground</a>, particularly in Europe and the North Atlantic, a fact that is now routinely <a href="https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/quasi-biennial-oscillation">exploited in making long range forecasts for the weather</a>.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/5wCq2Y9CB6Y?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>Bishop’s contribution was <a href="https://archive.org/details/eruptionkrakato00whipgoog/page/n18/mode/2up">acknowledged</a> by the <a href="https://archive.org/details/eruptionkrakato00whipgoog/page/n12/mode/2up">scientists who first followed him,</a> and he won a prize from New York’s Warner Observatory in a contest for <a href="https://www.nature.com/articles/045422a0">essays explaining the post-Krakatoa glows</a>. Bishop even merited a brief <a href="https://journals.ametsoc.org/downloadpdf/journals/mwre/37/2/1520-0493_1909_37_47b_dsb_2_0_co_2.pdf">obituary</a> in an American meteorological science journal.</p>
<p>Bishop, who was the son of missionaries, could also be a divisive figure in Hawaii. He supported the U.S. annexation of the islands, and his religious views opposed some native Hawai'ian traditions, <a href="http://www.ulukau.org/elib/cgi-bin/library?e=d-0voicesofeden-000Sec--11en-50-20-contact-book--1-010escapewin&a=d&d=D0.18&toc=0">such as the hula dance</a>. His contributions to science were largely forgotten in the 20th century.</p>
<p>An international scientific committee’s celebration of the <a href="https://www.sparc-climate.org/meetings/qbo60-celebrating-60-years-of-discovery-within-the-tropical-stratosphere/">60th anniversary of the Quasi-biennial Oscillation discovery</a> is an opportunity to <a href="http://iprc.soest.hawaii.edu/users/kph/zoom_0.mp4">remember Bishop</a> and his discovery.</p>
<p>[<em>You’re smart and curious about the world. So are The Conversation’s authors and editors.</em> <a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=youresmart">You can read us daily by subscribing to our newsletter</a>.]</p><img src="https://counter.theconversation.com/content/164299/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kevin Hamilton does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The Rev. Sereno Edwards Bishop mobilized ship captains to track the extraordinary sunsets appearing around the world after Krakatau erupted in 1883.Kevin Hamilton, Emeritus Professor of Atmospheric Sciences, University of HawaiiLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1511802021-01-25T13:29:51Z2021-01-25T13:29:51ZWhy does it take longer to fly from east to west on an airplane?<figure><img src="https://images.theconversation.com/files/378155/original/file-20210111-17-hrdaro.jpg?ixlib=rb-1.1.0&rect=0%2C50%2C1533%2C970&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The jet stream can have a big impact on how long a plane ride will last.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/w/index.php?title=Special:Search&limit=500&offset=500&profile=default&search=boeing+747&advancedSearch-current=%7B%7D&ns0=1&ns6=1&ns12=1&ns14=1&ns100=1&ns106=1&searchToken=6ddmgacgqvctsdphhxwo6ty0n#%2Fmedia%2FFile%3AG-CIVA_Boeing_747_British_Airways_%288401802240%29.jpg">Aeroprints via Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
<hr>
<blockquote>
<p><strong>Why does it take longer to fly from east to west on an airplane? – Henry D, Age 7, Cambridge, Massachusetts</strong></p>
</blockquote>
<hr>
<p>I am a retired <a href="https://www.unomaha.edu/college-of-public-affairs-and-community-service/aviation/about-us/faculty-staff/skip-bailey.php">United States Air Force pilot and flight instructor</a>, and a few years ago I was sitting in the cockpit of a Boeing 747 airplane. I was 29,000 feet in the sky, flying from New Jersey to Sacramento, California, and then to Hawaii. It took six hours to fly and land the plane safely in Sacramento. After a few hours in California, I continued to Hawaii,which took almost another five hours of flying. That was 11 total hours of flying. </p>
<p>After enjoying the sunshine in Hawaii, it was time to fly back to New Jersey. This trip went much faster. I didn’t stop in California this time, but flying back only took about eight and a half hours. I was still flying the same airplane, and New Jersey wasn’t any closer to California or Hawaii than it had been a few days before. </p>
<p>So why was my flight to Hawaii, from east to west, so much longer than my flight home?</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/378134/original/file-20210111-13-mc63ul.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A graphic showing the rough locations of jet streams around the globe." src="https://images.theconversation.com/files/378134/original/file-20210111-13-mc63ul.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/378134/original/file-20210111-13-mc63ul.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=345&fit=crop&dpr=1 600w, https://images.theconversation.com/files/378134/original/file-20210111-13-mc63ul.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=345&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/378134/original/file-20210111-13-mc63ul.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=345&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/378134/original/file-20210111-13-mc63ul.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=433&fit=crop&dpr=1 754w, https://images.theconversation.com/files/378134/original/file-20210111-13-mc63ul.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=433&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/378134/original/file-20210111-13-mc63ul.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=433&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Jet streams always blow from west to east and can found in many places around the world.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Jet_stream#/media/File:Jetstreamconfig.jpg">Lyndon State College Meteorology via Wikipedia</a></span>
</figcaption>
</figure>
<h2>Riding on a river of air</h2>
<p>The reason it took so much longer to fly back is the <a href="https://en.wikipedia.org/wiki/Jet_stream">jet stream</a>, a river of fast-moving air high up in the sky. </p>
<p>Jet streams are usually about 100 miles wide. They can be thousands of miles long and are found all over the earth. To be called a jet stream, the wind must be moving faster than 60 mph. </p>
<p>Jet streams generally blow from the west to the east around the Earth, often <a href="https://doi.org/10.1029/2008GL033614">following a meandering, curved path</a> just like a river on land. The jet stream over the United States never stays in one place – it tends to move <a href="https://doi.org/10.1175/1520-0469(1961)018%3C0172:TSJSOW%3E2.0.CO;2">farther south and blow stronger in the winter</a>, and to move farther north and not blow as strong in the summer. </p>
<p>So what does this have to do with planes?</p>
<h2>Flying into the wind</h2>
<p>Airplane pilots measure speed in two different ways. First is airspeed – how fast the wind would feel if you stuck your hand out the window. The second is ground speed – how fast the plane is moving over the ground. When you fly in the jet stream, your airspeed always stays the same, but your ground speed can change a lot because the air around the plane is moving.</p>
<p>On the way to Hawaii, I was flying with an airspeed of 562 mph. But because the jet stream was blowing against my airplane – called a headwind – at 140 mph, I was actually only moving across the ground at 422 mph.</p>
<p>But flying from Hawaii to New Jersey, the jet stream blows from behind the plane and pushes it forward. I was still flying with an airspeed of 562 mph, but the 140 mph tailwind meant that my airplane was moving across the ground at 702 mph.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/378135/original/file-20210111-15-4p9wus.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A graphic showing how routes differ when a plan flies from east to west or west to east." src="https://images.theconversation.com/files/378135/original/file-20210111-15-4p9wus.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/378135/original/file-20210111-15-4p9wus.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=275&fit=crop&dpr=1 600w, https://images.theconversation.com/files/378135/original/file-20210111-15-4p9wus.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=275&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/378135/original/file-20210111-15-4p9wus.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=275&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/378135/original/file-20210111-15-4p9wus.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=346&fit=crop&dpr=1 754w, https://images.theconversation.com/files/378135/original/file-20210111-15-4p9wus.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=346&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/378135/original/file-20210111-15-4p9wus.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=346&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Planes will try to avoid the jet stream when flying east to west, but try to hitch a ride on the way back.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Jet_stream#/media/File:Greatcircle_Jetstream_routes.svg">ChaosNil via Wikipedia</a></span>
</figcaption>
</figure>
<h2>Pilots plan to go fast</h2>
<p>When pilots plan the route of their flights, they often use weather forecasts <a href="https://doi.org/10.1175/1520-0493(1981)109%3C2450:COTWPJ%3E2.0.CO;2">to find where the jet stream is blowing</a>. When they fly from the east to the west, they try <a href="https://simpleflying.com/why-aircraft-dont-fly-in-a-straight-line-from-origin-to-destination/">to plan their flight</a> so the jet stream isn’t blowing against their airplane and giving them a bad headwind. When they plan their flight from the west to the east, they look for the jet stream and try to fly so it can give them a big tailwind and help them fly faster. A good plan can help conserve fuel too.</p>
<p>The next time you are flying high across the country from east to west, don’t be surprised when it takes a little longer than you expect. But be excited knowing that when you fly back your pilot is probably hitching a ride along the jet stream to get you home fast.</p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/151180/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Skip Bailey does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>When planes fly from east to west, they are flying against a river of air called a jet stream. These air currents can make your flight longer or shorter, depending on which way you are going.Skip Bailey, Aviation Institute Flight Training Coordinator and Instructor, University of Nebraska OmahaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1528522021-01-12T15:13:44Z2021-01-12T15:13:44ZHow mapping the weather 12,000 years ago can help predict future climate change<figure><img src="https://images.theconversation.com/files/378283/original/file-20210112-19-z7apog.jpg?ixlib=rb-1.1.0&rect=31%2C4%2C2964%2C1958&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">As the jet stream moves northwards, the UK can expect more storms and flooding in the winter.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/largs-scotland-uk-february-08-2019-1307925253">James McDowall/Shutterstock</a></span></figcaption></figure><p>The end of the last <a href="https://www.livescience.com/40311-pleistocene-epoch.html#:%7E:text=The%20Pleistocene%20Epoch%20is%20typically,parts%20of%20the%20planet%20Earth.%20could%20I%20suggest%20using%20https://qra.org.uk/what-is-the-quaternary/%20as%20the%20site%20you%20suggested%20has%20some%20irrelevant%20info%20and%20videos?">ice age</a>, around 12,000 years ago, was characterised by a final cold phase called the <a href="https://www.britannica.com/science/Younger-Dryas-climate-interval">Younger Dryas</a>. Scandinavia was still mostly covered in ice, and across Europe the mountains had many more, and larger, glaciers than today. There was a substantial icefield in the west of Scotland and glaciers could be found on many mountains across the British Isles.</p>
<p>Not surprisingly, the climate was colder back then, especially in winter, with temperatures in the UK getting down to -30°C or lower. Despite these freezing ice-age winters, differences in the Earth’s orbit around the Sun meant the summers were relatively warm, with an average temperature in July between 7°C and 10°C across most of the UK and Ireland.</p>
<p>Then, as now, the <a href="https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/wind/what-is-the-jet-stream">polar front jet stream</a> (a high-altitude fast-moving wind belt) had a major influence on the weather across Europe, bringing precipitation (rain and snow) from the Atlantic across the continent. However, before the time of written climate records, the timing, quantity and pattern of precipitation are poorly understood.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Lg91eowtfbw?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>Our <a href="https://advances.sciencemag.org/content/6/50/eaba4844">new study</a> has used glaciers that existed during the Younger Dryas to determine the precipitation patterns and path of the jet stream across Europe at that time. We identified glacial landforms called <a href="https://www.nationalgeographic.org/encyclopedia/moraine/">moraines</a> at 122 sites from Morocco in the south to Norway in the north, and from Ireland in the west to Turkey in the east, which demonstrated the presence of glaciers some 12,000 years ago.</p>
<p>We reconstructed the 3D geometry of each of these glaciers using knowledge of the way that ice flows across the landscape. From the reconstructed ice surfaces, we could determine an important point on each of these glaciers, the <a href="http://www.antarcticglaciers.org/glacier-processes/mass-balance/introduction-glacier-mass-balance/,">equilibrium line altitude</a> which is linked to climate via yearly precipitation and average summer temperature.</p>
<p>It is essentially the altitude on the glacier where snow accumulation and snow melt are equal at the end of September and can be seen as the snowline. The results provided a map of precipitation across Europe about 12,000 years ago which was controlled by the jet stream.</p>
<h2>Jet stream weather</h2>
<p>What the results showed was that the UK, Ireland, Portugal and Spain were mostly wetter than the present day, as was the Mediterranean, especially in the east – the Balkans, Greece and Turkey. It was relatively drier across much of France, Belgium, the Netherlands, Germany and farther east across Europe. These areas of wetter and drier climate allowed us to identify the location of the jet stream.</p>
<p>We surmised that the jet stream passed over the wetter regions bringing with it the storms (known as <a href="https://www.coolgeography.co.uk/A-level/AQA/Year%2013/Weather%20and%20climate/British%20Isles/British-Depressions.htm">mid-latitude depressions</a>) we are all familiar with in the UK – especially Scotland – and also potentially generated other smaller, more intense storms. Based on the path of the jet stream it is believed that the autumn and spring were wettest in the UK and Ireland and that the winters were drier.</p>
<p>Across Portugal, Spain and the Mediterranean, the winter months were probably the wettest, with autumn and spring being somewhat drier. This is the first time that we have had an insight into the seasonal weather patterns across Europe during the Younger Dryas, and indeed such glimpses of past climate, beyond the period for which we have recorded climate observations, are rare.</p>
<p>Normally it is only <a href="https://www.metoffice.gov.uk/weather/climate/science/climate-modelling">numerical climate models</a> that reveal such a regional scale view on past atmospheric circulation, storm tracks and precipitation. Numerical climate models plot our weather and climate by dividing the atmosphere, Earth’s surface and ocean into multiple interconnected cells, vertically and horizontally, in a three-dimensional grid, and solve complex mathematical equations to determine how energy and matter move through the system. </p>
<figure class="align-center ">
<img alt="A computer model of a 12,000-year-old glacier in the Iberian peninsula." src="https://images.theconversation.com/files/378292/original/file-20210112-23-8wk2gt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/378292/original/file-20210112-23-8wk2gt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=315&fit=crop&dpr=1 600w, https://images.theconversation.com/files/378292/original/file-20210112-23-8wk2gt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=315&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/378292/original/file-20210112-23-8wk2gt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=315&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/378292/original/file-20210112-23-8wk2gt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=396&fit=crop&dpr=1 754w, https://images.theconversation.com/files/378292/original/file-20210112-23-8wk2gt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=396&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/378292/original/file-20210112-23-8wk2gt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=396&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A 3D reconstruction of the Cuerpo de Hombre palaeoglacier in the Central Range of the Iberian Peninsula.</span>
<span class="attribution"><span class="source">Brice Rea, University of Aberdeen</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>Changing jet stream</h2>
<p>In our study, a comparison of the glacier-derived precipitation from 12,000 years ago was made with the outputs from several <a href="https://www.nature.com/subjects/palaeoclimate%20how%20about%20https://www.climate.gov/maps-data/primer/past-climate">palaeoclimate</a> (the study of climate in the past) computer simulations. Numerical climate models are extremely complex, yet they remain a simplification of reality, so different models inevitably generate outputs which variously agree and disagree.</p>
<p>The general pattern of precipitation determined from our study of the palaeo-glaciers agreed with some parts of the climate model outputs, but in disagreement with others – for example, none of the climate models identified all of the UK, Ireland, Portugal, Spain and Mediterranean as being wetter in the past.</p>
<p>We are already seeing signs that the <a href="https://theconversation.com/a-battle-for-the-jet-stream-is-raging-above-our-heads-125906">jet stream may be changing as the climate warms</a> and it is thought that it will probably move northwards and become wavier. These ripples could lead to more extremes, for example, heatwaves in summer and more storms and flooding in the winter.</p>
<p>To understand how climate will change in the future we rely on computer models, but these models do not yet agree on what happened in the past nor on exactly what will happen in the future. To make better future predictions from ongoing climate warming, palaeoclimate datasets, such as the glacier-derived precipitation determined from our study, can be used to test the computer models.</p>
<p>When the models can better reproduce precipitation patterns reconstructed from past climates, especially in periods when the jet stream has moved, then our confidence in their predictions of future climate will also be boosted.</p><img src="https://counter.theconversation.com/content/152852/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>This work was funded by the Leverhulome Trust International Network Grant IN-2012-140. </span></em></p>Ice Age glaciers can help us track the jet stream 12,000 ago, and by comparing its path today we can see how it’s moving northwards, changing weather patterns and indicating climate change.Brice Rea, Professor, Geography, University of AberdeenLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1528762021-01-11T13:36:23Z2021-01-11T13:36:23ZBeast from the East 2? What ‘sudden stratospheric warming’ involves and why it can cause freezing surface weather<figure><img src="https://images.theconversation.com/files/377756/original/file-20210108-13-1xu9zrw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Darryl Fonseka / shutterstock</span></span></figcaption></figure><p>A “sudden stratospheric warming” event took place in early January 2021, according to the <a href="https://blog.metoffice.gov.uk/2021/01/05/new-year-begins-with-a-sudden-stratospheric-warming/">Met Office</a>, the UK’s national weather service. These events are some of the most extreme of atmospheric phenomena, and I study them as part of my academic research. The stratosphere is the layer of the atmosphere from around 10km to 50km above the Earth’s surface, and sudden warming up there can lead to very cold weather over Europe and Siberia, with an increased possibility of snow storms.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1346393319010693122"}"></div></p>
<p>In winter the polar regions are in darkness 24 hours a day, and so the stratosphere over the north pole drops to -60°C or even lower. The pole is surrounded by strong westerly winds, forming what is known as the polar vortex, a normal occurrence which develops every winter. However, about six times a decade, this vortex can break down in dramatic fashion. This can lead to temperatures over the pole increasing by up to 50°C over a few days, although temperatures are so low that they still remain below freezing. The average wind direction around the pole may also reverse, in which case a “sudden stratospheric warming” event has occurred.</p>
<p>The disturbance in the stratosphere can then be transmitted downward through the atmosphere. If this disturbance reaches the lower levels of the atmosphere it can affect the jet stream, a current of air which normally snakes eastwards around the planet, dividing colder polar air from warmer air to the south.</p>
<p>Where the jet stream crosses the Atlantic it usually points towards the British Isles, but sudden stratospheric warming can lead it to shift towards the equator. As air currents are temporarily rearranged, warmer Atlantic air is replaced by cold air from Siberia or the Arctic, and Europe and Northern Asia may experience unusually cold weather. This is what happened when the infamous “<a href="https://theconversation.com/beast-from-the-east-the-science-behind-europes-siberian-chill-92385">Beast from the East</a>” passed through Europe in 2018, causing huge snowstorms and dozens of deaths.</p>
<p>It can take a number of weeks for the impact of stratospheric warming to reach the surface, or the process may only take a few days. These events are hard to predict in advance. Some can only be predicted a few days ahead while others may be forecast from around two weeks before. </p>
<p>A number of factors including a <a href="https://theconversation.com/our-new-model-shows-australia-can-expect-11-tropical-cyclones-this-season-146318">La Niña event</a> in the tropical Pacific contributed to a strong vortex in early winter 2020/21. Strong vortices are hard to shift, meaning a sudden stratospheric warming event was not looking particularly likely. However, from just before Christmas, weather forecast model predictions began to converge on a likely stratospheric warming event in early January.</p>
<h2>From stratosphere to surface</h2>
<p>Around <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018JD030077">two thirds</a> of stratospheric warming events have a detectable surface impact, up to 40 days after the onset of the event. This is usually marked by lower than normal temperatures across Northern Europe and Asia, extending into western Europe, but with warmer temperatures over the eastern Canadian Arctic. </p>
<p>It’s not yet clear why some stratospheric warming events take weeks to impact the surface while others are felt days later, but it may be related to how the polar vortex changes around the onset of a warming event. The vortex can split into two smaller “child vortices”, or it can be displaced from its more usual position centred near the pole, to being over northern Siberia. </p>
<p>Early indications suggested that 2021’s event was more likely to be split, but it subsequently showed more features of <a href="https://www.stratobserve.com/prs_gpht_maps">a displacement</a>. It is not unusual for the vortex to show such mixed signals.</p>
<p>Colleagues and I recently developed a new method for <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JD033881">tracking the impact of a warming event</a> from its onset in the stratosphere to when its effect reaches the surface. We analysed 40 such events from the past 60 years, to try and figure out when we might expect extreme surface weather. </p>
<p>Most importantly, we found that warming events in which the stratospheric polar vortex splits in two generally lead to surface impacts appearing faster and stronger. So although there is an increased chance of snow and extreme cold in mid to late January 2021, other confounding factors may act to reduce this impact. </p>
<p>There are always competing forces at work in the atmosphere. Few people noticed the sudden stratospheric warming of January 2019 for example, which had little impact on the European winter. In that instance, there was a <a href="https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/asl.1009">westerly influence on the North Atlantic winds</a>, which originated in the tropics. This may have acted to oppose any stratospheric effect favouring easterly winds. In 2021, the battle is between the stratospheric warming and La Niña.</p>
<p>Sudden stratospheric warming events are a natural atmospheric fluctuation, not caused by climate change. So even with climate change, these events will still occur, which means that we need to be adaptable to an even more extreme range of temperatures.</p><img src="https://counter.theconversation.com/content/152876/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Richard Hall receives funding from NERC. </span></em></p>Sudden warming more than 10 km above the north pole can mean sudden freezing down here.Richard Hall, Research Associate, Climate Dynamics Group, University of BristolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1520382020-12-17T10:39:27Z2020-12-17T10:39:27ZWhy snow days are becoming increasingly rare in the UK<figure><img src="https://images.theconversation.com/files/375372/original/file-20201216-19-ddxaks.jpeg?ixlib=rb-1.1.0&rect=0%2C0%2C2464%2C1630&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A snowy start to the day at Watlington station, King's Lynn. December 18 2009.</span> <span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Winter_of_2009%E2%80%9310_in_Great_Britain_and_Ireland#/media/File:Snowy_365_at_Watlington.JPG">Lewis Collard/Wikipedia</a></span></figcaption></figure><p><a href="https://www.historic-uk.com/HistoryUK/HistoryofEngland/The-Thames-Frost-Fairs/">Winter frost fairs</a> were common on the frozen River Thames between the 17th and 19th centuries, but they’ve become unimaginable in our lifetime. Over decades and centuries, natural variability in the climate has plunged the UK into sub-zero temperatures from time to time. But global warming is tipping the odds away from the weather we once knew.</p>
<p>These days, people in the UK have become accustomed to much <a href="https://rmets.onlinelibrary.wiley.com/toc/10970088/2020/40/S1">warmer, wetter winters</a>. In fact, winter is warming faster than any other season. This is bad news for those holding out for a white Christmas – the <a href="https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/snow/white-christmas">Met Office reports</a> that only four Christmases in over five decades recorded snow at more than 40% of UK weather stations.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/375339/original/file-20201216-13-xs58p7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Painting of people, tents and horse-drawn carriages on the frozen river." src="https://images.theconversation.com/files/375339/original/file-20201216-13-xs58p7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/375339/original/file-20201216-13-xs58p7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=627&fit=crop&dpr=1 600w, https://images.theconversation.com/files/375339/original/file-20201216-13-xs58p7.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=627&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/375339/original/file-20201216-13-xs58p7.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=627&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/375339/original/file-20201216-13-xs58p7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=787&fit=crop&dpr=1 754w, https://images.theconversation.com/files/375339/original/file-20201216-13-xs58p7.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=787&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/375339/original/file-20201216-13-xs58p7.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=787&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A frost fair on the River Thames, painted by Thomas Wyke (1683-1684).</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Thomas_Wyke-_Thames_frost_fair.JPG">Thomas Wyke/Wikipedia</a></span>
</figcaption>
</figure>
<p>Christmas is a magical day for many, but meteorologically, it’s no different from other winter periods, when snow and ice are also becoming less common. The Met Office definition of a snow day at a given location in the UK is when snow lies on at least 50% of the ground at 9am. Currently, the Cairngorms around Aviemore receive over 70 snow-lying days per year – the most in the UK. </p>
<p>This amount is smaller than in previous decades though. <a href="https://www.metoffice.gov.uk/research/climate/maps-and-data/data/haduk-grid/haduk-grid">Met Office data</a> shows that, since 1979, the number of snow-lying days has generally decreased by up to five days per decade, and up to ten days per decade in the North Pennines, near Penrith. Around a fifth of the total area of the UK has experienced a significant drop in the prevalence of days with snow lying on the ground.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/374778/original/file-20201214-13-ikelqr.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two maps of the UK depicting the change in prevalence of snow days throughout the UK from 1971-2019." src="https://images.theconversation.com/files/374778/original/file-20201214-13-ikelqr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/374778/original/file-20201214-13-ikelqr.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=270&fit=crop&dpr=1 600w, https://images.theconversation.com/files/374778/original/file-20201214-13-ikelqr.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=270&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/374778/original/file-20201214-13-ikelqr.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=270&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/374778/original/file-20201214-13-ikelqr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=340&fit=crop&dpr=1 754w, https://images.theconversation.com/files/374778/original/file-20201214-13-ikelqr.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=340&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/374778/original/file-20201214-13-ikelqr.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=340&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Snow days are a rarer occasion in the UK today than they were five decades ago.</span>
<span class="attribution"><a class="source" href="https://www.metoffice.gov.uk/research/climate/maps-and-data/data/haduk-grid/haduk-grid">Met Office</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>What causes snow days?</h2>
<p>Snow days are often the result of a meandering <a href="https://theconversation.com/uk/topics/jet-stream-14987">jet stream</a>, the fast-flowing current of air that’s between 9km and 16km above the Earth’s surface. The jet stream normally transports temperate weather from the Atlantic across the UK, but if it’s displaced southwards, it allows persistent high pressure systems of colder air from the north and east, originating in the Arctic or over the Eurasian continent, known as blocking high pressures, to settle over the UK for extended periods. </p>
<p>A number of atmospheric processes can cause the jet stream to meander, but perhaps the most dramatic is when the stratospheric polar vortex, a huge rotating air mass in the middle atmosphere, breaks down. This disruption causes the jet stream to weaken, leading to events such as the infamous 2018 <a href="https://theconversation.com/beast-from-the-east-the-science-behind-europes-siberian-chill-92385">Beast from the East</a>, which brought widespread snowfall to the UK.</p>
<p>The winter of 2018 was not unique in this sense – 2009-2010 and 2013 both brought snowfall because of these dynamic “beasts”. So why is there still a decline in winter snow days in the UK?</p>
<h2>The snows of yesteryear</h2>
<p>There’s no <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL073071">strong evidence</a> for a long-term trend in polar vortex disruptions, or other atmospheric processes that influence the jet stream. So the fact that people in the UK have fewer snow days to enjoy each year than they did in the past can’t be blamed on the invisible twists and turns above their heads.</p>
<p>But as the concentration of CO₂ in the atmosphere climbs, disruptions that do occur sit on top of increasing background temperatures, reducing the likelihood of the cold spells that bring widespread snowfall. Just as natural climate trends have lowered the severity of winters since the days of the frost fairs, man-made climate change will increasingly keep the UK’s average temperature above zero.</p>
<p>A heavy covering of snow can transform the country and our perception of it. Snow days, with the closures of schools and workplaces that they bring, evoke fond memories and bring out the child in many as hillslopes and parks become sledging highways. More tangibly, in Scotland, the snowsports industry is estimated to be worth <a href="https://www.pressandjournal.co.uk/fp/news/politics/scottish-politics/1639897/snow-sports-contribute-31m-to-scottish-economy/">over £30 million</a> a year. </p>
<p>But wintry weather can be dangerous too. The cold affects our <a href="https://jech.bmj.com/content/68/7/641.short?casa_token=o9ihfq8efcUAAAAA:bOGF90i4RAl0xKUjj4kx4_Q5D5V0x1364f7nrDXchvqRXC4G90QQroOMV9mp0mfq1BlLi-wrhO0">health</a>, exacerbating heart and lung conditions and <a href="https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/748492/the_cold_weather_plan_for_england_2018.pdf">the spread of infectious diseases</a>. In extreme cases, heavy snowfall can cause widespread livestock deaths, which happened in <a href="https://www.belfasttelegraph.co.uk/news/northern-ireland/blizzard-death-toll-for-livestock-hits-44000-29229931.html">Northern Ireland in 2013</a>. The inevitable disruption to travel and businesses can cause economic damage running into <a href="https://www.theguardian.com/uk-news/2018/mar/03/freezing-weather-storm-emma-cost-uk-economy-1-billion-pounds-a-day">billions of pounds</a>, with sectors like the construction industry halted entirely.</p>
<p>While the falling chances of a white Christmas might disappoint many, the current trajectory of less and less snow will at least come as a relief to some.</p><img src="https://counter.theconversation.com/content/152038/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alan Thomas Kennedy-Asser receives funding from NERC.</span></em></p><p class="fine-print"><em><span>Dann Mitchell receives funding from NERC. </span></em></p><p class="fine-print"><em><span>Eunice Lo receives funding from NERC. </span></em></p>Since 1979, the average number of snow days has fallen by about five per decade.Alan Thomas Kennedy-Asser, Research Associate in Climate Science, University of BristolDann Mitchell, Met Office Co-Chair in Climate Hazards, University of BristolEunice Lo, Research Associate in Climate Science, University of BristolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1463812020-09-22T12:21:46Z2020-09-22T12:21:46ZHow can smoke from West Coast fires cause red sunsets in New York?<figure><img src="https://images.theconversation.com/files/359137/original/file-20200921-16-16qwk42.jpg?ixlib=rb-1.1.0&rect=73%2C0%2C4297%2C2507&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A red hazy sunset over Indiana caused by wildfire smoke from the Western U.S.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/the-sun-sets-beyond-kirkwood-avenue-in-a-haze-of-smoke-news-photo/1228538354?adppopup=true"> SOPA Images/LightRocket va Getty Images</a></span></figcaption></figure><p>If you are one of the millions of people in the Midwest and Eastern U.S. who turned your gaze toward the sky recently, you may have noticed the Sun shining through an odd, milky haze. This widespread opaque veil was caused not by clouds, but rather by smoke from wildfires in the Western U.S.</p>
<p>The smoke was cruising by in the middle levels of the atmosphere many thousands of feet above the ground. While far too high to smell, it caused spectacularly hued sunsets from New York to D.C. to Missouri.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/359082/original/file-20200921-16-a2ozww.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A satellite photo of the Eastern U.S. with a haze of smoke visible." src="https://images.theconversation.com/files/359082/original/file-20200921-16-a2ozww.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/359082/original/file-20200921-16-a2ozww.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=602&fit=crop&dpr=1 600w, https://images.theconversation.com/files/359082/original/file-20200921-16-a2ozww.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=602&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/359082/original/file-20200921-16-a2ozww.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=602&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/359082/original/file-20200921-16-a2ozww.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=757&fit=crop&dpr=1 754w, https://images.theconversation.com/files/359082/original/file-20200921-16-a2ozww.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=757&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/359082/original/file-20200921-16-a2ozww.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=757&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The thin haze, easily visible in this satellite photo over Pennsylvania and New York, is smoke that traveled over a thousand miles on air currents from the fires on the West Coast.</span>
<span class="attribution"><a class="source" href="https://www.noaa.gov/">NOAA</a></span>
</figcaption>
</figure>
<h2>Red sky at night</h2>
<p>Lasting for about a week from Sept. 12 to Sept. 19, the smoke could be seen in satellite images as wisps and patches of light gray and was especially apparent over the darkly contrasting water of the Atlantic.</p>
<p>Soot particles are much larger than air molecules and are more adept at <a href="https://edu.rsc.org/soundbite/why-smoke-particles-turned-the-sky-red/4010543.article">scattering the yellow, orange and red wavelengths of sunlight</a>. The enhanced oranges, pinks and reds during sunset occur when the Sun’s rays have to travel through more smoke. That happens when the Sun is very low near the horizon rather than when it is directly overhead, hence the fiery sunsets. </p>
<h2>Riding the jet stream</h2>
<p>The smoke on the East Coast is coming from the horrifically <a href="https://www.washingtonpost.com/weather/2020/09/16/wildfire-smoke-reaches-europe/">large and persistent wildfires</a> in the Western states. Smoke from those fires is showing up not only on the Eastern Seaboard, but even <a href="https://abcnews.go.com/US/smoke-west-coast-wildfires-travels-europe/story?id=73071098">across the Atlantic in Europe</a>. </p>
<p>How has all that smoke migrated so far? Blame this on the vagaries of the jet stream. </p>
<p>The jet stream is a high-altitude belt of fast wind that sails from west to east around the hemisphere in the middle latitudes. The jet stream is always present, but its wind speeds and exact path around the globe vary daily. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/359083/original/file-20200921-16-1d083al.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A map of the U.S. showing concentrations of smoke traveling across the Northern U.S. from the West to the East Coast." src="https://images.theconversation.com/files/359083/original/file-20200921-16-1d083al.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/359083/original/file-20200921-16-1d083al.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=419&fit=crop&dpr=1 600w, https://images.theconversation.com/files/359083/original/file-20200921-16-1d083al.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=419&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/359083/original/file-20200921-16-1d083al.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=419&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/359083/original/file-20200921-16-1d083al.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=527&fit=crop&dpr=1 754w, https://images.theconversation.com/files/359083/original/file-20200921-16-1d083al.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=527&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/359083/original/file-20200921-16-1d083al.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=527&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Smoke, highlighted by the darker colors, follows the course of the jet stream once it gets high enough into the atmosphere.</span>
<span class="attribution"><a class="source" href="https://www.noaa.gov/">NOAA</a></span>
</figcaption>
</figure>
<p>In early September, the jet stream’s path abruptly dipped south, draping it through the Western states. When this happened, the air current picked up the rising plumes of smoke and transported them across the U.S. in a layer of air between 10,000 and 20,000 feet above sea level. As the smoke layer raced eastward at up to 100 mph, it spread over cities along the way – dimming the Sun and creating red sunsets. </p>
<h2>A connected world</h2>
<p>Smoke isn’t the only aerosol that can sail around the Earth on wind currents. Pollution from China regularly travels to the U.S., where it’s <a href="https://weather.com/science/environment/news/china-emissions-reach-america">been detected along the East Coast</a>. Fine dusts lofted from the Sahara Desert in Africa can be <a href="https://theconversation.com/a-massive-saharan-dust-plume-is-moving-into-the-southeast-us-bringing-technicolor-sunsets-and-suppressing-tropical-storms-141494">swept westward to the Southeastern U.S.</a>, as happened just a few months ago.</p>
<p>After a week of hazy skies, a large air mass from Canada blew into the East Coast bringing in smoke-free air. But the Western U.S. fire season continues, and if the jet stream dips south again, the East could experience additional blasts of smoke-laden air. The globe may be vast, but wind currents connect us all.</p>
<p>[<em>Deep knowledge, daily.</em> <a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=deepknowledge">Sign up for The Conversation’s newsletter</a>.]</p><img src="https://counter.theconversation.com/content/146381/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jeffrey B. Halverson does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Last week, much of the Midwest and eastern US experienced hazy skies and red sunsets. The cause was smoke transported from the Western US by the jet stream and spread as far as Boston and even Europe.Jeffrey B. Halverson, Professor of Geography & Environmental Systems, Associate Dean of the Graduate School, University of Maryland, Baltimore CountyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1259062019-11-14T17:23:20Z2019-11-14T17:23:20ZA battle for the jet stream is raging above our heads<figure><img src="https://images.theconversation.com/files/301498/original/file-20191113-77320-129x3r5.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C5095%2C3813&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The northern hemisphere jet stream crossing Cape Breton Island in the Maritime Provinces of Eastern Canada.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:STS039-601-049_Jet_Stream_(cropped).jpg">NASA/Wikimedia Commons</a></span></figcaption></figure><p>When prolonged periods of severe weather strike, two things often get the blame these days: climate change and the jet stream. Many have expressed concerns that the rapidly melting Arctic is now disturbing the jet stream, bringing more frequent bouts of wild weather. But potentially even more powerful changes are afoot in the tropics – and the consequences could be severe.</p>
<p>The northern hemisphere’s jet stream is a current of fast-moving air encircling the globe from west to east in the middle latitudes – the zone between the baking tropics and the freezing Arctic. The strongest winds are about ten kilometres high, near the altitudes at which planes fly, but the bottom of the jet can reach all the way down to the ground, forming the prevailing westerly winds familiar to many. The southern hemisphere’s counterpart is what gives rise to the <a href="https://www.britannica.com/science/roaring-forties">Roaring Forties</a> – similarly treacherous winds between latitudes 40° and 50°.</p>
<p>The jet forms a relatively sharp dividing line between the warm tropical and cold polar air masses. The strongest winds are concentrated in a band several hundred kilometres wide. But this band is not fixed. It meanders and snakes its way around the globe, sometimes touching the edge of the tropics and at other times scraping the polar regions</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/301818/original/file-20191114-26250-9nuy9t.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/301818/original/file-20191114-26250-9nuy9t.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=822&fit=crop&dpr=1 600w, https://images.theconversation.com/files/301818/original/file-20191114-26250-9nuy9t.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=822&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/301818/original/file-20191114-26250-9nuy9t.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=822&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/301818/original/file-20191114-26250-9nuy9t.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1033&fit=crop&dpr=1 754w, https://images.theconversation.com/files/301818/original/file-20191114-26250-9nuy9t.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1033&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/301818/original/file-20191114-26250-9nuy9t.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1033&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">In November 2019 (top), the jet shifted southwards from its usual position (bottom), leaving the UK on the cusp of its cold side, where storms often intensify.</span>
<span class="attribution"><a class="source" href="http://www.esrl.noaa.gov/psd/">ESRL/NOAA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>As a result, the jet can have a wide array of impacts across the hemisphere. If it passes over your location, expect to be repeatedly bombarded by the whirling storms that are carried along by it. As a recent example, the severe flooding in the North of England in November 2019 arose in part from a shift of the jet, which put the UK right in the middle of a region where storms tend to grow.</p>
<p>If the jet shifts to pass north of you, you’ll find yourself under the warm, dry zone of the atmosphere which lies south of the jet. This brings generally settled and pleasant weather in summer, but can set the scene for droughts and heatwaves. And if the jet moves south instead, you’ll be on its cold polar side, so you’d better hope this doesn’t happen too much during winter.</p>
<h2>Weather worries</h2>
<p>The jet has always varied – and has always affected our weather patterns. But now climate change is affecting our weather too. As I explore <a href="https://global.oup.com/academic/product/jet-stream-9780198828518">in my latest book</a>, it’s when the wanderings of the jet and the hand of climate change add up that we get record-breaking heatwaves, floods and droughts – but not freezes.</p>
<p>The coldest weeks of any given winter will occur when the jet brings masses of cold air directly from the polar regions. But severe though this may feel, <a href="https://iopscience.iop.org/article/10.1088/1748-9326/ab4867/meta">records show</a> that similar events in past decades were even colder than they are now. While the jet is largely doing the same as it always has, the planet-heating greenhouse gases we’ve added to our atmosphere mean that invasions of polar air these days are just that bit milder.</p>
<p>The flip side, of course, is that when the jet moves north in summer, bringing warm air from the south, we often have to endure temperatures beyond anything in living memory.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/301736/original/file-20191114-26211-k71up1.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/301736/original/file-20191114-26211-k71up1.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=398&fit=crop&dpr=1 600w, https://images.theconversation.com/files/301736/original/file-20191114-26211-k71up1.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=398&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/301736/original/file-20191114-26211-k71up1.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=398&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/301736/original/file-20191114-26211-k71up1.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=500&fit=crop&dpr=1 754w, https://images.theconversation.com/files/301736/original/file-20191114-26211-k71up1.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=500&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/301736/original/file-20191114-26211-k71up1.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=500&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Mount Everest (top middle) is so high that it grazes the jet stream, blowing snow off its peak.</span>
<span class="attribution"><a class="source" href="https://eol.jsc.nasa.gov/SearchPhotos/photo.pl?mission=ISS008&roll=E&frame=13304">NASA</a></span>
</figcaption>
</figure>
<p>It is clear and well understood how climate change and the jet can combine like this to cause <a href="https://doi.org/10.1029/2019GL084601">truly extreme weather events</a>. But whether climate change is directly changing the jet’s behaviour is a much harder question to answer.</p>
<p><a href="https://www.nature.com/articles/ngeo2234">Some have suggested</a> that the rapidly warming Arctic is weakening the jet, by reducing the temperature contrast between the tropical and polar air to either side of it. As a result, the jet meanders more to the north and south, and these meanders can remain fixed over one location for longer – as happened when the <a href="https://theconversation.com/beast-from-the-east-and-freakishly-warm-arctic-temperatures-are-no-coincidence-92774">“Beast from the East”</a> placed much of Northern Europe under a bitter chill.</p>
<p>There are certainly some interesting ideas here, but many still <a href="https://science.sciencemag.org/content/343/6172/729">do not find the logic compelling</a>, and more convincing evidence from observations and computer models will be needed for these theories to become widely accepted.</p>
<p>Scientists are however increasingly confident that important changes are afoot in the tropics. Driven by the vast quantities of energy pouring in from the Sun directly overhead, these are the great powerhouses of Earth’s climate. Indeed, the power of the tropics is evident in the <a href="https://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina/enso-impacts">worldwide weather disruption</a> caused by <a href="https://www.esrl.noaa.gov/psd/enso/">El Niño</a> events – subtle increases or decreases in temperatures in the equatorial Pacific Ocean, that in turn disturb the jet stream.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/300340/original/file-20191105-88368-yn9lp9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/300340/original/file-20191105-88368-yn9lp9.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/300340/original/file-20191105-88368-yn9lp9.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/300340/original/file-20191105-88368-yn9lp9.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/300340/original/file-20191105-88368-yn9lp9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/300340/original/file-20191105-88368-yn9lp9.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/300340/original/file-20191105-88368-yn9lp9.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">If El Niño causes equatorial Pacific Ocean temperatures to warm, the jet stream brings stormier weather in winter.</span>
<span class="attribution"><a class="source" href="https://www.esrl.noaa.gov/psd/enso/">climate.gov</a></span>
</figcaption>
</figure>
<p>Over the past few years, it has become apparent that at high altitudes, the Earth’s tropical regions are <a href="https://doi.org/10.1002/grl.50465">heating up more quickly than the rest of the world</a>. At least partly because of this, the tropical regions of the atmosphere have been widening, expanding ever so slightly away from the equator, and impinging more on the jet stream. </p>
<h2>Tug of war</h2>
<p>We are in the early days of a great battle in the air above our heads between the Arctic and the tropics, for the future of the jet stream. At best, there might be a stalemate, leaving the jet stream <a href="https://www.nature.com/articles/s41586-019-1465-z">distorted</a> but otherwise unmoved.</p>
<p>However, if one of the competitors outweighs the other, regional climate patterns could be severely altered as the climate zones shift along with the jet. It’s too early to say with any confidence which of these will win out, but many computer models predict the jet will <a href="https://doi.org/10.1175/JCLI-D-12-00536.1">shift a little towards the pole</a>, consistent with a greater influence of the tropics. </p>
<p>In this case, we should expect to see the warm, dry regions at the edge of the tropics extend a little further out from the equator. The strongest impacts of this would likely be felt in regions such as the <a href="https://iopscience.iop.org/article/10.1088/1748-9326/10/10/104012/meta">Mediterranean</a>, which are already highly sensitive to fluctuations in rainfall. A northward jet shift would act to steer much needed rainstorms towards central Europe instead, leaving the Mediterranean at greater risk of drought.</p>
<p>So, the jet may not become more erratic as the Arctic warms, but it may well change profoundly. And one thing is clear: the stress of increased temperatures and altered rainfall patterns from our destabilising climate will leave us even more vulnerable to the weather patterns brought by the whim of the wandering jet stream.</p>
<hr>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/263883/original/file-20190314-28475-1mzxjur.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/263883/original/file-20190314-28475-1mzxjur.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=140&fit=crop&dpr=1 600w, https://images.theconversation.com/files/263883/original/file-20190314-28475-1mzxjur.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=140&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/263883/original/file-20190314-28475-1mzxjur.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=140&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/263883/original/file-20190314-28475-1mzxjur.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=176&fit=crop&dpr=1 754w, https://images.theconversation.com/files/263883/original/file-20190314-28475-1mzxjur.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=176&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/263883/original/file-20190314-28475-1mzxjur.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=176&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/imagine-newsletter-researchers-think-of-a-world-with-climate-action-113443?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=Imagineheader1125906">Click here to subscribe to our climate action newsletter. Climate change is inevitable. Our response to it isn’t.</a></em></p><img src="https://counter.theconversation.com/content/125906/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tim recently published a popular science book 'Jet Stream: A Journey Through our Changing Climate', which provides an introduction to weather and climate dynamics for a lay audience, and discusses the ways in which climate change is affecting the jet and our weather patterns. Tim Woollings receives funding from the UK Natural Environment Research Council.</span></em></p>The jet stream is being distorted on both sides by fast-warming tropical and Arctic air. Should the tropics win out, weather patterns could change profoundly.Tim Woollings, Associate Professor in Physical Climate Science, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1194042019-06-26T08:58:28Z2019-06-26T08:58:28ZEverest: I journeyed into the ‘death zone’ to install the world’s highest weather station<p>Perched at almost 8,500 metres on Everest, we paced back-and-forth, attempting to stave off frostbite as temperatures hovered close to -30°C and our drill batteries became too cold to work. Our ambition to install the highest automatic weather station in history looked destined for failure.</p>
<p>We were at the end of an almost two-month expedition, led by National Geographic, to conduct a hugely ambitious scientific survey of the 8,848-metre Mount Everest. I am a climate scientist who specialises in <a href="https://www.lboro.ac.uk/departments/geography/staff/tom-matthews/">extreme environments</a>, and together with Baker Perry (a geographer at Appalachian State University), I was trying to install the highest weather station in the world.</p>
<p>Weeks of sickness had plagued the expedition (from diarrhoea to full-on Influenza-A), but we had so far succeeded. Four stations were under our belt, including at Everest Basecamp (5,315 metres), and Camp II (6,464 metres) – perched high above the infamous Khumbu Icefall.</p>
<p>The day before we had celebrated installing the highest operating weather station in the world, near Camp IV at almost 8,000 metres. Only one team of <a href="http://www.evk2cnr.org/cms/files/evk2cnr.org/CS%20South%20Col.pdf">Italian scientists</a> had deployed equipment this high before.</p>
<p>Any celebrations, however, were short-lived.</p>
<p>We filled the rest of that day eating, melting snow and sleeping, squeezing in about two hours of sleep before climbing out of Camp IV as midnight approached. We were determined to get our final station as close to the summit as possible, capturing the first continuous measurements of the weather in the thin air of the “death zone” above 8,000 metres.</p>
<p>Such data increases our understanding of the climates possible on Earth. Were we going to find the strongest near-surface winds on the planet? Just how cold and oxygen deprived is it up there during a winter storm? Could a human theoretically survive these conditions? Beyond this frontier meteorology and insight into life at the extremes, the station data could help improve weather forecasts on the mountain, hopefully making Everest climbers less susceptible to deadly surprises from <a href="https://journals.ametsoc.org/doi/abs/10.1175/BAMS-87-4-465">extreme events</a>.</p>
<p>Led by a superhuman Sherpa team from the nearby village of Phortse – carrying the disassembled weather station, other science equipment and the normal climbing paraphernalia – we made good progress from Camp IV, climbing faster than we had all expedition. The atmosphere, however, was tense.</p>
<p>Above 8,000 metres there is little room for failure, and the 2019 Spring climbing season provided too many reminders of our vulnerability. Unfavourable weather concentrated a record number of climbers into just a few days for summit attempts, making parts of the mountain <a href="https://www.bbc.co.uk/news/world-asia-48423738">unusually crowded</a>. Tragically, many more climbers than normal would never make it back down alive.</p>
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Read more:
<a href="https://theconversation.com/everest-11-climbers-dead-in-16-days-how-should-we-deal-with-the-bodies-on-the-mountain-118374">Everest: 11 climbers dead in 16 days – how should we deal with the bodies on the mountain?</a>
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<p>These crowds also impacted our expedition.</p>
<p>After around three hours of good progress towards the summit, we hit the back of a queue of climbers all clipped to the rope above. Our pace was cut by more than half. Hands and feet began to cool. The fear of frostbite grew. After hours more of shuffling, collective frustration was palpable. Our lead Sherpa (Panuru) – a charismatic veteran of 14 successful Everest summit – declared that we must reassess our options. So it was there, at Everest’s “Balcony” (8,430 metres), that we peeled off from the back of the pack and found a spot for the weather station. </p>
<p>Continuing any higher would have been a dangerous gamble against the odds. Ours was not a typical climbing expedition: we needed to perform hours of work on the summit and, given the slow progress, it was extremely unlikely our oxygen supplies would have lasted the round trip.</p>
<p>Our Sherpa team leapt into action at the Balcony. They had trained with us over the past two months for this very moment. For Baker and I this represented the climax of almost eight months’ preparation.</p>
<p>We were desperate when the drill (that was needed to help bolt the station to the rock) refused to be coaxed to life.</p>
<p>It was only with the sustained body heat from our warmest Sherpa that the batteries eventually warmed enough for the drilling and installation to commence.</p>
<p>The Sherpa were immense: a lifetime spent at high altitude meant they were quicker witted in the low oxygen environment, stronger, with hands more tolerant of the cold. It was thanks to their efforts that we successfully broke new ground with the installation of this weather station – nearly half a vertical kilometre higher than anything that had gone before.</p>
<p>Now safely back at sea-level, Baker and I have been watching the near-real time, <a href="https://www.natgeo.com/everest">satellite-streamed weather data</a> with interest. We have already seen winds near Camp IV equivalent to a category 2 hurricane – and this is the period normally known for its settled weather.</p>
<p>These stations owe their severe winds to the subtropical jetstream – a high-altitude ribbon of fast-moving air which influences weather across large swathes of the Northern Hemisphere. Placing a weather station at such height means scientists can now monitor the jet directly and learn more about it. And the fact it is on the relatively well-trodden Mount Everest means passing climbers and Sherpas may be able to help with maintenance.</p>
<p>If the new weather stations can survive the extreme conditions long enough, we hope that they will also give us a more complete picture of the high-altitude Himalayan climate, including how it may be changing. This is urgent because it is here that glaciers – the source of freshwater to <a href="https://www.independent.co.uk/environment/himalayas-glacier-melting-climate-change-global-warming-everest-a8965276.html">hundreds of millions of people</a> – are retreating, and as we improve our understanding of the climate, we increase our ability to outline the changes (and challenges) that may be ahead.</p>
<p>There is of course much to do, with far more monitoring needed to understand the spatial diversity in climate and its rate of change across the Earth’s mountains. It is difficult and risky to install weather stations in high-altitude locations, but the stakes are now very high. As scientists we must therefore accept the challenge and continue venturing upward.</p><img src="https://counter.theconversation.com/content/119404/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tom Matthews's expedition to Everest was led by National Geographic in partnership with Tribhuvan University and Rolex.</span></em></p>A climate scientist goes to work – at 8,500 metres.Tom Matthews, Lecturer in Climate Science, Loughborough UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1013242018-08-10T05:17:18Z2018-08-10T05:17:18ZJupiter’s magnetic fields may stop its wind bands from going deep into the gas giant<figure><img src="https://images.theconversation.com/files/231414/original/file-20180810-30443-kel1zv.jpg?ixlib=rb-1.1.0&rect=1441%2C0%2C1891%2C1287&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The colorful cloud belts dominate Jupiter’s southern hemisphere in this image captured by NASA’s Juno spacecraft.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/jpl/pia21974/jupiter-s-colorful-cloud-belts">NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill</a></span></figcaption></figure><p>One of the most striking features of <a href="https://solarsystem.nasa.gov/planets/jupiter/overview/">Jupiter</a> – a gaseous giant with no solid surface – is the coloured bands that encircle the planet. </p>
<p>These bands are so large and distinct that they can be seen from here on Earth using a modest telescope, and thus have fascinated astronomers since the era of Galileo.</p>
<p>In research <a href="https://doi.org/10.3847/1538-4357/aace53">published today in The Astrophysical Journal</a>, Jeffrey Parker, from Lawrence Livermore National Laboratory in the United States, and I have developed a theory that could help explain what is going on beneath these bands and why they only go so deep into the planet.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/231413/original/file-20180810-30467-n6lq9a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/231413/original/file-20180810-30467-n6lq9a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/231413/original/file-20180810-30467-n6lq9a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/231413/original/file-20180810-30467-n6lq9a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/231413/original/file-20180810-30467-n6lq9a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/231413/original/file-20180810-30467-n6lq9a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/231413/original/file-20180810-30467-n6lq9a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/231413/original/file-20180810-30467-n6lq9a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The bands of Jupiter captured by an Earth-based astronomer.</span>
<span class="attribution"><span class="source">NASA/Freddy Willems</span></span>
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</figure>
<h2>Winds on Jupiter</h2>
<p>These bands are actually strong steady winds, or jets, that flow in Jupiter’s atmosphere, carrying with them clouds of ammonia and other colourful elements. These jets are similar to the jet streams that flow high up in Earth’s atmosphere. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/jupiters-new-moons-an-irregular-bunch-with-an-extra-oddball-thats-the-smallest-discovered-so-far-100160">Jupiter's new moons: an irregular bunch with an extra oddball that's the smallest discovered so far</a>
</strong>
</em>
</p>
<hr>
<p>But there is more to these jets than meets the eye. What goes on below Jupiter’s clouds is, to a large extent, still a mystery.</p>
<p>Although there exist many theories for how the jets on Jupiter form and how deep they penetrate beneath the clouds, until recently we had no direct observations to support them.</p>
<p>In mid-2016, NASA’s spacecraft Juno headed to Jupiter with a mission to approach the planet closer that any probe has done before. It reached distances of less than 4,500km above Jupiter’s clouds at its closest approach (about the distance from New York to Los Angeles).</p>
<p>Upon arrival, Juno began to make precise measurements of Jupiter’s gravitational and magnetic fields. </p>
<p>When the data started pouring in, it was found that the jets go <a href="https://www.theguardian.com/science/2018/mar/07/nasa-spacecraft-reveals-jupiters-interior-in-unprecedented-detail">as deep as 3,000km</a> beneath Jupiter’s clouds, and then terminate. (This is about 5% of the planet’s <a href="https://solarsystem.nasa.gov/planets/jupiter/by-the-numbers/">radius at the equator</a>.)</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/hF0UjhPSS3A?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Only so deep for Jupiter’s bands.</span></figcaption>
</figure>
<p>This created a new puzzle for scientists: why do the jets penetrate as deep as they do, but no deeper?</p>
<p>Here is where <a href="https://doi.org/10.3847/1538-4357/aace53">our research</a> comes into the picture. We have developed a theory that explains how magnetic fields have a tendency to shut down the jets.</p>
<p>What does this have to do with Jupiter?</p>
<h2>Inside the gas giant</h2>
<p>Jupiter’s gaseous bulk consists mostly of hydrogen and helium. As you go deep beneath the clouds into the interior, the pressure of the gas increases (similar to how the pressure increases when you dive deep into the ocean here on Earth).</p>
<p>Scientists understand that at about 3,000km below Jupiter’s clouds, the pressure is so high that electrons can get loose from the molecules of hydrogen and helium and start to move around freely, creating electric and magnetic fields.</p>
<p>Is it just a coincidence that this happens at about the same depth that the jets break down? Scientists speculate that it is not. As Steve Levin, Juno project scientist at NASA’s Jet Propulsion Laboratory, <a href="https://youtu.be/S6Joupv6f-M?t=37m3s">explains</a>:</p>
<blockquote>
<p>It’s very interesting that (the jets disappear at) about 3,000km, because that’s about where it might be conducting electricity enough to make a magnetic field.</p>
<p>So, it could be that the magnetic field has something to do with why the belts and zones only go that deep (…) But we don’t know this yet; this is just speculation.</p>
</blockquote>
<p>Here is how our theory ties in. Using principles from statistical physics of turbulent systems, we devised a mathematical model which predicts that when magnetic fields are strong enough, the jets shut down.</p>
<p>Specifically, within our model a jet organises magnetic fluctuations in such a manner so that the coherent effect of these fluctuations acts to dampen the jet itself.</p>
<p>This offers a partial explanation as to why the jets terminate at about 3,000km below the clouds.</p>
<p>It’s hoped that theory and observation together will continue to give deeper insight on the physics of the universe as Juno and other probes, such as <a href="https://blogs.nasa.gov/parkersolarprobe/2018/08/09/parker-solar-probe-proceeds-toward-launch-aug-11/">NASA’s new Parker Solar Probe</a>, explore and gather data from our Solar system and beyond.</p><img src="https://counter.theconversation.com/content/101324/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Research was supported by the U.S. National Science Foundation grants PHY-1607611 and OCE-1357047. Also by the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. DE-AC52-07NA27344.</span></em></p>Jupiter’s bands are one of its most striking features – and can be seen from Earth – but they only go so deep within the giant planet. Now scientists think they know why.Navid Constantinou, Research fellow and researcher in climate and fluid physics, Australian National UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1011362018-08-06T13:50:08Z2018-08-06T13:50:08ZParts of Spain and Portugal are more than 46°C – here’s what is going on<p>Wildfires, drought and extreme heat have been the talk of the town and country across Europe this summer. Attention has now turned to Portugal and Spain, where temperatures at the weekend reached <a href="https://www.ft.com/content/1769faec-988d-11e8-ab77-f854c65a4465">more than 46°C</a> in some parts of both countries – close to the all-time European record of 48°C, set in Greece in 1977. Records aside, the obvious question is what is causing the current Iberian heatwave and whether this might be a harbinger of the future.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/230770/original/file-20180806-191035-16djmnh.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/230770/original/file-20180806-191035-16djmnh.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/230770/original/file-20180806-191035-16djmnh.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=476&fit=crop&dpr=1 600w, https://images.theconversation.com/files/230770/original/file-20180806-191035-16djmnh.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=476&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/230770/original/file-20180806-191035-16djmnh.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=476&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/230770/original/file-20180806-191035-16djmnh.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=598&fit=crop&dpr=1 754w, https://images.theconversation.com/files/230770/original/file-20180806-191035-16djmnh.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=598&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/230770/original/file-20180806-191035-16djmnh.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=598&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Maximum temperatures for August 6, with large areas well into the 40s.</span>
<span class="attribution"><a class="source" href="http://www.aemet.es/es/eltiempo/prediccion/temperaturas?dia=hoy&zona=penyb&img=maxima">Agencia Estatal de Meteorología (AEMET)</a></span>
</figcaption>
</figure>
<p>A number of factors can be identified. These include unusually warm sea surface temperatures in the North Atlantic, a wandering jet stream and associated “blocking” pattern of high pressure, a very dry land surface, and climate change. </p>
<p>The anomalous size and position of warm water areas in the North Atlantic this summer have shifted the so-called “polar front” northwards. This is the point where warm air from the south meets cold polar air, and any movement in the front will affect the distribution of high and low atmospheric pressure right across the Atlantic. This in turn influences the flow of westerly winds across the Atlantic and over Western and Southern Europe, especially the thin and fast “jet stream” in the upper atmosphere.</p>
<p>This summer, an area of persistent high pressure or “blocking” has become established over Western Europe and the eastern parts of the Atlantic. Such blocking causes the jet stream to appear “lazy” and wander much further north and south than its average position. </p>
<p>The upshot of all of this is that atmospheric blocking and a very snake-like jet stream prevents low pressure systems, and the “bad” weather they bring, from heading eastwards across Western and Southern Europe. In such a situation, the usual fluctuations between good, and not so good, summer weather are largely put on hold. Instead, as Portugal, Spain and much of Europe have experienced, clear skies, lots of heat, and very dry surface conditions become the norm. </p>
<p>In certain circumstances, persistent blocking can even draw in very warm air from elsewhere. This is what happened in Portugal and Spain, after intense heat caused an area of low pressure to form over Iberia. This “heat low” created the conditions for the flow of hot dry air from the Saraha Desert. Currently life in Portugal and Spain is not just in an oven, but more like a convection oven.</p>
<h2>A warmer baseline means hotter extremes</h2>
<p>Heatwaves in Portugal and Spain are not uncommon because this type of extreme weather is characteristic of the hot and dry summers in the Mediterranean climate region. Yet there is <a href="https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013JD020590">convincing observational evidence</a> that heatwaves are happening more frequently across the Iberian Peninsula. Logically the question arises as to what extent the current heat is associated with climate change. </p>
<p>Although answering this question thoroughly would involve undertaking some well-designed climate modelling experiments, it’s safe to say that there is indeed a fair chance the current heatwave is associated with climate change. That is because heatwaves are now happening on a background of rising global temperatures so the base level of background temperature on which extremes are occurring has lifted somewhat compared to pre-industrial levels.</p>
<p>And what of the question on everyone’s parched lips: are the current extremes the “new norm”? The short answer is no, not right now, as extremes of over 46°C still constitute rare events. However, analyses of the pronounced <a href="http://www.nature.com.ezproxy.auckland.ac.nz/articles/nclimate2468.pdf">2003 European heat wave</a>, which affected both Portugal and Spain, indicate that the very similar extremes of August 2003 could be fairly normal by the 2040s.</p>
<p>This of course raises questions as to the habitability of places that already possess harsh summer climates. Most likely their sustainability will depend on the extent to which traditional climate adaptation strategies related to building and lifestyles can be pushed to the limit to cope with a new climate future typified by summers with temperatures in and over the mid 40s and how flexible people and businesses might be to the idea of going elsewhere or literally underground during summer.</p><img src="https://counter.theconversation.com/content/101136/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Glenn McGregor does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>And how long before such extreme heatwaves become the ‘new norm’ across the region?Glenn McGregor, Professor of Climatology, Durham UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1004182018-07-25T10:25:06Z2018-07-25T10:25:06ZThree (and a half) reasons why it has been so hot and dry in the UK and Ireland<figure><img src="https://images.theconversation.com/files/229030/original/file-20180724-194137-1tckjw2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Michaelasbest / shutterstock</span></span></figcaption></figure><p>The UK and Ireland have been experiencing a prolonged hot and dry spell since June, with the first half of summer being the UK’s <a href="https://www.bbc.co.uk/news/uk-44903861">driest on record</a>. The lack of rainfall has led to hosepipe bans in Northern Ireland and the north-west of England, while the weather is also playing havoc with farming. A shortage of <a href="https://theconversation.com/why-the-2018-heatwave-will-not-be-as-bad-as-1976-for-your-fruit-and-vegetables-99241">lettuce and broccoli</a> is expected in the next few months, and grass isn’t growing fast enough to feed Ireland’s <a href="https://www.irishtimes.com/business/agribusiness-and-food/farmers-face-winter-fodder-crisis-following-drought-1.3573000">sheep and cattle</a> through the winter.</p>
<p>The hot and dry weather is associated with a high pressure weather system situated over the UK. The high pressure means that the storms the UK occasionally gets at this time of year are being steered much further northwards towards Iceland. While the UK and Ireland have been wilting in the sunshine, Reykjavík has recorded its <a href="http://icelandreview.com/news/2018/05/29/reykjaviks-rain-record-broken">wettest</a> (May) and <a href="http://icelandreview.com/news/2018/07/03/reykjavik-has-least-sunny-june-thirty-years">cloudiest</a> (June) months on record. </p>
<p>This high pressure system is unusually persistent and has been building up over Europe throughout spring and early summer. In April it was over Central Europe and in May shifted northwards towards Scandinavia. Subsequently it was the hottest April and May in <a href="https://www.dw.com/en/may-and-april-2018-hottest-in-germany-since-1881/a-44074472">Germany</a> and the hottest May in <a href="http://cphpost.dk/news/record-may-set-to-end-on-hot-note.html">Denmark</a> since observations began in the 1880s.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1014034632575213569"}"></div></p>
<p>But why has it been so dry and warm? Here is a shortlist of candidates that could be playing a role – or not – in this unusual summer:</p>
<h2>1. Climate change</h2>
<p>Let’s start with the obvious: temperatures are increasing globally due to the burning of fossil fuels, which is increasing concentrations of atmospheric carbon dioxide. The global rise in temperatures means that heatwaves are occurring against a warmer background and so are more likely to become extreme.</p>
<p>The past few years have seen some record-breaking temperatures in Europe, for example the 2015 heatwave and the 2017 <a href="https://uk.reuters.com/article/uk-europe-weather-heatwave/lucifer-heat-wave-holds-italy-eastern-europe-in-fiery-grip-idUKKBN1AK1IE">“Lucifer” heatwave</a> in Central Europe. Unusually warm summer temperatures have also been recorded elsewhere, for example in Canada and <a href="https://twitter.com/metoffice/status/1021440303432003585">Japan</a>, and climate change is very likely to have played a role in the UK and Ireland as well.</p>
<h2>2. North Atlantic temperatures</h2>
<p>Temperatures over the North Atlantic Ocean can play a role in setting the position of a narrow band of strong wind in the upper atmosphere known as the jet stream. The position of the jet stream in turn has a profound impact on the weather experienced in the UK and Ireland. </p>
<iframe src="https://giphy.com/embed/11DtsEhZBhCYJG" width="100%" height="259" frameborder="0" class="giphy-embed" allowfullscreen=""></iframe>
<p><a href="https://giphy.com/gifs/climate-change-11DtsEhZBhCYJG">The jet stream can affect weather in Europe.</a></p>
<p>This summer, ocean temperatures have been relatively warm between the Gulf of Mexico and North Africa while temperatures south of Greenland have been unusually cold. This is thought to have pushed the jet stream further northwards, sending bad weather towards Iceland while allowing areas of high pressure to linger over Europe.</p>
<h2>3. La Niña</h2>
<p>Every few years, ocean temperatures in the tropical Pacific swing between being relatively warm (known as El Niño) and cool (La Niña). Since October 2017 the area has been in a La Niña phase. This is sometimes associated with cold winters in north-western Europe (for example the winter of 2010-11 and the recent “<a href="https://theconversation.com/beast-from-the-east-and-freakishly-warm-arctic-temperatures-are-no-coincidence-92774">Beast from the East</a>” cold spell in March 2018). However, La Niña cannot really be blamed here – this year’s event had started to weaken around April and had almost gone by June when the UK’s current dry spell began. </p>
<h2>4. It’s the weather</h2>
<p>The above factors influence the type of weather the UK and Ireland will get but, within these broad possibilities, good or bad luck also plays a role. This is especially the case for very unusual weather such as the current hot and dry spell. This summer is no different and the hot and dry weather is partly due a combination of Atlantic temperatures, climate change and annual weather patters. Should weather patterns continue as they are then 2018 may turn out to be as hot and dry as the extreme <a href="http://www.bbc.co.uk/newsbeat/article/40358961/what-the-drought-of-1976-looked-like-as-this-years-heatwave-continues">summer of 1976</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/229065/original/file-20180724-194134-1wsi4u5.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/229065/original/file-20180724-194134-1wsi4u5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/229065/original/file-20180724-194134-1wsi4u5.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=299&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229065/original/file-20180724-194134-1wsi4u5.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=299&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229065/original/file-20180724-194134-1wsi4u5.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=299&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229065/original/file-20180724-194134-1wsi4u5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=375&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229065/original/file-20180724-194134-1wsi4u5.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=375&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229065/original/file-20180724-194134-1wsi4u5.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=375&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Green and pleasant lands … turned yellow.</span>
<span class="attribution"><a class="source" href="https://twitter.com/metoffice/status/1019552615263277059">Met Office</a></span>
</figcaption>
</figure>
<p>This raises the question of how predictable it is. The science of forecasting the weather a few months ahead is still in its infancy, and such “seasonal forecasts” are subject to lots of uncertainty. However, the UK Met Office did predict back in <a href="https://blog.metoffice.gov.uk/2018/06/11/will-summer-be-a-washout-or-a-scorcher-2/">early June</a> that there was an increased probability of a drier and warmer summer. While meteorologists are not yet able to predict prolonged hot and dry spells months in advance there were useful indicators of an increased chance of extreme weather.</p>
<p>Advancing the science of seasonal forecasting requires a deeper understanding of the different factors than can influence the weather, from ocean temperatures and jet streams through to changes in Arctic sea ice. As part of the <a href="http://aboutdrought.info">UK Drought and Water Scarcity programme</a> research institutions in the UK are being funded to further our knowledge of these influences and to better understand how the UK can respond to and mitigate the impacts of prolonged dry weather.</p><img src="https://counter.theconversation.com/content/100418/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Len Shaffrey receives funding from Natural and Environmental Research Council, Innovate UK, and the European Commission.</span></em></p>A climate scientist explains what is going on with this heatwave.Len Shaffrey, Professor of Climate Science, National Centre for Atmospheric Science, University of ReadingLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/923852018-02-28T18:49:18Z2018-02-28T18:49:18Z‘Beast from The East’ – the science behind Europe’s Siberian chill<figure><img src="https://images.theconversation.com/files/208297/original/file-20180228-36706-185c0cg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://twitter.com/NPASLondon/status/968522249711116288">National Police Air Service</a></span></figcaption></figure><p>The so-called “Beast from the East” has arrived in the UK, bringing unusually cold weather – <a href="https://www.accuweather.com/en/gb/london/ec4a-2/february-weather/328328">about 7°C colder</a> than the historical average for this time of year. Wind chill is making temperatures feel particularly arctic. So how did the Siberian gusts come to arrive on Europe’s doorstep?</p>
<p>The movement of air across the globe, and the weather it brings with it, is governed by three major influences: gravity, the sun, and something called the <a href="https://www.metoffice.gov.uk/learning/learn-about-the-weather/how-weather-works/coriolis-effect">Coriolis effect</a>. The influence of gravity is simple, constantly pulling air towards the Earth’s surface.</p>
<p>The rise and fall of the sun dictates whether the air stays there. During the day, radiation from the sun heats the Earth, warming air directly above the surface and causing it to rise, leaving behind a region of low pressure (a low density of air particles). As the air rises, it cools and spreads outwards. This mass of air, now denser than the air below it, sinks back down under the force of gravity, and naturally flows back towards the lower pressure region of air, creating a cycle of air circulation. These circulating patterns of wind exist <a href="https://www.metoffice.gov.uk/learning/learn-about-the-weather/how-weather-works/global-circulation-patterns">on an intercontinental scale</a>, transporting heat all the way from the tropics to the poles.</p>
<p>However, thanks to the <a href="https://www.metoffice.gov.uk/learning/learn-about-the-weather/how-weather-works/coriolis-effect">Coriolis effect</a> – the deflection of objects moving in a straight path due to the Earth’s rotation – the winds do not travel directly north or south. To illustrate this effect, imagine a spinning top. Parts of the spinning top closer to the spindle rotate at slower speeds than parts further away, as they have less distance to travel to complete a full circle. Similarly, the equator <a href="http://www.physicstutorials.org/home/rotational-motion/tangential-speed-velocity">has to travel much faster than the poles do</a> as the Earth rotates. As air travels north from the equator, its extra momentum compared to the slower rotating land that it is moving over makes it curve across to the east, while air travelling to the south pole curves westward.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/rdGtcZSFRLk?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
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<p>In the northern hemisphere, this interaction between the Coriolis effect and the circulation systems produces the <a href="https://www.metoffice.gov.uk/learning/wind/what-is-the-jet-stream">northern polar jet stream</a>: high altitude currents of air blowing eastwards at hundreds of miles per hour, moving weather systems around the globe. This causes the UK’s prevailing westerly and south-westerly winds, which usually draw weather systems in from the relatively warm Atlantic and shield us from colder air masses to the east.</p>
<p>The shape of the jet streams is not rigid – it follows a meandering path, much like a slithering snake. Occasionally, the jet stream path can become so twisted that it folds back upon itself, reversing the direction of the prevailing wind, and drawing in cold air from the east. <a href="https://theconversation.com/beast-from-the-east-and-freakishly-warm-arctic-temperatures-are-no-coincidence-92774">A complex cascade of events</a> associated with a phenomenon called Stratospheric Sudden Warming led to exactly this happening. In the last couple of days, the bitterly cold front combined with water vapour in the air to carpet the country in a blanket of brilliant white. </p>
<p>As the warmth of the sun disappears each night, the cold can feel all the more biting. But in the absence of the sun’s heat, the smaller difference in temperature between air near the ground and higher up makes air circulate more slowly. This often creates calmer conditions that might just provide a brief respite from the extra chill of the wind. For this same reason, air passengers generally experience <a href="https://www.skyscanner.net/news/guide-turbulence">smoother flying conditions when flying at night</a>.</p>
<p>If you live in the city however, your experience of the “beast” can vary wildly from place to place. Cities continue to produce heat at night, <a href="https://www.nationalgeographic.org/encyclopedia/urban-heat-island/">generating their own microclimates</a>. This man-made heat keeps air moving, and warms city dwellers up more than those in rural areas. At the same time, the ordered formation of buildings in cities <a href="http://urban-climate.org/documents/TonyChandler_TheClimateOfLondon.pdf">creates strong wind corridors</a> that are certainly best avoided at times like these.</p>
<p>Wherever you are experiencing this freezing weather, you can at least be thankful that you are here on Earth. Wind circulation patterns on other planets produce far more extreme weather than we will ever experience. Visitors to Venus, for example, would experience some serious turbulence when approaching landing, as <a href="http://venus.aeronomie.be/en/venus/temperaturegreenhouseeffect.htm">the 500°C difference between surface and cloud</a> generates extreme air circulation. </p>
<p>However, if you were lucky enough to touch down and survive the experience of the crushing pressure found on Venus’ surface, you would feel nothing more than a gentle breeze, thanks to the planet’s very slow rotation, weak Coriolis effect, and dense air. You might want to seek shelter though – at close to 460°C, suddenly a cold chill doesn’t seem so bad.</p><img src="https://counter.theconversation.com/content/92385/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gareth Dorrian receives funding from Nottingham Trent University and has previously received research support funding from the Natural Environment Research Council. </span></em></p><p class="fine-print"><em><span>Ian Whittaker does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>No matter how cold it is, you’re lucky you don’t live on Venus.Gareth Dorrian, Post Doctoral Research Associate in Space Science, Nottingham Trent UniversityIan Whittaker, Lecturer, Nottingham Trent UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/897402018-01-11T15:12:01Z2018-01-11T15:12:01ZIs warming in the Arctic behind this year’s crazy winter weather?<figure><img src="https://images.theconversation.com/files/201651/original/file-20180111-101511-sa3hd1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Seriously cold: The 'bomb cyclone' freezes a fountain in New York City.</span> <span class="attribution"><span class="source">AP Photo/Mark Lennihan</span></span></figcaption></figure><p>Damage from extreme weather events during 2017 racked up the <a href="http://www.ncdc.noaa.gov/billions">biggest-ever bills for the U.S.</a> Most of these events involved conditions that align intuitively with global warming: heat records, drought, wildfires, coastal flooding, hurricane damage and heavy rainfall. </p>
<p>Paradoxical, though, are possible ties between climate change and the recent spate of frigid weeks in eastern North America. A very new and “hot topic” in climate change research is the notion that rapid warming and wholesale melting of the Arctic may be playing a role in causing persistent cold spells. </p>
<p>It doesn’t take a stretch of the imagination to suppose that losing <a href="https://doi.org/10.1002/2013RG000431">half the Arctic sea-ice cover in only 30 years</a> might be wreaking havoc with the weather, but exactly how is not yet clear. As a research atmospheric scientist, I <a href="https://theconversation.com/a-melting-arctic-demands-more-not-less-research-on-earth-science-46118">study</a> how <a href="https://theconversation.com/a-melting-arctic-and-weird-weather-the-plot-thickens-37314">warming in the Arctic is affecting temperature regions around the world</a>. Can we say changes to the Arctic driven by global warming have had a role in the freakish winter weather North America has experienced? </p>
<h2>A ‘dipole’ of abnormal temperatures</h2>
<p>Weird and destructive weather was in the news almost constantly during 2017, and 2018 seems to be following the same script. Most U.S. Easterners <a href="https://ny.curbed.com/2018/1/3/16844890/winter-storm-2018-new-york-bomb-cyclone-snow">shivered their way through the end of 2017</a> into the New Year, while Westerners longed for rain to dampen parched soils and extinguish wildfires. Blizzards have plagued the Eastern Seaboard – notably the “bomb cyclone” storm on Jan. 4, 2018 – while California’s Sierra Nevada stand nearly bare of snow. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/201548/original/file-20180110-46706-1e6gl5x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/201548/original/file-20180110-46706-1e6gl5x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/201548/original/file-20180110-46706-1e6gl5x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=648&fit=crop&dpr=1 600w, https://images.theconversation.com/files/201548/original/file-20180110-46706-1e6gl5x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=648&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/201548/original/file-20180110-46706-1e6gl5x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=648&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/201548/original/file-20180110-46706-1e6gl5x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=814&fit=crop&dpr=1 754w, https://images.theconversation.com/files/201548/original/file-20180110-46706-1e6gl5x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=814&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/201548/original/file-20180110-46706-1e6gl5x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=814&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A study in contrasts: Warming near Alaska and the Pacific Ocean are ‘ingredients’ to a weather pattern where cold air from the Arctic plunges deep into North America.</span>
<span class="attribution"><a class="source" href="https://earthobservatory.nasa.gov/IOTD/view.php?id=91517">NASA Earth Observatory</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>This story is becoming a familiar one, as similar conditions have played out in four of the past five winters. Some politicians in Washington D.C., <a href="https://weather.com/science/environment/news/2017-12-29-trump-tweet-global-warming-weather-climate">including President Trump</a>, have used the unusual cold to question global warming. But if they looked at the big picture, they’d see that eastern cold spells are a relative fluke in the Northern Hemisphere as a whole and that most areas are warmer than normal. </p>
<p>A warm, dry western North America occurring in combination with a cold, snowy east is not unusual, but the prevalence and persistence of this pattern in recent years have piqued the interests of climate researchers. </p>
<p>The jet stream – a fast, upper-level river of wind that encircles the Northern Hemisphere – plays a critical role. When the jet stream swoops far north and south in a big wave, extreme conditions can result. During the past few weeks, a big swing northward, forming what’s called a “ridge” of persistent atmospheric pressure, persisted off the West Coast along with a deep southward dip, or a “trough,” over the East. </p>
<p>New terms have been coined to describe these stubborn features: <a href="http://dx.doi.org/10.1002/2016JD025116">“The North American Winter Temperature Dipole,”</a> the <a href="http://dx.doi.org/10.1126/sciadv.1501344">“Ridiculously Resilient Ridge”</a> over the West, and the <a href="https://theconversation.com/a-melting-arctic-and-weird-weather-the-plot-thickens-37314">“Terribly Tenacious Trough”</a> in the East. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/201541/original/file-20180110-46712-s7k7xs.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/201541/original/file-20180110-46712-s7k7xs.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/201541/original/file-20180110-46712-s7k7xs.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=260&fit=crop&dpr=1 600w, https://images.theconversation.com/files/201541/original/file-20180110-46712-s7k7xs.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=260&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/201541/original/file-20180110-46712-s7k7xs.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=260&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/201541/original/file-20180110-46712-s7k7xs.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=327&fit=crop&dpr=1 754w, https://images.theconversation.com/files/201541/original/file-20180110-46712-s7k7xs.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=327&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/201541/original/file-20180110-46712-s7k7xs.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=327&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">While the eastern U.S. suffered very cold temperatures in the recent cold snap, much of the rest of the Northern Hemisphere saw higher-than-average air temperatures.</span>
<span class="attribution"><a class="source" href="https://www.esrl.noaa.gov/psd/">NOAA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Regardless what it’s called, this dipole pattern – abnormally high temperatures over much of the West along with chilly conditions in the East – has dominated North American weather in four of the past five winters. January 2017 was a stark exception, when a strong El Niño flipped the ridge-trough pattern, dumping record-breaking rain and snowpack on California while the east enjoyed a mild month. </p>
<p>Two other important features are conspicuous in the dipole temperature pattern: extremely warm temperatures in the Arctic near Alaska and warm ocean temperatures in the eastern Pacific. Several <a href="http://iopscience.iop.org/article/10.1088/1748-9326/11/8/084016/meta">new studies</a> point to these “ingredients” as key to the recent years with a persistent dipole.</p>
<h1>It takes two to tango</h1>
<p>What role does warming – specifically the warming ocean and air temperatures in the Arctic – play in this warm-West/cool-East weather pattern? The explanation goes like this. </p>
<p>Pacific Ocean temperatures fluctuate naturally owing to short-lived phenomena such as El Niño/La Niña and <a href="http://dx.doi.org/10.1023/A:1015820616384">longer, decades-length patterns</a>. Scientists have long recognized that <a href="https://www.ncbi.nlm.nih.gov/pubmed/25323549">those variations affect weather patterns</a> across North America and beyond.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/201552/original/file-20180110-46706-1x2d6qk.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/201552/original/file-20180110-46706-1x2d6qk.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/201552/original/file-20180110-46706-1x2d6qk.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=334&fit=crop&dpr=1 600w, https://images.theconversation.com/files/201552/original/file-20180110-46706-1x2d6qk.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=334&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/201552/original/file-20180110-46706-1x2d6qk.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=334&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/201552/original/file-20180110-46706-1x2d6qk.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=419&fit=crop&dpr=1 754w, https://images.theconversation.com/files/201552/original/file-20180110-46706-1x2d6qk.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=419&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/201552/original/file-20180110-46706-1x2d6qk.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=419&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">When a persistent area of atmospheric pressure stays in the western U.S., air from the Arctic pours into the U.S, causing a split between the warm and dry West and the cold East.</span>
<span class="attribution"><a class="source" href="https://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/The_Ridiculously_Resilient_Ridge.pdf/page1-1200px-The_Ridiculously_Resilient_Ridge.pdf.jpg">Mesocyclone2014 and David Swain</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The new twist in this story is that the Arctic has been warming at at least double the pace of the rest of the globe, meaning that the difference in temperature between the Arctic and areas farther south has been shrinking. This matters because the north/south temperature difference is one of the main drivers of the jet stream. The jet stream creates the high- and low-pressure systems that dictate our blue skies and storminess while also steering them. Anything that affects the jet stream will also affect our weather.</p>
<p>When ocean temperatures off the West Coast of North America are warmer than normal, as they have been most of the time since winter 2013, the jet stream tends to form a ridge of high pressure along the West Coast, causing storms to be diverted away from California and leaving much of the West high and dry. </p>
<p>If these warm ocean temperatures occur in combination with abnormally warm conditions near Alaska, the extra heat from the Arctic can intensify the ridge, causing it to reach farther northward, become more persistent, and pump even more heat into the region near Alaska. And in recent years, Alaska has experienced periods of record warm temperatures, owing in part to reduced sea ice.</p>
<p>My colleagues and I have called this combination of natural and climate change-related effects “<a href="http://dx.doi.org/10.1175/BAMS-D-17-0006.1">It Takes Two to Tango</a>,” a concept that may help explain the Ridiculously Resilient Ridge observed frequently since 2013. <a href="http://dx.doi.org/10.1038/s41467-017-01907-4">Several</a> <a href="http://dx.doi.org/10.1038/ngeo2517">new</a> <a href="http://dx.doi.org/10.1088/1748-9326/11/8/084016">studies</a> support this human-caused boost of a natural pattern, though <a href="https://apnews.com/fbafc62d4a81492fb0e54bac3329f81b/US-cold-snap-was-a-freak-of-nature,-quick-analysis-finds">controversy still exists</a> regarding the mechanisms linking rapid Arctic warming with <a href="http://dx.doi.org/10.1002/wcc.474">weather patterns farther south in the mid-latitudes</a>.</p>
<h2>More extreme weather ahead?</h2>
<p>In response to the strengthened western ridge of atmospheric pressure, the winds of the jet stream usually also form a deeper, stronger trough downstream. Deep troughs act like an open refrigerator door, allowing frigid Arctic air to plunge southward, bringing misery to areas ill-prepared to handle it. Snowstorms in Texas, ice storms in Georgia and chilly snowbirds in Florida can all be blamed on the Terribly Tenacious Trough of December 2017 and January 2018. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/201652/original/file-20180111-101495-di88qu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/201652/original/file-20180111-101495-di88qu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/201652/original/file-20180111-101495-di88qu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=374&fit=crop&dpr=1 600w, https://images.theconversation.com/files/201652/original/file-20180111-101495-di88qu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=374&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/201652/original/file-20180111-101495-di88qu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=374&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/201652/original/file-20180111-101495-di88qu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=470&fit=crop&dpr=1 754w, https://images.theconversation.com/files/201652/original/file-20180111-101495-di88qu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=470&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/201652/original/file-20180111-101495-di88qu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=470&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Cold weather from the Arctic combined with warm tropical air fueled a storm that produced well over a foot of snow and spots of flooding in Boston.</span>
<span class="attribution"><span class="source">AP Photo/Michael Dwyer</span></span>
</figcaption>
</figure>
<p>Adding icing on the cake is the tendency for so-called “nor’easters,” such as the “bomb cyclone” that struck on Jan. 4, to form along the East Coast when the trough’s southwest winds align along the Atlantic Seaboard. The resulting intense contrast in temperature between the cold land and Gulf Stream-warmed ocean provides the fuel for these ferocious storms.</p>
<p>The big question is whether climate change will make dipole patterns – along with their attendant tendencies to produce extreme weather – more common in the future. The answer is yes and no. </p>
<p>It is widely expected that global warming will produce fewer low-temperature records, a tendency already observed. But it may also be true that cold spells will become more persistent as <a href="http://dx.doi.org/10.1126/sciadv.1501344">dipole patterns intensify</a>, a tendency that also <a href="http://dx.doi.org/10.1002/2016JD025116">seems to be occurring</a>.</p>
<p>It’s hard to nail down whether this weather pattern – overall warmer winters in North America but longer cold snaps – will persist. Understanding the mechanisms behind these complex interactions between natural influences and human-caused changes is challenging.</p>
<p>Nevertheless, research is moving forward rapidly as creative new metrics are developed. Our best tools for looking into the future are sophisticated computer programs, but they, too, struggle to simulate these complicated behaviors of the climate system. Given the importance of predicting extreme weather and its impacts on many aspects of our lives, researchers must continue to unravel connections between climate change and weather to help us prepare for the likely ongoing tantrums by Mother Nature.</p><img src="https://counter.theconversation.com/content/89740/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jennifer Francis receives funding from the National Science Foundation and the National Aeronautics and Space Administration. </span></em></p>An atmospheric scientist who studies the Arctic explains why – because of global warming – the U.S. may be in for longer cold spells in the winter.Jennifer Francis, Research Professor, Rutgers UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/858512017-10-17T13:22:41Z2017-10-17T13:22:41ZWhy (ex)Hurricane Ophelia took a wrong turn towards Ireland and Britain – and carried all that dust<figure><img src="https://images.theconversation.com/files/190632/original/file-20171017-30390-5euzt5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.metoffice.gov.uk/barometer/uk-storm-centre/ophelia">Met Office</a></span></figcaption></figure><p>Hurricane Ophelia, by then downgraded to Storm Ophelia, reached Ireland on Monday October 16. At the time of writing there had already been three deaths and countless reports of severe damage to buildings and fallen trees. Unlike other severe storms to reach Ireland and Britain, such as the <a href="https://theconversation.com/sting-jet-the-mysterious-cause-of-the-1987-great-storms-worst-winds-85620">1987 Great Storm</a>, Ophelia was in many ways a typical tropical cyclone with a tight spiral of cloud, powerfully strong winds, and a cloud-free eye.</p>
<p>But what set it apart from other Atlantic hurricanes was its direct route to Europe. While hurricanes do sometimes take a circuitous route westward across the ocean and loop back again towards Europe, this one took a short cut.</p>
<p>Ophelia began as a rather innocuous looking group of clouds in the Atlantic Ocean, several hundred miles south-west of the Azores and roughly on the same latitude as Morocco or northern Florida. Yet even at this stage it was unusual. Most named storms in the Atlantic are generated in warmer waters much further south and, as such, they are generally driven across the ocean by the easterly (blowing westwards) trade winds. They eventually dissipate as they curve north into the Atlantic or make landfall in the Caribbean, Mexico or the US. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190634/original/file-20171017-30417-d50w2d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190634/original/file-20171017-30417-d50w2d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190634/original/file-20171017-30417-d50w2d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=371&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190634/original/file-20171017-30417-d50w2d.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=371&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190634/original/file-20171017-30417-d50w2d.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=371&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190634/original/file-20171017-30417-d50w2d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=466&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190634/original/file-20171017-30417-d50w2d.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=466&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190634/original/file-20171017-30417-d50w2d.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=466&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Tracks of all Atlantic hurricanes from 1851 to 2012.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/File:Atlantic_hurricane_tracks.jpg">Nilfanion / US National Hurricane Centre</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>In Ophelia’s case, moderate atmospheric shear (changes in direction and strength of wind with height) and relatively cool sea surface temperatures meant it took several days to develop the well-defined low pressure centre, strong winds and spiral clouds of a hurricane. Then, instead of travelling west, like most Atlantic hurricanes, Ophelia began to head north-east.</p>
<p>This can be explained by the position and strength of the mid-latitude jet stream, an atmospheric feature that plays a major role in determining the weather over Western Europe. When its path loops north over the UK it can produce <a href="https://theconversation.com/explainer-the-omega-shaped-jet-stream-responsible-for-europes-heatwave-44268">stable warm conditions</a> in the summer (as in the record-breaking heatwave of July 2015), and bright cold days in the winter. When its path west to east is more direct, it guides low pressure storm systems across the UK and Ireland and can be responsible for the rapid strengthening of storms in a short period of time (described colloquially by forecasters <a href="https://theconversation.com/weather-bomb-scary-new-name-for-common-winter-storm-35413">as a “weather bomb”</a>). </p>
<figure>
<iframe src="https://giphy.com/embed/uDOWJ8J8zD1fy" width="100%" height="324" frameborder="0" style="max-width: 100%" class="giphy-embed" webkitallowfullscreen="" mozallowfullscreen="" allowfullscreen=""></iframe>
<figcaption>A northwards loop of the jet stream (far right) guided Ophelia from the Azores to Ireland.</figcaption>
</figure>
<p>It is this that produced such a rapid change in direction for Ophelia. Such waves on the mid-latitude jet stream are not unusual, however, the combination of both the jet’s and Ophelia’s position produced the conditions to guide the ex-hurricane toward the British Isles.</p>
<h2>Adrift in the Azores</h2>
<p>Tropical cyclones that form in or travel to the region near the Azores can become trapped. Stronger vertical wind shear to the north and south as well as colder sea surface temperatures in the surroundings can lead to storms that travel very slowly and eventually dissipate due to unfavourable conditions. </p>
<p>If there is no external force that can help to steer the storm, but conditions remain favourable, then tropical cyclones can persist for a long time. An example of this was <a href="http://www.nhc.noaa.gov/data/tcr/AL142012_Nadine.pdf">Hurricane Nadine</a> which circled the same part of the ocean where Ophelia formed for almost a month in 2012, the fourth longest-lived Atlantic hurricane on record.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190635/original/file-20171017-30410-1a324p0.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190635/original/file-20171017-30410-1a324p0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190635/original/file-20171017-30410-1a324p0.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=439&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190635/original/file-20171017-30410-1a324p0.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=439&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190635/original/file-20171017-30410-1a324p0.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=439&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190635/original/file-20171017-30410-1a324p0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=551&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190635/original/file-20171017-30410-1a324p0.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=551&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190635/original/file-20171017-30410-1a324p0.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=551&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">First part of Ophelia’s journey. Compare to the map above – very few hurricanes take this route.</span>
<span class="attribution"><a class="source" href="http://www.nhc.noaa.gov/refresh/graphics_at2+shtml/025759.shtml?swath#contents">US National Hurricane Centre</a></span>
</figcaption>
</figure>
<p>That may have been Ophelia’s fate too, had it not been for the mid-latitude jet stream which instead guided the hurricane directly toward Ireland. It is partly due to this behaviour that the forecasts have been so accurate. Jet streams are generally well represented in numerical weather models, and so their influence on a storm’s path can be well predicted.</p>
<p>Ahead of Ophelia’s arrival the UK had a weekend of unseasonably balmy temperatures thanks to warm tropical air driven northwards. This is partly due to the winds circulating around the Ophelia low pressure centre, but also the positioning of the jet stream helping to draw air up from the tropics. </p>
<h2>What about that dust?</h2>
<p>On Monday much of the UK looked far less like a hurricane had arrived, and much more like the whole country had been put through a rosy Instagram filter. The sun was particularly red at dawn and throughout much of the day the whole sky glowed a yellowy-orange. </p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"920033678872285184"}"></div></p>
<p>This effect was partly thanks to the southerly winds on Ophelia’s eastern side, which transported Saharan dust and smoke from Iberian forest fires. In fact several flights over the UK were forced to make <a href="http://www.bbc.co.uk/news/uk-england-41639386">emergency landings</a> when smoke could be smelt in the cabin. Increased number of particles in the atmosphere then scattered light preferentially from the blue end of the spectrum, leaving the more orange and red colours to reach our eyes. </p>
<p>But Ophelia also produced a layer of upper level clouds, thick enough to block out much of the sun’s rays directly but thin enough to allow a large amount of diffuse, scattered light to pass through. On a day when the sky was not full of smoke and dust particles, this would have appeared like a run-of-the-mill white skied, overcast day. However, on Monday it led to Facebook feeds being filled with photos of a bright orange sun at midday and yellow clouds.</p><img src="https://counter.theconversation.com/content/85851/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alexander Roberts receives funding from Natural Environment Research Council. </span></em></p>Very few Atlantic hurricanes travel northwards like this.Alexander Roberts, Researcher, Institute for Climate and Atmospheric Science (ICAS), University of LeedsLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/643372016-08-25T13:30:27Z2016-08-25T13:30:27ZBritain’s recent wet summers can be blamed on the Atlantic jet stream, says new study<figure><img src="https://images.theconversation.com/files/135348/original/image-20160824-30249-lezcfy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Chris Mole / shutterstock</span></span></figcaption></figure><p>Weather forecasters can predict Britain’s notoriously variable weather as it changes from day to day or even hour to hour. But what about year to year? After all, 2013 saw weeks of unbroken sunshine whereas summer this year has barely got going – despite some blazing hot days now and again – even by late August. Some winters are wet and stormy, others mild and dry.</p>
<p>While we can now predict winter seasonal weather with some accuracy, our summer efforts are less advanced. But why shouldn’t we be able to forecast a scorching summer with more accuracy? A new study, published in <a href="http://link.springer.com/article/10.1007/s00382-016-3307-0">Climate Dynamics</a> with colleagues at the University of Sheffield, should help.</p>
<p>It all depends on the jet stream, the ribbon of strong westerly winds which blows across the Atlantic 10km or more above the surface. Britain is located near the easterly end of the North Atlantic jet and so the jet stream influences much of the <a href="http://www.metoffice.gov.uk/media/pdf/h/9/Drivers_and_impacts_of_seasonal_weather_in_the_UK_archive_CS01_Tagged.pdf">variability in its weather</a>.</p>
<p>In summer, a jet lying to the north of the British Isles steers any rain-bearing, low-pressure systems northward towards Norway and away from Britain. This results in the UK experiencing the sort of warm dry weather seen for weeks on end in 2013. </p>
<p>However, when the jet stream lies further to the south, those same low-pressure systems tend to hit north-western Europe, resulting in wet weather. A difference in rainfall between north-west Scotland and south-east England highlights this difference. In 2012, for example, while most of the British Isles experienced well above average rainfall, north-west Scotland was unusually dry. In drier summers, however, north-west Scotland tends to experience the higher rainfall associated with a northward jet displacement.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/135505/original/image-20160825-6593-i7j7hn.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/135505/original/image-20160825-6593-i7j7hn.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/135505/original/image-20160825-6593-i7j7hn.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=376&fit=crop&dpr=1 600w, https://images.theconversation.com/files/135505/original/image-20160825-6593-i7j7hn.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=376&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/135505/original/image-20160825-6593-i7j7hn.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=376&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/135505/original/image-20160825-6593-i7j7hn.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=473&fit=crop&dpr=1 754w, https://images.theconversation.com/files/135505/original/image-20160825-6593-i7j7hn.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=473&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/135505/original/image-20160825-6593-i7j7hn.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=473&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Rainfall in wet June 2012 and dry June 2006 compared to the long-term average. North-west Scotland bucks the trend.</span>
<span class="attribution"><a class="source" href="http://www.metoffice.gov.uk/climate/uk/summaries/anomacts">Met Office</a></span>
</figcaption>
</figure>
<p>In most recent years, this southerly shift in the jet stream has prevailed. This has in part been attributed to warm sea surface temperatures in the Atlantic which lead to lower pressure and increased summer rainfall.</p>
<p>A number of recent summer extremes such as the record-breaking wet summers of <a href="http://news.bbc.co.uk/1/hi/uk/6971370.stm">2007</a> or <a href="http://www.metoffice.gov.uk/climate/uk/summaries/2012/summer">2012</a> and the <a href="http://www.metoffice.gov.uk/climate/uk/interesting/2013-heatwave">warm dry summer of 2013</a> have heightened the need for improved understanding of summertime jet stream variability. After all, improvements in seasonal forecasting would lead to great socio-economic benefits – imagine being able to plan your summer months in advance, already knowing whether rain and cloud or endless sunshine is more likely.</p>
<h2>Shifting streams</h2>
<p>Ultimately, our research found that there was no single factor that causes the jet stream to shift around. Changes in the sun’s strength from year to year play a role, as does the extent of sea-ice in the Arctic and sea surface temperatures in both the North Atlantic and, perhaps more surprisingly, the tropics near Indonesia. However there is little evidence that the well-known El Niño phenomenon has any influence in summer.</p>
<p>It takes quite a while for changes to have an impact on the jet stream. For example, it would seem that solar variability <a href="http://onlinelibrary.wiley.com/doi/10.1002/qj.2782/pdf">three to five years previously</a> is more significant than solar variability at the time of any given summer. </p>
<p>Likewise, our work found an intriguing and as yet unexplained link between sea ice extent in the previous autumn in the Barents and Kara seas, north of Norway and Russia, and the position of the jet stream in the following summer. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/135509/original/image-20160825-6630-1tc70i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/135509/original/image-20160825-6630-1tc70i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/135509/original/image-20160825-6630-1tc70i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=398&fit=crop&dpr=1 600w, https://images.theconversation.com/files/135509/original/image-20160825-6630-1tc70i9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=398&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/135509/original/image-20160825-6630-1tc70i9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=398&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/135509/original/image-20160825-6630-1tc70i9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=500&fit=crop&dpr=1 754w, https://images.theconversation.com/files/135509/original/image-20160825-6630-1tc70i9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=500&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/135509/original/image-20160825-6630-1tc70i9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=500&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Arctic sea ice is disappearing … bad news for British summers?</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/usgeologicalsurvey/4371011380/">US Geological Survey</a></span>
</figcaption>
</figure>
<p>Increased sea ice is associated with a northward shift in the jet, which means drier summers in the UK. Given how fast the <a href="https://www.theguardian.com/environment/climate-consensus-97-per-cent/2016/aug/22/historical-documents-reveal-arctic-sea-ice-is-disappearing-at-record-speed">Arctic is melting</a>, less ice could contribute to a southward shift in the jet and an increased probability of wet British summers. However, we still don’t know exactly how this will play out in future, and it’s possible that continued sea-ice decline will have some unexpected effects.</p>
<p>At present, <a href="http://onlinelibrary.wiley.com/doi/10.1002/2014GL059637/abstract">dynamical forecasting models</a> are showing some skill in making seasonal predictions for the North Atlantic winter, but demonstrate almost no skill for the summer months. Our work found that, while much of the jet variability is attributed to seemingly random atmospheric “noise”, the predictable component of summer jet stream variability may be as high as 35%. This may seem quite small but ensuring such factors are adequately represented in the forecasts will help us predict what’s in store next summer.</p><img src="https://counter.theconversation.com/content/64337/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Richard Hall does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Will 2017 be a ‘bbq summer’? It depends on changes in the jet stream.Richard Hall, PhD researcher in atmospheric jet streams and climate variability, University of SheffieldLicensed as Creative Commons – attribution, no derivatives.