tag:theconversation.com,2011:/fr/topics/thunderstorm-3292/articlesThunderstorm – The Conversation2023-03-28T12:22:16Ztag:theconversation.com,2011:article/2027042023-03-28T12:22:16Z2023-03-28T12:22:16ZWhy tornadoes are still hard to forecast – even though storm predictions are improving<figure><img src="https://images.theconversation.com/files/518829/original/file-20230331-18-ihfljb.jpg?ixlib=rb-1.1.0&rect=0%2C315%2C1649%2C1048&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A tornado touches down.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/noaanssl/47953539496">Morgan Schneider/OU CIMMS/NOAA NSSL</a></span></figcaption></figure><p><em>Meteorologists <a href="https://www.spc.noaa.gov">began warning about severe weather</a> with the potential for tornadoes <a href="https://www.wpc.ncep.noaa.gov/threats/threats.php">several days before</a> storms tore across <a href="https://earthobservatory.nasa.gov/images/151138/tornado-leaves-path-of-destruction-in-mississippi">the Southeast</a> and the <a href="https://www.nytimes.com/2023/03/31/us/midwest-storms-flood-weather.html">Central U.S.</a> in late March 2023. At one point, <a href="https://twitter.com/NWS/status/1641890452562403328">more than 28 million people</a> were under a <a href="https://www.weather.gov/safety/tornado-ww">tornado watch</a>. But pinpointing exactly where a tornado will touch down – like the tornadoes that hit <a href="https://www.bbc.com/news/world-us-canada-65072195">Rolling Fork, Mississippi</a>, on March 24, and towns in <a href="https://apnews.com/article/tornado-arkansas-storm-concert-79fe2da8a6b8bd92970032530b760d20">Arkansas</a>, <a href="https://www.bbc.com/news/world-us-canada-65072195">Illinois</a> and <a href="https://www.spc.noaa.gov/climo/reports/230331_rpts.html">multiple other states</a> on March 31 – still relies heavily on seeing the storms developing on radar. <a href="https://atmo.tamu.edu/people/profiles/faculty/nowotarskichristopher.html">Chris Nowotarski</a>, an atmospheric scientist, explains why, and how forecast technology is improving.</em></p>
<h2>Why are tornadoes still so difficult to forecast?</h2>
<p>Meteorologists have gotten a lot better at forecasting the conditions that make tornadoes more likely. But predicting exactly which thunderstorms will produce a tornado and when is harder, and that’s where a lot of severe weather research is focused today.</p>
<p>Often, you’ll have a line of thunderstorms in an environment that looks favorable for tornadoes, and one storm might produce a tornado but the others don’t. </p>
<p>The differences between them could be due to small differences in meteorological variables that aren’t resolved by our current observing networks or computer models. Even changes in the land surface conditions – fields, forested regions or urban environments – could affect whether a tornado forms. These small changes in the storm environment can have large impacts on the processes within storms that can make or break a tornado.</p>
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<img alt="Scientists stand near a truck outfitted with measuring devices with a dramatic storm on the horizon." src="https://images.theconversation.com/files/517815/original/file-20230327-18-egyw14.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/517815/original/file-20230327-18-egyw14.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/517815/original/file-20230327-18-egyw14.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/517815/original/file-20230327-18-egyw14.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/517815/original/file-20230327-18-egyw14.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/517815/original/file-20230327-18-egyw14.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/517815/original/file-20230327-18-egyw14.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">
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<span class="caption">One way scientists gather data for understanding tornadoes is by chasing storms.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/noaanssl/52201476520/">Annette Price/CIWRO</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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<p>One of the strongest predictors of whether a thunderstorm produces a tornado relates to <a href="https://www.weather.gov/jetstream/tornado">vertical wind shear</a>, which is how the wind changes direction or speed with height in the atmosphere.</p>
<p>How wind shear interacts with rain-cooled air within storms, which we call “outflow,” and how much precipitation evaporates can influence whether a tornado forms. If you’ve ever been in a thunderstorm, you know that right before it starts to rain, you often get a gust of cold air surging out from the storm. The characteristics of that cold air outflow are important to whether a tornado can form, because tornadoes typically form in that cooler portion of the storm.</p>
<h2>How far in advance can you know if a tornado is likely to be large and powerful?</h2>
<p>The vast majority of violent tornadoes form from <a href="https://www.weather.gov/ama/supercell">supercells</a>, thunderstorms with a deep rotating updraft, called a “mesocyclone.” Vertical wind shear can enable the midlevels of the storm to rotate, and upward suction from this mesocyclone can intensify the rotation within the storm’s outflow into a tornado.</p>
<p>If you have a supercell on radar and it has strong rotation above the ground, that’s often a precursor to a tornado. Some research suggests that <a href="https://doi.org/10.1175/WAF-D-19-0099.1">a wider mesocyclone is more likely to create a stronger</a>, longer-lasting tornado than other storms.</p>
<p>Forecasters also look at the storm’s environmental conditions – temperature, humidity and wind shear. Those offer more clues that a storm is likely to produce a significant tornado.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/R7CD6MpTefs?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">What radar showed as a tornado headed toward Rolling Fork, Mississippi, on March 24, 2023.</span></figcaption>
</figure>
<p>The percentage of tornadoes that <a href="https://www.weather.gov/safety/tornado-ww">trigger a warning</a> has increased over recent decades, due to <a href="https://www.weather.gov/jetstream/how">Doppler radar</a>, improved modeling and better understanding of the storm environment. About <a href="https://doi.org/10.1175/WAF-D-19-0119.1">87% of deadly tornadoes</a> from 2003 to 2017 had an advance warning.</p>
<p>The lead time for warnings has also improved. In general, it’s <a href="https://community.fema.gov/ProtectiveActions/s/article/Tornado-Alerts-and-Warnings">about 10 to 15 minutes</a> now. That’s enough time to get to your basement or, if you’re in a trailer park or outside, to find a safe facility. Not every storm will have that much lead time, so it’s important to get to shelter fast.</p>
<h2>What are researchers discovering today about tornadoes that can help protect lives in the future?</h2>
<p>If you think back to the <a href="https://www.imdb.com/title/tt0117998/">movie “Twister</a>,” in the early 1990s we were starting to do more field work on tornadoes. We were taking radar out in trucks and driving vehicles with roof-mounted instruments into storms. That’s when we really started to appreciate what we call the storm-scale processes – the conditions inside the storm itself, how variations in temperature and humidity in outflow can influence the potential for tornadoes.</p>
<p>Scientists can’t launch a weather balloon or send instruments into every storm, though. So, we also use computers to model storms to understand what’s happening inside. Often, we’ll run several models, referred to as ensembles. For instance, if nine out of 10 models produce a tornado, we know there’s a good chance the storm will produce tornadoes.</p>
<p>The National Severe Storms Laboratory has recently been experimenting with tornado warnings based on these models, called <a href="https://www.nssl.noaa.gov/projects/wof/">Warn-on-Forecast</a>, to increase the lead time for tornado warnings.</p>
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<a href="https://images.theconversation.com/files/517845/original/file-20230328-490-c5aoro.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A destroyed home with just one wall standing and furniture strewn about in Rolling Fork, Mississippi, after the tornado March 24, 2023." src="https://images.theconversation.com/files/517845/original/file-20230328-490-c5aoro.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/517845/original/file-20230328-490-c5aoro.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/517845/original/file-20230328-490-c5aoro.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/517845/original/file-20230328-490-c5aoro.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/517845/original/file-20230328-490-c5aoro.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/517845/original/file-20230328-490-c5aoro.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/517845/original/file-20230328-490-c5aoro.jpg?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"></a>
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<span class="caption">An early warning can be the difference between life and death for people in homes without basements or cellars.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/aerial-view-of-a-destroyed-neighborhood-in-rolling-fork-news-photo/1249647508">Chandan Khanna/AFP via Getty Images</a></span>
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<p>There are a lot of other areas of research. For example, to better understand how storms form, <a href="http://people.tamu.edu/%7Ecjnowotarski/research.html">I do a lot of idealized computer modeling</a>. For that, I use a model with a simplified storm environment and make small changes to the environment to see how that changes the physics within the storm itself. </p>
<p>There are also new tools in storm chasing. There’s been an explosion in the use of drones – scientists are putting sensors into unmanned aerial vehicles and <a href="https://www.colorado.edu/aerospace/2021/12/08/designing-flying-ai-systems-study-supercell-thunderstorms-close">flying them close to</a> and sometimes into the storm.</p>
<p>The focus of tornado research has also shifted from the Great Plains – the traditional “tornado alley” – <a href="https://doi.org/10.1175/JAMC-D-15-0342.1">to the Southeast</a>.</p>
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<h2>What’s different about tornadoes in the Southeast?</h2>
<p>In the Southeast there are some different influences on storms compared with the Great Plains. The Southeast has more trees and more varied terrain, and also more moisture in the atmosphere because it’s close to the Gulf of Mexico. There tend to be <a href="https://doi.org/10.1175/2008WAF2222132.1">more fatalities</a> in the Southeast, too, because <a href="https://theconversation.com/tornadoes-that-strike-at-night-are-more-deadly-and-require-more-effective-warning-systems-132955">more tornadoes form at night</a>.</p>
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<img alt="US map showing highest number of tornadoes in Mississippi, Alabama and western Tennessee." src="https://images.theconversation.com/files/517812/original/file-20230327-18-9tncri.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/517812/original/file-20230327-18-9tncri.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=458&fit=crop&dpr=1 600w, https://images.theconversation.com/files/517812/original/file-20230327-18-9tncri.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=458&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/517812/original/file-20230327-18-9tncri.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=458&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/517812/original/file-20230327-18-9tncri.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=575&fit=crop&dpr=1 754w, https://images.theconversation.com/files/517812/original/file-20230327-18-9tncri.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=575&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/517812/original/file-20230327-18-9tncri.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=575&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">A map of severe tornado days from 1986 to 2015 shows a large number in the Southeast.</span>
<span class="attribution"><a class="source" href="https://www.spc.noaa.gov/">NOAA Storm Prediction Center</a></span>
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<p>We tend to see more tornadoes in the Southeast that are in lines of thunderstorms called “quasi-linear convective systems.” The processes that lead to tornadoes in these storms can be different, and scientists are learning more about that.</p>
<p>Some research has also suggested the start of <a href="https://doi.org/10.1002/joc.5285">a climatological shift</a> in tornadoes toward the Southeast. It can be difficult to disentangle an increase in storms from better technology spotting more tornadoes, though. So, more research is needed.</p>
<p><em>This article was updated March 31, 2023, with tornadoes in Arkansas and the central U.S.</em></p><img src="https://counter.theconversation.com/content/202704/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Nowotarski receives funding from NSF, NOAA, DOE, and NASA.</span></em></p>Researchers are turning to computer models, drones and other methods to improve tornado forecasting.Chris Nowotarski, Associate Professor of Atmospheric Science, Texas A&M UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1888622022-08-17T13:54:17Z2022-08-17T13:54:17ZWhy it’s not safe to shower during a thunderstorm<p>The Met Office has issued several “yellow thunderstorm warnings” for the UK, highlighting the <a href="https://www.metoffice.gov.uk/about-us/press-office/news/weather-and-climate/2022/days-of-thunder-ahead-for-some">potential for frequent lightning</a>. While your chance of getting struck by lightning is low, it’s important to know how to stay safe during a thunderstorm. Globally, about 24,000 people each year are <a href="https://www.vaisala.com/sites/default/files/documents/Annual_rates_of_lightning_fatalities_by_country.pdf">killed by lightning</a> and another 240,000 are injured.</p>
<p>Most people are familiar with basic thunderstorm safety, such as avoiding standing under trees or near a window, and not speaking on a corded phone (mobile phones are safe). But did you know you should avoid taking a shower, a bath or washing the dishes during a thunderstorm?</p>
<p>To understand why, you first need to know a bit about how thunderstorms and lightning work.</p>
<p>Two basic elements cause a thunderstorm to thrive: moisture and rising warm air, which of course go hand in hand with summertime. The high temperatures and humidity create large amounts of moist air that rises into the atmosphere, where it can form into a thunderstorm.</p>
<p>Clouds contain millions of water and ice droplets and the interaction of these is what leads to <a href="https://www.weather.gov/safety/lightning-science-electrification">lightning generation</a>. The rising water drops collide with the falling ice drops, passing them a negative charge and leaving themselves with a positive charge. In a thunderstorm, clouds act as enormous Van de Graaff generators, separating the positive and negative charges to create massive charge separations inside the clouds.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/laDmuQFmK3Y?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">How a Van de Graaff generator works.</span></figcaption>
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<p>As thunderclouds move over the Earth, they generate an opposite charge in the ground, and this is what attracts a lighting strike towards the ground. The thunderstorm wants to balance its charges, and it does this by discharging between positive and negative regions. The path of this discharge is usually the one of least resistance, so things that are more conductive (like metal) are more likely to be struck during a storm. </p>
<p>The most useful advice for a thunderstorm is: when thunder roars, go indoors. However, this does not mean you are completely safe from the storm. There are some activities inside that can be almost as risky as staying outside in the storm. </p>
<h2>Path of least resistance</h2>
<p>Unless you’re sitting in a bath outside or showering in the rain, you’re incredibly unlikely to be struck by lightning. But if lightning strikes your house, the electricity would follow the path of least resistance to the ground. Things such as metal wires or water in your pipes provide a convenient conductive path for the electricity to follow to the ground. </p>
<p>The shower provides both of those things (water and metal), making it an ideal path for the electricity to take. It could turn that nice relaxing shower into something much less relaxing. The US <a href="https://www.cdc.gov/disasters/lightning/safetytips.html#:%7E:text=Stay%20away%20from%20open%20spaces,strike%20the%20tallest%20object%20around.">Centers for Disease Control and Prevention</a> strongly encourage people to avoid all water-based activities during a thunderstorm – even the washing up – to reduce your risk of a strike. </p>
<p>There are other risks to look out for during a thunderstorm. One that may not seem obvious is leaning on a concrete wall. While concrete itself isn’t that conductive, if it has been reinforced with metal beams (called “rebar”), these can provide a conductive path for the lightning. Also avoid using anything plugged into an electrical outlet (computers, TVs, washing machines, dishwashers) as all of these can provide pathways for the lightning strike to take. </p>
<p>As a rule of thumb, if you can hear thunder in the distance, then you’re close enough to the storm to have lightning reach you, even if there is no rain. Lightning strikes can happen as far as ten miles away from the parent storm. Typically, half an hour after hearing that final thunderclap is a safe time to venture back into the shower. Thunderstorms usually like to save a big one for the end, and you don’t want to end up part of the fireworks!</p><img src="https://counter.theconversation.com/content/188862/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>James Rawlings 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>Lightning kills 24,000 people each year. Here’s how to stay safe during an electrical storm.James Rawlings, Physics Lecturer, Nottingham Trent UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1434022020-08-13T12:07:07Z2020-08-13T12:07:07ZWhy does some rain fall harder than other rain?<figure><img src="https://images.theconversation.com/files/352560/original/file-20200812-18-1v5ghbk.jpg?ixlib=rb-1.1.0&rect=20%2C0%2C2674%2C1729&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A downpour or a drizzle: What causes the difference?</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/rainbow-in-rain-royalty-free-image/122286751?adppopup=true"> David Pinzer Photography/Moment via Getty Images</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">
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<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>
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<blockquote>
<p><strong>Why does some rain fall harder than other rain? – Naomi B., age 9, San Fancisco, California</strong></p>
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<hr>
<p>There are some days when the rain falls peacefully and gently, nourishing the Earth. But on some other days the rain comes down in a torrential downpour that <a href="https://ges.umbc.edu/halverson/">meteorologists like me</a> call a cloudburst. Standing outside in one of these intense rainstorms can feel like being smothered in a heavy, wet towel. These storms can flood the lands below them and lead to great destruction.</p>
<p>So what causes this difference?</p>
<p>All rain comes from a combination of two things: moisture in the air – usually in the form of clouds – and currents of air moving upwards. As moist air rises up through a cloud, the air cools and the water in it turns into tiny raindrops.</p>
<p>This is the same thing that happens when you can <a href="https://www.loc.gov/everyday-mysteries/item/why-do-i-see-my-breath-when-its-cold-outside/">see your breath on a cold evening</a>. The temperature change from warm to cool causes water droplets to form in your breath. </p>
<p>In a cloud, these tiny raindrops are very light and float as the rising air pushes them up. But the higher they go, the larger and heavier they get. Eventually, they get so heavy that they fall to the Earth as rain.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/352061/original/file-20200810-18-1sa6tnv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="People cross a street holding umbrellas and in long coats during a winter storm in Philadelphia." src="https://images.theconversation.com/files/352061/original/file-20200810-18-1sa6tnv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/352061/original/file-20200810-18-1sa6tnv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/352061/original/file-20200810-18-1sa6tnv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/352061/original/file-20200810-18-1sa6tnv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/352061/original/file-20200810-18-1sa6tnv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/352061/original/file-20200810-18-1sa6tnv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/352061/original/file-20200810-18-1sa6tnv.jpg?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"></a>
<figcaption>
<span class="caption">All-day drizzles come from steady storms that don’t have much upward wind flow.</span>
<span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/Winter-Storm/78d6c8efa1954f2cb9decc3bb2b770fd/89/0">AP Photo/Matt Rourke</a></span>
</figcaption>
</figure>
<h2>Cold air storms are steady and slow</h2>
<p>Cold air can hold much less moisture than warm air, so wintertime clouds don’t have much water in them; they are <a href="https://en.wikipedia.org/wiki/Nimbostratus_cloud">thin and layered rather</a> than puffy and tall and full of water.</p>
<p>Since cold air likes to sink to the ground, it’s difficult to get that air to rise quickly, so these wintertime clouds have only gentle upward air currents. As these slow currents sweep up through the thin clouds that don’t have much moisture, small raindrops form. Gravity easily pulls them down against the air current before they get too big. When clouds are thin and the air is moving slowly, you get nice calm rain.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/352058/original/file-20200810-20-1zyxdz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A large mushroom shaped cloud formation against a blue sky in New Mexico" src="https://images.theconversation.com/files/352058/original/file-20200810-20-1zyxdz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/352058/original/file-20200810-20-1zyxdz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/352058/original/file-20200810-20-1zyxdz.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/352058/original/file-20200810-20-1zyxdz.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/352058/original/file-20200810-20-1zyxdz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/352058/original/file-20200810-20-1zyxdz.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/352058/original/file-20200810-20-1zyxdz.jpeg?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">Massive thunderstorms called supercells can form when the right weather ingredients come together.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Thunderstorm#/media/File:Chaparral_Supercell_2.JPG">Greg Lundeen/NOAA</a></span>
</figcaption>
</figure>
<h2>Thunderstorms and big winter storms are quick and intense</h2>
<p>Hard rainstorms happen when there is a lot of moisture in the air and the air moves upwards very fast. Summer thunderstorms are the perfect example. </p>
<p>The warm, moist air rises very quickly – like a hot air balloon – and can be moving <a href="https://www.koaa.com/news/covering-colorado/just-how-fast-does-an-updraft-need-to-be-to-create-colorados-record-breaking-hailstone">as fast as 30 to 40 miles per hour</a>. The air also holds <a href="https://www.lsop.colostate.edu/wp-content/uploads/sites/6/2014/10/WhyDoesWarmAirHoldMoreWater.pdf">much more moisture than winter clouds</a> – up to five times as much.</p>
<p>All of this creates <a href="https://en.wikipedia.org/wiki/Cumulonimbus_cloud">very tall, thick clouds that are full of moisture</a>. Water droplets form quickly as the air moves up through the clouds. But since the wind is blowing upwards so fast, the droplets can get huge before gravity drags them down to Earth. When the weight of all the water droplets gets to be too much for the wind, the wind current collapses, and all the raindrops in the cloud come crashing down at once. These are summer thunderstorms.</p>
<p>Thunderstorms can drop one, two or even three inches of rain in <a href="https://en.wikipedia.org/wiki/Flash_flood">less than an hour</a>. These sudden torrential downpours, called cloudbursts, can lead to flash flooding that can overflow streams and roads and trap people wherever they are.</p>
<p>[<em>The Conversation’s science, health and technology editors pick their favorite stories.</em> <a href="https://theconversation.com/us/newsletters/science-editors-picks-71/?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=science-favorite">Weekly on Wednesdays</a>.]</p>
<p>Thankfully, because thunderstorms are so violent and relatively small, they don’t last very long. Once the rain falls from the clouds and squashes the upward air currents, the clouds disappear and you often see a nice blue sky. </p>
<p>Of course, winter can deliver some strong storms too – especially over the warmer ocean water. When strong winter storms drop lots of heavy rain, the same principles are at work: lots of moisture in the air, fast upwards wind currents and tall clouds. </p>
<p>No two rainstorms are ever the same. Sometimes clouds can rain so hard it feels like you are standing in the shower. Other times they bring only a nice peaceful drizzle. Now, whether you are soaked or singing in the rain, you’ll know why. </p>
<hr>
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<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>Some rainstorms drench you in a second, while others drop rain in a nice peaceful drizzle. A meteorologist explains how rainstorms can be so different.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/1403192020-06-15T12:19:55Z2020-06-15T12:19:55ZWhat is a derecho? An atmospheric scientist explains these rare but dangerous storm systems<figure><img src="https://images.theconversation.com/files/341358/original/file-20200611-80784-qr10wr.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C5122%2C3379&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A derecho moves across central Kansas on July 3, 2005.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/deadly-derecho-strikes-central-kansas-news-photo/595068428?adppopup=true">Jim Reed/Corbis via Getty Images</a></span></figcaption></figure><p>Thunderstorms are common across North America, especially in warm weather months. About 10% of them <a href="https://www.nssl.noaa.gov/education/svrwx101/thunderstorms/">become severe</a>, meaning they produce hail 1 inch or greater in diameter, winds gusting in excess of 50 knots (57.5 miles per hour), or a tornado.</p>
<p>The U.S. occasionally experiences rarer events: organized lines of thunderstorms with widespread damaging winds, known as derechos.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/341562/original/file-20200612-153862-17rxzao.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/341562/original/file-20200612-153862-17rxzao.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/341562/original/file-20200612-153862-17rxzao.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/341562/original/file-20200612-153862-17rxzao.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/341562/original/file-20200612-153862-17rxzao.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/341562/original/file-20200612-153862-17rxzao.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=504&fit=crop&dpr=1 754w, https://images.theconversation.com/files/341562/original/file-20200612-153862-17rxzao.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=504&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/341562/original/file-20200612-153862-17rxzao.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=504&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Derechos occur fairly regularly over large parts of the U.S. each year, most commonly from April through August.</span>
<span class="attribution"><a class="source" href="https://www.spc.noaa.gov/misc/AbtDerechos/images/Jet_Stream_figs/derechoclimo.png">Dennis Cain/NOAA</a></span>
</figcaption>
</figure>
<p>Derechos occur mainly across the central and eastern U.S., where many locations are affected one to two times per year on average. They can produce significant damage to structures and sometimes cause “blowdowns” of millions of trees. Pennsylvania and New Jersey <a href="https://www.inquirer.com/weather/derecho-power-outages-peco-supercell-thunderstorms-philadelphia-weather-20200606.html">received the brunt</a> of a derecho on June 3, 2020, that killed four people and left nearly a million without power across the mid-Atlantic region.</p>
<p>In the West, derechos are less common, but Colorado – where I serve as state climatologist and <a href="https://scholar.google.com/citations?user=vfbhQHkAAAAJ&hl=en">director of the Colorado Climate Center</a> – experienced a <a href="https://www.coloradoan.com/story/news/2020/06/08/colorado-experiences-rare-derecho-produced-hurricane-force-winds/5317767002/">rare and powerful derecho</a> on June 6, 2020 that generated winds exceeding 100 miles per hour in some locations. And on August 10, 2020 a derecho rolled across Iowa, Wisconsin, Illinois and Indiana, generating rare “<a href="https://twitter.com/CNN/status/1292898789649461248">particularly dangerous situation” warnings</a> from forecasters and registering wind gusts as high as <a href="https://twitter.com/pppapin/status/1292874234725912577">130 miles per hour</a>.</p>
<p>Derechos have also been observed and analyzed in many other parts of the world, including Europe, Asia and South America. They are an important and active research area in meteorology. Here’s what we know about these unusual storms.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/-6695cCRKmY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A massive derecho in June 2012 developed in northern Illinois and traveled to the mid-Atlantic coast, killing 22 and causing $4 billion to $5 billion in damages.</span></figcaption>
</figure>
<h2>Walls of wind</h2>
<p>Scientists have long recognized that organized lines of thunderstorms can produce widespread damaging winds. <a href="http://www.spc.noaa.gov/misc/AbtDerechos/hinrichs/JohnsDerechoStory.pdf">Gustav Hinrichs</a>, a professor at the University of Iowa, analyzed severe winds in the 1870s and 1880s and identified that many destructive storms were produced by straight-line winds rather than by tornadoes, in which winds rotate. Because the word “tornado,” of Spanish origin, was already in common usage, Hinrichs proposed “derecho” – Spanish for “straight ahead” – for damaging windstorms not associated with tornadoes. </p>
<p>In 1987, meteorologists defined what <a href="https://doi.org/10.1175/1520-0434(1987)002%3C0032:DWCIW%3E2.0.CO;2">qualified as a derecho</a>. They proposed that for a storm system to be classified as a derecho, it had to produce severe winds – 57.5 mph (26 meters per second) or greater – and those intense winds had to extend over a path at least 250 miles (400 kilometers) long, with no more than three hours separating individual severe wind reports. </p>
<p>Derechos are almost always caused by a type of weather system known as a <a href="https://www.weather.gov/jetstream/derecho_bowecho">bow echo</a>, which has the shape of an archer’s bow on radar images. These in turn are a specific type of <a href="https://doi.org/10.1175/AMSMONOGRAPHS-D-18-0001.1">mesoscale convective system</a>, a term that describes <a href="https://doi.org/10.1038/s43017-020-0057-7">large, organized groupings of storms</a>. </p>
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<p>Researchers are studying whether and how climate change is affecting weather hazards from thunderstorms. Although some aspects of mesoscale convective systems, such as the amount of rainfall they produce, are very likely to change with continued warming, it’s not yet clear how future climate change may affect the likelihood or intensity of derechos. </p>
<h2>Speeding across the landscape</h2>
<p>The term “derecho” vaulted into public awareness in June 2012, when one of the most destructive derechos in U.S. history formed in the Midwest and <a href="https://www.spc.noaa.gov/misc/AbtDerechos/casepages/jun292012page.htm">traveled some 700 miles in 12 hours</a>, eventually making a direct impact on the Washington, D.C. area. This event killed 22 people and caused millions of power outages. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/341563/original/file-20200612-153822-1boc4jz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/341563/original/file-20200612-153822-1boc4jz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/341563/original/file-20200612-153822-1boc4jz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=665&fit=crop&dpr=1 600w, https://images.theconversation.com/files/341563/original/file-20200612-153822-1boc4jz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=665&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/341563/original/file-20200612-153822-1boc4jz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=665&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/341563/original/file-20200612-153822-1boc4jz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=835&fit=crop&dpr=1 754w, https://images.theconversation.com/files/341563/original/file-20200612-153822-1boc4jz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=835&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/341563/original/file-20200612-153822-1boc4jz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=835&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Top: Radar imagery every two hours, from 1600 UTC 29 June to 0400 UTC 30 June 2012, combined to show the progression of a derecho-producing bow echo across the central and eastern US. Bottom: Severe wind reports for the 29-30 June 2012 derecho, colored by wind speed.</span>
<span class="attribution"><a class="source" href="https://doi.org/10.1038/s43017-020-0057-7">Schumacher and Rasmussen, 2020, adapted from Guastini and Bosart 2016</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Only a few recorded derechos had occurred in the western U.S. prior to June 6, 2020. On that day, a line of strong thunderstorms developed in eastern Utah and western Colorado in the late morning. This was unusual in itself, as storms in this region tend to be less organized and occur later in the day. </p>
<p>The thunderstorms continued to organize and moved northeastward across the Rocky Mountains. This was even more unusual: Derecho-producing lines of storms are driven by a pool of cold air near the ground, which would typically be disrupted by a mountain range as tall as the Rockies. In this case, the line remained organized.</p>
<p>As the line of storms emerged to the east of the mountains, it caused widespread wind damage in the Denver metro area and northeastern Colorado. It then strengthened further as it proceeded north-northeastward across eastern Wyoming, western Nebraska and the Dakotas. </p>
<p>In total there were nearly 350 reports of severe winds, including 44 of 75 miles per hour (about 34 meters per second) or greater. The strongest reported gust was 110 mph at Winter Park ski area in the Colorado Rockies. Of these reports, 95 came from Colorado – by far the most severe wind reports ever from a single thunderstorm system. </p>
<figure>
<img src="https://cdn.theconversation.com/static_files/files/1070/06jun20_warnings.gif?1591986529">
<figcaption><span class="caption">Animation showing the development and evolution of the 6-7 June 2020 western derecho. Radar reflectivity is shown in the color shading, with National Weather Service warnings shown in the colored outlines (yellow polygons indicate severe thunderstorm warnings). Source: Iowa Environmental Mesonet.</span></figcaption>
</figure>
<p>Coloradans are accustomed to big weather, including strong winds in the mountains and foothills. Some of these winds are generated by flow <a href="http://glossary.ametsoc.org/wiki/Downslope_windstorm">down mountain slopes</a>, localized thunderstorm <a href="https://www.weather.gov/bmx/outreach_microbursts">microbursts</a>, or even “<a href="https://theconversation.com/when-does-a-winter-storm-become-a-bomb-cyclone-113452">bomb cyclones</a>.” Western thunderstorms more commonly produce hailstorms and tornadoes, so it was very unusual to have a broad swath of the state experience damaging straight-line winds that extended from west of the Rockies all the way to the Dakotas. </p>
<h2>Damage comparable to a hurricane</h2>
<p>Derechos are challenging to predict. On days when derechos form, it is often uncertain whether any storms will form at all. But if they do, the chance exists for explosive development of intense winds. Forecasters did not anticipate the historic June 2012 derecho until it was already underway.</p>
<p>For the western derecho on June 6, 2020, outlooks showed an enhanced potential for severe storms in Nebraska and the Dakotas two to three days in advance. However, the outlooks didn’t highlight the potential for destructive winds farther south in Colorado until the morning that the derecho formed.</p>
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<p>Once a line of storms has begun to develop, however, the National Weather Service routinely issues highly accurate severe thunderstorm warnings 30 to 60 minutes ahead of the arrival of intense winds, alerting the public to take precautions. </p>
<p>Communities, first responders and utilities may have only a few hours to prepare for an oncoming derecho, so it is important to know <a href="https://www.weather.gov/media/top/Methods%20to%20Receive%20Warnings_monday.pdf">how to receive severe thunderstorm warnings</a>, such as TV, radio and smartphone alerts, and to take these warnings seriously. Tornadoes and tornado warnings often get the most attention, but lines of severe thunderstorms can also pack a major punch.</p>
<p><em>This is an updated version of an article originally published on June 15, 2020.</em></p><img src="https://counter.theconversation.com/content/140319/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Russ Schumacher receives funding from the National Science Foundation, National Oceanic and Atmospheric Administration for research on high impact weather, and from the Colorado Agricultural Experiment Station for research, education, and outreach related to Colorado's climate.</span></em></p>Hurricane and tornado winds spin in circles, but there’s another, equally dangerous storm type where winds barrel straight ahead. They’re called derechos, and are most common in summer.Russ Schumacher, Associate Professor of Atmospheric Science and Colorado State Climatologist, Colorado State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1028792018-09-20T10:37:31Z2018-09-20T10:37:31ZDestructive 2018 hail season a sign of things to come<figure><img src="https://images.theconversation.com/files/237201/original/file-20180919-143281-xzckd3.jpg?ixlib=rb-1.1.0&rect=0%2C38%2C4920%2C3251&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Icy hailstones can do major damage, depending where they land.</span> <span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/Severe-Weather-Nebraska/1e8e19915cb142f28ee3a7c111ab3d5d/1/0">AP Photo/Nati Harnik</a></span></figcaption></figure><p>As ominous skies moved overhead just after noon on Aug. 6, the <a href="https://www.denverpost.com/2018/08/07/cheyenne-mountain-zoo-hail-storm/">small splash of a hailstone</a> was heard in the pool of the bear exhibit at the Cheyenne Mountain Zoo in Colorado Springs. Moments later, a barrage of ice baseballs began falling from the sky, with one or two softball-sized hailstones in the mix. People and animals scrambled for cover.</p>
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<p>In the aftermath, the <a href="https://kdvr.com/2018/08/10/cheyenne-mountain-zoo-to-reopen-after-hail-damages-buildings-kills-several-animals/">hail storm’s wrath was revealed</a>: at least eight people injured, five animals killed and hundreds of cars severely damaged in the zoo parking lot.</p>
<p>Whether you’re a farmer managing hundreds of acres of corn, a school teacher in care of young children at recess, or an unsuspecting tourist excited to watch the dolphin show, hail storms matter. This summer’s Colorado Springs event is yet another reminder of the destruction that large hail can bring. These large hail events seem to be more and more common, which has prompted <a href="https://scholar.google.com/citations?user=mW88UtIAAAAJ&hl=en">atmospheric scientists like me</a> to investigate trends in hail storms, as well as other severe weather phenomena. A better scientific understanding of hail storms can help increase public awareness, including of how best to protect one’s life and property.</p>
<h2>A record-setting year</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/237159/original/file-20180919-158240-1ih9r4f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/237159/original/file-20180919-158240-1ih9r4f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/237159/original/file-20180919-158240-1ih9r4f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=599&fit=crop&dpr=1 600w, https://images.theconversation.com/files/237159/original/file-20180919-158240-1ih9r4f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=599&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/237159/original/file-20180919-158240-1ih9r4f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=599&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/237159/original/file-20180919-158240-1ih9r4f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=753&fit=crop&dpr=1 754w, https://images.theconversation.com/files/237159/original/file-20180919-158240-1ih9r4f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=753&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/237159/original/file-20180919-158240-1ih9r4f.jpg?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"></a>
<figcaption>
<span class="caption">The more times a hailstone cycles through a cloud, the larger it can grow before falling out.</span>
<span class="attribution"><a class="source" href="https://scijinks.gov/rain/">SciJinks</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Hail is precipitation that <a href="https://www.wunderground.com/prepare/hail">falls in the form of ice</a>. In basic terms, hailstones form as water is lofted into the upper cold regions of a thunderstorm, where it freezes. Supercooled liquid water at these heights can continue to add mass to a small hailstone – eventually it becomes too heavy and falls out of the cloud.</p>
<p>The largest hailstones are found in strong thunderstorms, <a href="https://www.weather.gov/ama/supercell">called supercells</a>, which have sufficiently strong updrafts to allow hailstones to reach the ice region of the cloud, thus <a href="https://www.nssl.noaa.gov/education/svrwx101/hail/forecasting/">acquiring more mass before falling out</a>. Supercells, and thus large hail, are aided by warm and moist conditions that promote strong, juicy updrafts and also a wind field that strengthens and turns with height.</p>
<p>For anyone living in the Great Plains, this year has been a hail season to remember, with many more large hailstones being reported than usual. Neither the total number of severe hail reports – defined by the National Weather Service’s <a href="https://www.spc.noaa.gov">Storm Prediction Center</a> (SPC) as hail in excess of 1 inch in diameter – nor the number of days in which severe hail has fallen, were out of the ordinary. But in 2018, the percentage of hail greater than 2 inches in diameter certainly was.</p>
<p>Here in Colorado, over 20 percent of severe hail reports through the beginning of September have been at least 2 inches. Three percent have been at least 3 inches – bigger than a standard 2.75-inch baseball. These are the highest such percentages in state history. Moreover, Colorado saw a new record, with hail greater than 3 inches in diameter reported 10 times, over seven different days.</p>
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<p>And these trends hold nationally, too, according to <a href="https://www.spc.noaa.gov/wcm/">preliminary SPC reports</a>. Across the country, percentages of large hailstones are among the highest seen since the turn of this century. Geographically, very large hail reports for 2018 stretch from Idaho to Florida to Connecticut, with a maximum in the western Great Plains.</p>
<p><iframe id="63AWu" class="tc-infographic-datawrapper" src="https://datawrapper.dwcdn.net/63AWu/3/" height="400px" width="100%" style="border: none" frameborder="0"></iframe></p>
<p>So what’s going on?</p>
<h2>A new normal?</h2>
<p>Having an increasingly higher proportion of large hail is not an encouraging trend for home or automobile owners.</p>
<p>The <a href="https://www.spc.noaa.gov/wcm/">SPC hail database</a> is the country’s primary source of hail reports. They come from a variety of sources including trained spotters, meteorologists, law enforcement and the general public. As such, the database has <a href="http://www.ejssm.org/ojs/index.php/ejssm/article/view/149/104">some inherent reporting biases</a>. For example, reports tend to cluster around places where people live. And people tend to report hail of sizes they associate with common objects, such as golf balls or baseballs.</p>
<p>But it’s hard to believe people are suddenly becoming more excited to report only large hail, while ignoring the quarter-sized hail that can still be a nuisance. In fact, this year may be a hint of what will become commonplace in the future.</p>
<p>Effectively simulating the growth of hail in a model is nearly impossible due to the intricate microphysical processes involved. Imagine the difficulty of trying to account for hail’s erratic wobbling and tumbling within a cloud and the constant addition and loss of water on its surface. As icing on the cake, each hailstone has a unique shape, and it’s hardly ever spherical. Nevertheless, it is possible to look at changes in variables that are important for hail growth.</p>
<p>Recent research assessed the <a href="https://doi.org/10.1038/nclimate3321">effects of climate change on hail size and frequency</a> using a hail growth model with environmental variables adjusted for climate change. The relatively cool and dry Plains region is expected to become warmer and more moist in a future climate, leading to stronger updrafts and more moisture availability. Under these conditions, scientists found that both average hail diameter and frequency of large hail occurrence are expected to increase across the central part of the country.</p>
<p>Researchers at the National Center for Atmospheric Research have found that in a future climate, there will be <a href="https://doi.org/10.1007/s00382-017-4000-7">more strong thunderstorms and fewer weak ones</a>. This result again favors an increase in larger hailstone sizes, since stronger thunderstorms allow the hail to be cycled through the cloud layer for a longer time.</p>
<h2>Untangling what drives the trend</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/237200/original/file-20180919-158240-5x19vd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/237200/original/file-20180919-158240-5x19vd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/237200/original/file-20180919-158240-5x19vd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=800&fit=crop&dpr=1 600w, https://images.theconversation.com/files/237200/original/file-20180919-158240-5x19vd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=800&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/237200/original/file-20180919-158240-5x19vd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=800&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/237200/original/file-20180919-158240-5x19vd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1005&fit=crop&dpr=1 754w, https://images.theconversation.com/files/237200/original/file-20180919-158240-5x19vd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1005&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/237200/original/file-20180919-158240-5x19vd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1005&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Hard, dense hailstones can do some serious damage.</span>
<span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/Severe-Weather-Plains/b7de798aa0d240e4ae4e712b240ff500/2/0">Stacey Valdez via AP</a></span>
</figcaption>
</figure>
<p>Is a changing climate all that’s responsible for what appears to be a shift toward storms with greater hailstone sizes that occur more frequently? What about population dynamics and urbanization? After all, hailstones can only be reported where people are around to measure them.</p>
<p>These questions have prompted my current research that aims to differentiate the relative contributions from climate change and population change to future hail storm risk. I’m combining a weather model that projects future changes in variables that promote hail storms with spatial population projections from NCAR and the EPA. The goal is to assess where and how much the greatest human risk from large hail is expected to be in the future.</p>
<p>Compared to tornadoes and hurricanes, large hail has received relatively little research attention, but that’s starting to change: A <a href="https://www.mmm.ucar.edu/north-american-hail-workshop">major international workshop</a> was held in August 2018 to share research ideas and results. Achieving better understanding of how hail storms might look in the future, and which places might become more at risk, is of great worth for decision-makers, the insurance industry and the general public. Hopefully, such knowledge will spare everyone – including vacationing families at a popular zoo – the nightmare of dealing with destruction by softball-sized hail.</p><img src="https://counter.theconversation.com/content/102879/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Samuel Childs receives funding from National Science Foundation Grants DGE-1321845 and AGS-1637244. His PhD advisor is Dr. Russ Schumacher.</span></em></p>The future climate that scientists predict for the middle of the United States is one that will foster more hail events with bigger hailstones.Samuel Childs, PhD Student in Atmospheric Science, Colorado State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/972892018-06-07T10:53:35Z2018-06-07T10:53:35ZHow far away was that lightning?<figure><img src="https://images.theconversation.com/files/221632/original/file-20180604-175445-meat87.jpg?ixlib=rb-1.1.0&rect=587%2C71%2C3455%2C2583&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">One one thousand, two one thousand....</span> <span class="attribution"><a class="source" href="https://unsplash.com/photos/vP5Im4q8Z6g">Eric Ward/Unsplash</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>You probably do it. It might be ingrained from when you were a kid, and now it’s almost automatic. You see the flash of lightning – and you immediately start counting the seconds till it thunders.</p>
<p>But does counting really get you a good estimate for how far away the lightning is? Is this one of those old wives’ tales, or is it actually based on science? In this case, we have physics to thank for this quick and easy – and pretty accurate – calculation.</p>
<p>So what happens when a big storm rolls in?</p>
<p>The lightning you see is the <a href="https://www.nssl.noaa.gov/education/svrwx101/lightning/">discharge of electricity</a> that travels between clouds or to the ground. The thunder you hear is the rapid expansion of the air in response to the lightning’s intense heat.</p>
<p>If you’re really close to the lightning, you will see it and hear the thunder simultaneously. But when it’s far away, you see and hear the event at different times. That’s because <a href="https://morgridge.org/blue-sky/why-is-light-faster-than-sound/">light travels much faster than sound</a>. Think of sitting in the nosebleed seats at a baseball game. You see the batter hit the ball a second before you hear the crack of the bat. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/221638/original/file-20180604-175414-15qczrv.jpg?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"></a>
<figcaption>
<span class="caption">The visual part is instantaneous.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/noaaphotolib/27330291264">Pete Gregoire</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>When observing an event on Earth, you see things almost the instant they happen – the speed of light is so fast you can’t even detect the travel time. The speed of sound is much slower, which gives us time to do our calculation.</p>
<p>Let’s simplify the speed equation: <a href="https://en.wikipedia.org/wiki/Speed_of_sound">Sound travels a little over 700 miles per hour</a>, or 700 miles in 3,600 seconds. That means 7 miles traveled every 36 seconds. Make this even easier and round down to 7 miles every 35 seconds… or 1 mile every 5 seconds! Count to 5: If you hear thunder, the lightning occurred within 1 mile.</p>
<p>Now that you know how far away that lightning strike was, is it far enough to be <a href="https://www.weather.gov/safety/lightning-safety">a safe distance from the storm</a>? That’s actually a trick question. Thunder can be heard up to 25 miles away, and lightning strikes have been documented to occur as far as 25 miles from thunderstorms – known as a “<a href="https://www.nssl.noaa.gov/education/svrwx101/lightning/faq/">bolt from the blue</a>.” So if you can hear thunder, you’re close enough to be hit by lightning, and sheltering indoors or in an enclosed car is your safest bet.</p>
<p>And don’t count on the folk wisdom that lightning never strikes the same place twice to protect you. That one is just plain wrong. For example, lightning strikes the top of the <a href="https://www.livescience.com/13704-empire-state-building-lightning-strike.html">Empire State Building</a> an average of 23 times per year.</p><img src="https://counter.theconversation.com/content/97289/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Becky Bolinger receives funding from National Oceanic and Atmospheric Administration and the State of Colorado to monitor drought and climate conditions. </span></em></p>When you see a bolt of lightning, do you immediately start counting to see how far off a storm is? An atmospheric scientist parses the practice.Becky Bolinger, Assistant State Climatologist and Research Scientist in Atmospheric Science, Colorado State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/879462017-11-22T18:01:37Z2017-11-22T18:01:37ZThunderstorms create radioactivity, scientists discover<figure><img src="https://images.theconversation.com/files/195818/original/file-20171122-6072-skhc5p.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">Maxime Raynal/wikipedia</span></span></figcaption></figure><p>Thunder and lightning have sparked awe and fear in humans since time immemorial. In both modern and ancient cultures, these natural phenomena are often thought to be governed by some of the most important and powerful gods – <a href="http://www.sanatansociety.org/hindu_gods_and_goddesses/indra.htm#.WhVO8bSFhmA">Indra in Hinduism</a>, <a href="https://www.greekmythology.com/Olympians/Zeus/zeus.html">Zeus in Greek mythology</a> and <a href="https://www.ancient.eu/Thor/">Thor in Norse mythology</a>. </p>
<p>We know that thunderstorms can trigger a number of remarkable effects, most commonly power cuts, hailstorms and pets hiding under beds. But it turns out we still have things to learn about them. A new study, <a href="http://nature.com/articles/doi:10.1038/nature24630">published in Nature</a>, has now shown that thunderstorms can also produce radioactivity by triggering nuclear reactions in the atmosphere. </p>
<p>This may sound like the plot of a blockbuster science fiction disaster. But in reality, it’s nothing to worry about. Since the early 20th century, scientists have been aware of <a href="https://theconversation.com/explainer-how-much-radiation-is-harmful-to-health-17906">ionising radiation</a> – particles and electromagnetic waves that can damage cells – raining down into the Earth’s atmosphere from space. This radiation can react with atoms or molecules, carrying enough energy to liberate electrons from either atoms or molecules. It therefore leaves behind an “ion” with a positive electrical charge.</p>
<p>Just over a century ago, the Austrian physicist <a href="https://en.wikipedia.org/wiki/Victor_Francis_Hess">Victor Hess</a> made measurements of ionisation in a hot-air balloon five kilometres above the Earth’s surface. He noted that the ionisation rate increased rapidly with height, the opposite of what might be expected if the source of the ionising radiation was coming from the ground. Hess therefore concluded that there must be a source of radiation with very high penetrating power located above the atmosphere. He was named co-recipient of the <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1936/">Nobel Prize in Physics in 1936</a> for his discovery, later dubbed “cosmic rays”.</p>
<p>We now know that cosmic rays are made up of charged particles: primarily, electrons, atomic nuclei and protons – the latter make up the nucleus along with neutrons. Some originate from the sun, while others come from the <a href="https://theconversation.com/an-extragalactic-mystery-where-do-high-energy-cosmic-rays-come-from-6623">distant explosions of dead stars</a> in our galaxy, known as supernovas. When these cosmic rays enter the Earth’s atmosphere, they interact with atoms and molecules to produce a shower of <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">subatomic particles</a>. Among these are neutrons, which have no electric charge.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/195848/original/file-20171122-6044-cx0uhf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/195848/original/file-20171122-6044-cx0uhf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=456&fit=crop&dpr=1 600w, https://images.theconversation.com/files/195848/original/file-20171122-6044-cx0uhf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=456&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/195848/original/file-20171122-6044-cx0uhf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=456&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/195848/original/file-20171122-6044-cx0uhf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=573&fit=crop&dpr=1 754w, https://images.theconversation.com/files/195848/original/file-20171122-6044-cx0uhf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=573&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/195848/original/file-20171122-6044-cx0uhf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=573&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A simulation of a cosmic ray shower formed when a proton hits the atmosphere about 20km above the ground.</span>
<span class="attribution"><span class="source">wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>It is these neutrons that <a href="https://theconversation.com/explainer-what-is-radiocarbon-dating-and-how-does-it-work-9690">make radiocarbon dating possible</a>. Most carbon atoms have six protons and either six or seven neutrons in their nuclei (dubbed “isotopes <sup>12</sup>C and <sup>13</sup>C” respectively). However, neutrons produced by cosmic rays can react with atmospheric nitrogen to create <sup>14</sup>C, a heavy and unstable isotope of carbon that, over time, will “radioactively decay” (split up while emitting radiation) back into nitrogen. </p>
<p>In nature, <sup>14</sup>C is incredibly rare and makes up only about one in a trillion carbon atoms. But, apart from its weight and radioactive properties, 14C is basically identical to the more common carbon isotopes. It oxidises to form carbon dioxide and enters the food chain as plants absorb the radioactive CO<sub>2</sub>. </p>
<p>The ratio of <sup>12</sup>C to <sup>14</sup>C in a given organism will start to change when that organism dies and ceases to ingest carbon. The <sup>14</sup>C already in its system then starts to decay. It’s a slow process since <sup>14</sup>C has a radioactive half-life of 5,730 years, but it is predictable, meaning that organic samples can be dated by measuring the ratio of <sup>12</sup>C to <sup>14</sup>C still remaining.</p>
<p>In this way, cosmic rays are responsible for nuclear reactions in the Earth’s atmosphere. Until today, we thought it was the only natural channel producing radioactive elements such as <sup>14</sup>C. The word “nuclear”, so sinister when partnered with “bomb” or “waste”, simply refers to the changes that are brought about in an atomic nucleus. </p>
<h2>Chasing neutrons</h2>
<p>Almost 100 years ago, the renowned Scottish physicist and meteorologist <a href="https://en.wikipedia.org/wiki/Charles_Thomson_Rees_Wilson">Charles Wilson</a> proposed that thunderstorms could also trigger nuclear reactions in the atmosphere. Wilson, who undertook fieldwork at the isolated meteorological observatory on the summit of Ben Nevis, Britain’s highest mountain, was fascinated by thundercloud formation and atmospheric electricity. However, his suggestion predated the discovery of the neutron – one of the tell-tale products of nuclear reactions – by seven years, so his proposal could not be tested.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/195817/original/file-20171122-6020-a2u08.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/195817/original/file-20171122-6020-a2u08.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/195817/original/file-20171122-6020-a2u08.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/195817/original/file-20171122-6020-a2u08.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/195817/original/file-20171122-6020-a2u08.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/195817/original/file-20171122-6020-a2u08.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/195817/original/file-20171122-6020-a2u08.jpg?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">Lightning over the St Lawrence River on a stormy night in Quebec in 2010.</span>
<span class="attribution"><span class="source">Jp Marquis/wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Since Wilson’s time, there have been many studies that have claimed to have detected thunderstorm-produced neutrons, but <a href="https://www.nature.com/articles/313773a0">none have proven to be definitive</a>. Others have searched for energetic electomagnetic radiation (X-rays and gamma-rays) that accompanies the avalanche of high-energy electrons that we know is produced by lightning in thunderclouds. Calculations show that these electrons and gamma-rays can knock neutrons out of nitrogen and oxygen in the atmosphere. But although the X-ray and gamma-rays have been observed, there has never been a direct observation of the consequent nuclear reactions taking place in a thunderstorm.</p>
<p>The new study uses a different approach. Instead of searching for the elusive neutrons, the authors rely on other byproducts of the nuclear reactions. If electrons and gamma-rays cause unstable isotopes of nitrogen and oxygen to be formed by nuclear reactions following a lightning stroke, these should decay after a few minutes to form stable isotopes of carbon and nitrogen. </p>
<p>Crucially, this decay produces a particle known as a “positron”, the “<a href="https://theconversation.com/explainer-what-is-antimatter-53414">antimatter</a>” version of the electron. All particles have antimatter versions of themselves – these have the same mass but the opposite charge. When antimatter and matter come in contact, they annihilate in a flash of energy. This is the energy the researchers looked for. Using radiation detectors looking over the Sea of Japan, they observed the unambiguous gamma ray fingerprints of positron-electron annihilation taking place immediately after lightning strikes in low winter thunderclouds. This is clear evidence of nuclear reactions taking place in thunderclouds.</p>
<p>These results are important as they demonstrate a previously unknown source of isotopes in the Earth’s atmosphere. These include <sup>13</sup>C, <sup>14</sup>C and <sup>15</sup>N but future studies may also reveal others, such as isotopes of hydrogen, helium and beryllium. </p>
<p>The findings also have implications for astronomers and planetary scientists.
Other planets within our solar system have thunderstorms in their atmospheres that might contribute to the composition of their atmospheres. One of these planets is Jupiter, which is fittingly also the god of thunder in ancient Roman mythology.</p><img src="https://counter.theconversation.com/content/87946/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jim Wild receives funding from the Science and Technology Facilities Council (STFC) and the Natural Environment Research Council (NERC). He is a Fellow of the Royal Astronomical Society and a member of the American Geophysical Union. He is currently the Chairman of the STFC Astronomy Grants Panel. </span></em></p>Scientists have finally been able to prove that thunder and lightning drive nuclear reactions.Jim Wild, Professor of Space Physics, Lancaster UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/857452017-10-16T18:49:21Z2017-10-16T18:49:21ZTropical thunderstorms are set to grow stronger as the world warms<figure><img src="https://images.theconversation.com/files/190352/original/file-20171016-21986-74bzev.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A supercell thunderstorm in the US state of Oklahoma.</span> <span class="attribution"><span class="source">Hamish Ramsay</span>, <span class="license">Author provided</span></span></figcaption></figure><p>Thunderstorms are set to become more intense throughout the tropics and subtropics this century as a result of climate change, according to <a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1707603114">new research</a>.</p>
<p>Thunderstorms are among nature’s most spectacular phenomena, producing lightning, heavy rainfall, and sometimes awe-inspiring cloud formations. But they also have a range of important impacts on humans and ecosystems.</p>
<p>For instance, lightning produced by thunderstorms is an <a href="http://journals.ametsoc.org/doi/abs/10.1175/1520-0442(1994)007%3C1484:TIOACC%3E2.0.CO;2">important trigger for bushfires globally</a>, while the hailstorm that hit Sydney in April 1999 remains Australia’s <a href="http://www.nationalgeographic.com.au/nature/sydneys-apocalyptic-hailstorm.aspx">costliest ever natural disaster</a>.</p>
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Read more:
<a href="https://theconversation.com/to-understand-how-storms-batter-australia-we-need-a-fresh-deluge-of-data-68487">To understand how storms batter Australia, we need a fresh deluge of data</a>
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<p>Given the damage caused by thunderstorms in Australia and <a href="http://www.independent.co.uk/environment/world-deadliest-storms-tornadoes-cyclones-hail-lightning-bangladesh-india-egypt-zimbabwe-a7741261.html">around the world</a>, it is important to ask whether they will grow in frequency and intensity as the planet warms.</p>
<p>Our main tools for answering such questions are global climate models – mathematical descriptions of the Earth system that attempt to account for the important physical processes governing the climate. But global climate models are not fine-scaled enough to simulate individual thunderstorms, which are typically only a few kilometres across.</p>
<p>But the models can tell us about the ingredients that increase or decrease the power of thunderstorms.</p>
<h2>Brewing up a storm</h2>
<p>Thunderstorms represent the dramatic release of energy stored in the atmosphere. One measure of this stored energy is called “convective available potential energy”, or CAPE. The higher the CAPE, the more energy is available to power updrafts in clouds. Fast updrafts move ice particles in the cold, upper regions of a thunderstorm rapidly upward and downward through the storm. This helps to separate negatively and positively charged particles in the cloud and eventually leads to <a href="http://www.srh.noaa.gov/jetstream/lightning/lightning.html">lightning strikes</a>. </p>
<p>To create thunderstorms that cause damaging wind or hail, often referred to as severe thunderstorms, a second factor is also required. This is called “vertical wind shear”, and it is a measure of the changes in wind speed and direction as you rise through the atmosphere. Vertical wind shear helps to organise thunderstorms so that their updrafts and downdrafts become physically separated. This prevents the downdraft from cutting off the energy source of the thunderstorm, allowing the storm to persist for longer.</p>
<p>By estimating the effect of climate change on these environmental properties, we can estimate the likely effects of climate change on severe thunderstorms.</p>
<h2>Stormy forecast</h2>
<p>My research, carried out with US colleagues and <a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1707603114">published today in Proceedings of the National Academy of Sciences</a>, does just that. We examined changes in the energy available to thunderstorms across the tropics and subtropics in 12 global climate models under a “business as usual” scenario for greenhouse gas emissions. </p>
<p>In every model, days with high values of CAPE grew more frequent, and CAPE values rose in response to global warming. This was the case for almost every region of the tropics and subtropics.</p>
<p>These simulations predict that this century will bring a marked increase in the frequency of conditions that favour severe thunderstorms, unless greenhouse emissions can be significantly reduced.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/190353/original/file-20171016-21953-1j6qu1w.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/190353/original/file-20171016-21953-1j6qu1w.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190353/original/file-20171016-21953-1j6qu1w.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=320&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190353/original/file-20171016-21953-1j6qu1w.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=320&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190353/original/file-20171016-21953-1j6qu1w.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=320&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190353/original/file-20171016-21953-1j6qu1w.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=402&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190353/original/file-20171016-21953-1j6qu1w.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=402&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190353/original/file-20171016-21953-1j6qu1w.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=402&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Change in frequency (in days per year) of favourable conditions for severe thunderstorms for 2081-2100, compared with 1981-2000 averaged across 12 climate models under the RCP8.5 greenhouse-gas concentration scenario. Stippling indicates regions where 11 of the 12 models agree on the sign of the change.</span>
<span class="attribution"><span class="source">CREDIT</span>, <span class="license">Author provided</span></span>
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<p>Previous studies have made similar predictions for severe thunderstorms in <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00426.1">eastern Australia</a> and the <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00382.1">United States</a>. But ours is the first to study the tropics and subtropics as a whole, a region that is characterised by some of the <a href="http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-87-8-1057">most powerful thunderstorms on Earth</a>.</p>
<h2>What drives the increased energy?</h2>
<p>Different climate models, constructed by <a href="https://www.wcrp-climate.org/wgcm-cmip">different research groups around the world</a>, all agree that global warming will increase the energy available to thunderstorms – a prediction underlined by our new research. But we need to <a href="http://journals.ametsoc.org/doi/abs/10.1175/BAMS-86-11-1609">understand why this happens</a>, so as to be sure that the effect is real and not a product of faulty model assumptions.</p>
<p>My colleagues and I <a href="http://onlinelibrary.wiley.com/doi/10.1002/grl.50796/abstract">previously proposed</a> that high levels of CAPE can develop in the tropics as a result of the turbulent mixing that occurs when clouds draw in air from their surroundings. This mixing prevents the atmosphere from dissipating the available energy too quickly. Instead, the energy builds up for longer and is released in less frequent but more intense storms.</p>
<p>As the climate warms, the amount of water vapour required for cloud formation increases. This is the result of a well-known thermodynamic relationship called the <a href="http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/clausius.php">Clausius-Clapeyron relation</a>. In a warmer climate this means the difference in the humidity between the clouds and their surroundings becomes larger. As a result, the mixing mechanism becomes more efficient in building up the available energy. This, we argue, accounts for the increase in CAPE seen in our model simulations.</p>
<p>In our new study, we tested this idea in a global climate model by artificially increasing the strength of the mixing between clouds and their surroundings. As expected, this change produced a large increase in the energy available to thunderstorms in our model.</p>
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<a href="https://theconversation.com/australia-faces-a-stormier-future-thanks-to-climate-change-35327">Australia faces a stormier future thanks to climate change</a>
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<p>Another prediction of our hypothesis is that days with both high values of CAPE and heavy precipitation tend to occur when the atmosphere is least humid in its middle levels (at altitudes of a few kilometres). Using real data from <a href="http://www.abc.net.au/local/stories/2008/05/09/2240552.htm">weather balloons</a>, we confirmed that this is the case across the tropics and subtropics.</p>
<h2>What this means for future thunderstorms</h2>
<p>The models predict that the energy available for thunderstorms will increase as the Earth warms. But how much more intense will storms actually become as a result?</p>
<p>The answer to that question is currently uncertain, and answering it is the next job for <a href="http://onlinelibrary.wiley.com/doi/10.1002/qj.2567/full">me</a>, and <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-15-0623.1?journalCode=clim">other researchers around the world</a>.</p>
<p>But it is clear that through our continued greenhouse gas emissions, we are increasing the fuel available to the strongest thunderstorms. Exactly how much stronger our future thunderstorms will ultimately become remains to be seen.</p><img src="https://counter.theconversation.com/content/85745/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martin Singh receives support from the ARC Centre of Excellence for Climate System Science and has previously been supported by the US National Science Foundation.</span></em></p>The amount of atmospheric energy available to thunderstorms will increase in response to climate change, putting the tropics and subtropics at risk of being lashed with more intense storms.Martin Singh, Lecturer, School of Earth, Atmosphere and Environment, Monash UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/684872016-11-14T03:20:42Z2016-11-14T03:20:42ZTo understand how storms batter Australia, we need a fresh deluge of data<p><em>The journal <a href="http://link.springer.com/journal/10584">Climatic Change</a> has published a <a href="http://link.springer.com/journal/10584/139/1/page/1">special edition</a> of review papers discussing major natural hazards in Australia. This article is one of a <a href="https://theconversation.com/au/topics/australian-natural-hazards-series-32987">series</a> looking at those threats in detail.</em></p>
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<p>Storms, wind and hail do a lot of damage to Australians and their property. The <a href="http://www.bom.gov.au/nsw/sevwx/14april1999.shtml">1999 Sydney hailstorm</a>, for instance, cost <a href="http://forms2.rms.com/rs/729-DJX-565/images/scs_1999_sydney_hailstorm.pdf">A$1.7 billion</a> in insured losses. That makes it the biggest single insurance loss in Australian history; in today’s money it would have cost more than A$4 billion.</p>
<p>More recently, one of the most severe storms in decades caused a <a href="http://www.abc.net.au/news/2016-09-28/sa-weather-south-australia-without-power-as-storm-hits/7885930">statewide blackout</a> in South Australia in September. The intense low-pressure system featured <a href="https://www.theguardian.com/australia-news/2016/nov/14/south-australian-blackout-included-seven-tornadoes-bureau-of-meteorology-says">seven tornadoes</a> that tore down three major transmission lines.</p>
<p>Our understanding of wind and hail depends on the type of storm that generates them – and this is where it gets complicated. Thunderstorms can generate not just heavy rainfall but also high winds, lightning and hail, albeit in very localised areas. Large-scale storms such as tropical cyclones are a different phenomenon altogether, bringing not just destructive winds, but also storm surges and soaking rains, often over wide areas. </p>
<p>This complexity makes storms difficult to study, because limited research resources are spread across the many different storm types and their associated hazards.</p>
<p>To help address these issues, we <a href="http://link.springer.com/article/10.1007/s10584-016-1737-7">collated and reviewed</a> the latest knowledge and understanding of storms in Australia, covering the current scientific literature on the assessment, causes, observed trends and future projected changes of storm hazards, with a specific focus on severe wind and hail. We found that progress has been made in many areas, but also that much remains to be done.</p>
<h2>Are we getting more or less storms?</h2>
<p>In short - we don’t know with confidence. Despite the severity of the impacts wrought by storms, there is limited observational data for some types of storms and their associated hazards, particularly for the estimation of hail and wind. </p>
<p>Current estimates of the hail hazard in Australia, for example, are available only from the Bureau of Meteorology’s <a href="http://www.bom.gov.au/australia/stormarchive/">severe storm archive</a>, which suffers from large uncertainties associated with biases and changing reporting practices. This makes it unsuitable for assessing the climatology of hail storms on a national scale. </p>
<p>Similarly, issues such as changes to Automatic Weather Stations (AWS) and limited records of atmospheric pressure observations, have hampered efforts to develop <a href="http://www.bom.gov.au/climate/data-services/amoj_wind_2010.pdf">high-quality surface wind datasets</a> across Australia. Bob Dylan might have been right when he told us “<a href="http://bobdylan.com/songs/subterranean-homesick-blues/">you don’t need a weatherman to know which way the wind blows</a>,” but then again he didn’t win his Nobel Prize for meteorology.</p>
<p>European researchers have <a href="https://www.hindawi.com/journals/tswj/2013/494971/">analysed hailstorm trends</a> using networks of devices called “<a href="http://www.sciencedirect.com/science/article/pii/S0169809508002536">hailpads</a>”. But these records do not exist in Australia, and so there is a significant gap in our knowledge about hailstorm histories and trends.</p>
<p>The projections of future wind hazard in and around Australia are equally limited and differ from region to region. For example, in the tropics, research suggests that <a href="http://www.nature.com/nature/journal/v509/n7500/full/nature13278.html">extreme wind hazard may decrease in the future</a>, although confidence in this prediction is low. Meanwhile, <a href="http://www.geosci-model-dev.net/7/621/2014/gmd-7-621-2014.html">summer wind increases</a> are possible in those parts of Australia that are subjected to <a href="http://www.bom.gov.au/nsw/sevwx/facts/ecl.shtml">East Coast Lows</a>. </p>
<p>We also don’t really know what to expect from <a href="http://www.sciencedirect.com/science/article/pii/S0169809512000968">future severe thunderstorms</a>, and while <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00426.1?mobileUi=0&journalCode=clim">research suggests</a> that they may become more frequent in southeastern Australia, there is a wide range of uncertainty around this projection.</p>
<p>For future trends in hail, again there are only a few studies currently available, but there is at least an indication of <a href="https://blogs.csiro.au/climate-response/stories/four-degrees-of-global-warming-australia-in-a-hot-world/">increases in hail frequency</a> in southeastern regions.</p>
<p>But while the picture is very uncertain for now, we hope this uncertainty will be reduced with the help of improvements in both the observation and computational modelling of storms and their associated hazards. We are growing more confident in our <a href="http://onlinelibrary.wiley.com/doi/10.1002/wcc.371/references;jsessionid=A69988FD600B9BA5E9C478AD7DF99322.f01t01?globalMessage=0">predictions for tropical cyclone</a>, forecasting that the overall number will decline, but that the strongest storms will grow stronger still.</p>
<p>We also hope to improve our understanding of severe thunderstorms by using remote sensing platforms to record hail and extreme wind events right across Australia. These include the <a href="http://www.gpats.com.au">GPATS lightning-detection network</a>, the new <a href="http://ds.data.jma.go.jp/mscweb/data/himawari/sat_img.php?area=fd_">Himawari-8 and 9 satellites</a>, and the Bureau of Meteorology’s <a href="http://www.cawcr.gov.au/technical-reports/CTR_055.pdf">soon-to-be upgraded radar network</a>. Validation of these techniques, of course, will also require high-quality direct observations of these severe weather conditions – the very thing we currently lack.</p>
<h2>Is this where you come in?</h2>
<p>Citizen scientists may, however, help to fill some of these gaps. There are exciting prospects for improving severe weather observations, such as the success of the <a href="http://mping.nssl.noaa.gov/">mPING</a> crowdsourced weather reports project in the United States, which allows participants to use a mobile phone app to report severe weather, which then feeds into <a href="http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-13-00014.1">new research</a>. </p>
<p>This approach could prove to be an excellent way of getting data in such a vast and diverse landscape as Australia, while simultaneously engaging with both the public and the atmospheric science community. We could also enlist the help of <a href="http://www.turing-gateway.cam.ac.uk/mfsg_sep2015">scientific study groups</a>, which bring together academics, scientists and industry partners to exchange ideas and develop research techniques.</p>
<p>“<a href="http://www.william-shakespeare.info/act5-script-text-julius-caesar.htm">The storm is up, and all is on the hazard</a>,” cried Cassius in William Shakespeare’s <em>Julius Caesar</em>. How true that is of storms in Australia. </p>
<p>If we don’t increase our observational and research abilities, we might never fully understand the impacts of severe storms, much less be able to deal with them.</p><img src="https://counter.theconversation.com/content/68487/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris White receives funding from various Tasmanian State Government research funding programs, Wine Australia and the Bushfire and Natural Hazard CRC.</span></em></p><p class="fine-print"><em><span>Jason Evans receives funding from the Australian Research Council, the National Environmental Science Programme Earth Systems and Climate Change Hub, Sydney Water, Water Research Australia, and various NSW state government research funding programs.</span></em></p><p class="fine-print"><em><span>Kevin Walsh receives funding from the Australian Research Council and other international funding organizations.</span></em></p>Severe storms bring a complex mixture of weather conditions, often in a very localised area. This unpredictability can make them very damaging, and very hard to study too.Christopher J White, Lecturer in Environmental Engineering, University of TasmaniaJason Evans, Associate Professor, UNSW SydneyKevin Walsh, Reader, School of Earth Sciences, The University of MelbourneLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/684262016-11-10T19:14:16Z2016-11-10T19:14:16ZFirestorms: the bushfire/thunderstorm hybrids we urgently need to understand<figure><img src="https://images.theconversation.com/files/145372/original/image-20161110-26340-15r4qm0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The higher the plume, the bigger the problem.</span> <span class="attribution"><span class="source">Jim Peaco/Wikimedia Commons</span></span></figcaption></figure><p><em>The journal <a href="http://link.springer.com/journal/10584">Climatic Change</a> has published a <a href="http://link.springer.com/journal/10584/139/1/page/1">special edition</a> of review papers discussing major natural hazards in Australia. This article is one of a <a href="https://theconversation.com/au/topics/australian-natural-hazards-series-32987">series</a> looking at those threats in detail.</em></p>
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<p>Fire has been a driving force across Australia for millennia. Indeed, the health of many of our ecosystems is intrinsically dependent on fire. But bushfires are also one of our most frequent natural hazards, with a total cost estimated at <a href="http://www.tandfonline.com/doi/abs/10.1080/13669870802648528">A$340 million per year</a>. </p>
<p>In the past decade or so, extreme bushfires in southeastern Australia have burned <a href="http://link.springer.com/article/10.1007/s10584-016-1811-1">more than a million hectares, claiming more than 200 lives and over 4,000 homes</a>. Similar losses in other major urban areas have prompted questions about whether we are seeing a shift towards a significantly more hazardous fire regime, characterised by increasing fire frequency and intensity, and the development of catastrophic “firestorms”. </p>
<p>While these extreme bushfires account for only a very small percentage of fire events, they are responsible for the lion’s share of bushfire-related losses.</p>
<p>In contrast to typical bushfires, which spread across the landscape as well-defined burning fronts with smoke plumes perhaps a few kilometres high, extreme bushfires exhibit deep and widespread flaming and produce smoke plumes that can extend 10-15km into the atmosphere. </p>
<p>At these altitudes, bushfire plumes can actually develop into thunderstorms (hence the term “firestorm”). As such, extreme bushfires become much more difficult for emergency services to handle, making them all but impossible to suppress and their spread difficult to predict. </p>
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<a href="https://images.theconversation.com/files/145371/original/image-20161110-26318-5ykpxx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/145371/original/image-20161110-26318-5ykpxx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/145371/original/image-20161110-26318-5ykpxx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/145371/original/image-20161110-26318-5ykpxx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/145371/original/image-20161110-26318-5ykpxx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/145371/original/image-20161110-26318-5ykpxx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/145371/original/image-20161110-26318-5ykpxx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/145371/original/image-20161110-26318-5ykpxx.jpg?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"></a>
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<span class="caption">A firestorm blots out the sky in Victoria’s Grampians.</span>
<span class="attribution"><span class="source">Randall Bacon</span>, <span class="license">Author provided</span></span>
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<h2>Beyond hot, dry and windy</h2>
<p>Like other dangerous bushfires, firestorms are driven by hot, dry and windy weather. But to spawn a firestorm, a range of other conditions must also be met; these can include a rugged landscape, particularly nasty weather events that produce “spikes” in fire danger, and conditions in the upper atmosphere that allow fire plumes to grow to considerable heights. </p>
<p>While previous studies have considered past and projected changes in the hot, dry and windy aspect of fire danger, less research has been done on the future projections for these other types of conditions. This means that we have quite a poor understanding of how extreme bushfires might affect us in the future.</p>
<p>As part of a series of reviews produced by the <a href="http://ozewex.org/">Australian Energy and Water Exchange</a> initiative, my colleagues and I have taken a <a href="http://link.springer.com/article/10.1007/s10584-016-1811-1">closer look</a> at the most catastrophic bushfire cases and the factors that drive them, beyond the usual hot, dry and gusty weather.</p>
<p>There has been an overall increase in the frequency of major bushfire events in southeastern Australia since the mid-19th century. In particular, in the past 15 years a major fire event has occurred every 5 years or less. While some of this increase is due to changes in land use since European colonisation, there is also strong evidence of climate-driven changes. </p>
<p>We found that besides increases in dangerous surface fire danger conditions, upper atmospheric conditions have also become more conducive to explosive fire growth. High levels of the <a href="http://www.highfirerisk.com.au/tools/c_haines_flierA4.pdf">c-Haines index</a>, which signals greater potential for a fire’s plume to rise high into the atmosphere, have become considerably more prevalent since the 1980s. The effects of droughts and widespread heatwaves have also contributed to the occurrence of extreme bushfires.</p>
<p>Looking into the future, high c-Haines values are projected to grow more prevalent still, albeit more gradually than over recent decades. Frontal weather patterns associated with particularly bad fire days are also projected to become more frequent during this century, and rainfall is projected to decrease over southwest and southeastern Australia. </p>
<p>All of this suggests that extreme bushfires will become a more common occurrence into the future. </p>
<h2>What we still don’t know</h2>
<p>Our methods for assessing fire danger do not explicitly account for the effects of extended drought and heatwaves on larger fuel elements such as branches and logs, and so may not properly account for their effects on fire spread and heat release into the atmosphere.</p>
<p>There is also considerable uncertainty about how fuel loads will change into the future. It is possible that the higher fire intensities expected to result from the direct effects of a warmer, drier climate may be offset by lower fuel loads.</p>
<p>Our understanding of extreme fire occurrence is also hampered by the lack of long-term and prehistoric climate data, which makes it hard to work out what the “normal” level of extreme bushfires has been in the past. While charcoal records show promise in this regard, we still don’t know enough about how charcoal is generated, deposited and subsequently preserved during extreme fires.</p>
<p>To predict the future occurrence of extreme bushfires, we also have more work to do in understanding how the trends forecast by global climate models will play out in terms of creating regional-scale fire weather conditions. And we still need to figure out the likely effects of other large-scale patterns such as El Niño.</p>
<p>Given the relatively recent advances that have been made in understanding the key drivers of extreme bushfires, the field is now ready for targeted studies that will help us estimate the future risk of extreme bushfires – and how best we can confront the threat.</p>
<hr>
<p><em>This article was amended on November 11, 2016, to correct the figure for the cost of bushfire-related damage. The correct figure is A$340 million, not A$8.5 billion which is the annual cost of all fire-related damage in Australia.</em></p><img src="https://counter.theconversation.com/content/68426/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jason Sharples receives funding from the Australian Research Council and the Bushfire and Natural Hazards Cooperative Research Centre. </span></em></p>When a bushfire rages so high it creates its own thunderstorm, it becomes a ‘firestorm’ - and makes life much more difficult for firefighters. We still have a lot to learn about what triggers them.Jason Sharples, Associate Professor, School of Physical, Environmental and Mathematical Sciences, UNSW Australia, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/393152015-03-25T19:13:04Z2015-03-25T19:13:04ZThe tropics are getting wetter: the reason could be clumpy storms<figure><img src="https://images.theconversation.com/files/75937/original/image-20150325-14515-ueemoa.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Clumps of thunderstorms are driving increases in rain over tropical oceans. </span> <span class="attribution"><a class="source" href="http://eol.jsc.nasa.gov">Image courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center</a></span></figcaption></figure><p>For a long time climate models have predicted that wet and warm areas in the tropics are going to get wetter, especially over the oceans. Observations of the recent past are beginning to support this hypothesis. What we didn’t know is how this change might occur.</p>
<p>In <a href="http://www.nature.com/articles/doi:10.1038/nature14339">research</a> published today in Nature, our research team shows that the answer could be thunderstorms, specifically those that are gregarious and clump together, a behaviour referred to as “organisation” of convective storms.</p>
<h2>Warm and wet gets wetter</h2>
<p>In its <a href="http://www.climatechange2013.org/images/report/WG1AR5_Chapter12_FINAL.pdf">Fifth Assessment Report</a> the Intergovernmental Panel on Climate Change (IPCC) states that it is virtually certain that rainfall will increase as temperatures increase and that short periods of intense rainfall over the tropics will become more frequent and intense.</p>
<p>Rainfall in the tropics is driven by convection: the rapid upward movement of air over small areas, usually expressing itself in thunderstorm clouds. Warm, moist air rises and as it expands on its way up it cools and condenses to form clouds, and ultimately rain.</p>
<p>In weather and climate jargon we distinguish between “deep” and “shallow” convection. Deep convective clouds reach to 10-20 kilometres height (typically 12-13 kilometres), and it is those clouds that produce much of the rain-bearing systems of the tropics.</p>
<h2>Clumpy storms</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/75938/original/image-20150325-14526-2gauia.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/75938/original/image-20150325-14526-2gauia.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/75938/original/image-20150325-14526-2gauia.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=903&fit=crop&dpr=1 600w, https://images.theconversation.com/files/75938/original/image-20150325-14526-2gauia.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=903&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/75938/original/image-20150325-14526-2gauia.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=903&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/75938/original/image-20150325-14526-2gauia.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1135&fit=crop&dpr=1 754w, https://images.theconversation.com/files/75938/original/image-20150325-14526-2gauia.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1135&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/75938/original/image-20150325-14526-2gauia.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1135&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
<span class="attribution"><a class="source" href="http://eol.jsc.nasa.gov">Image courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center</a></span>
</figcaption>
</figure>
<p>So let’s look at deep convection, the one that makes the rain. Previous research has shown that there are essentially two types of these storm systems, both made up of several individual thunderstorm cells, each just a few kilometres wide.</p>
<p>In one observed scenario, the individual cells are isolated and relatively widely spaced, resulting in short sharp bursts of rainfall, thunder and lightning. Seen from above a field of thunderstorms looks much like popcorn, given this “disorganised” form of convection the nickname “popcorn convection”.</p>
<p>A second type of convective systems is characterised by a clumping together of individual cells over areas of a 100 kilometres or larger with some growing up to 1000 kilometres or more. This “organised” convection often forms lines, arcs or blobs. Because the storm systems are so big they change the air-flow around them and that’s what makes them such efficient rainfall producers.</p>
<p>In <a href="http://onlinelibrary.wiley.com/doi/10.1029/2005GL024584/abstract">previous research</a> we’ve developed a way of identifying whether convective systems are strongly clumped together or not by using their cloud signatures as seen from space.</p>
<p>We’ve also <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00624.1">found previously</a> that the ones that organise produce more precipitation than the ones that don’t. In fact we’ve found that organised systems only occur 5% of the time in the tropics over 25 years, but produce 50% of the rain.</p>
<h2>How does convection contribute to the rainfall changes?</h2>
<p>The Nature study combines these earlier findings with space-based observations of rainfall to identify the role of the different types of convection in observed increases of rainfall by posing questions such as:</p>
<ul>
<li><p>Do organised and disorganised convection contribute equally to rainfall increases?</p></li>
<li><p>Are there more storms with the same amount of rain or are the storms getting more intense?</p></li>
</ul>
<p>We used a 12-year record of daily rainfall observations derived from satellites over the tropics and combined it with our record on convection type. What we found is that the increase in rainfall observed over the tropical oceans is largely happening because of an increase in the occurrence of organised storms, rather than an increase in their intensity. </p>
<p>This was a somewhat surprising finding, and it doesn’t confirm the simple hypothesis that the increased water vapour content in a warmer atmosphere automatically implies an increase in rainfall intensity. As is often the case, nature follows a more complex path, in this case by increasing the number of heavily precipitating storms. Our next task is to find out why!</p><img src="https://counter.theconversation.com/content/39315/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Christian Jakob receives funding from the Australian Research Council and the US Department of Energy.</span></em></p>For a long time climate models have forecast increasing rainfall in tropics. Now we know part of the reason: clumpy thunderstorms.Christian Jakob, Professor in Atmospheric Science, Monash UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/360022015-01-08T19:24:58Z2015-01-08T19:24:58ZCould climate change have played a role in the AirAsia crash?<figure><img src="https://images.theconversation.com/files/68440/original/image-20150108-1974-x95xfv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Could climate change be making flying more unsafe?</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/kamgtr/7837931542">KamrenB Photography</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>On December 28, AirAsia flight QZ8501 crashed into the sea between Indonesia and Singapore. Shortly before the crash, the pilots requested an <a href="http://www.abc.net.au/news/2014-12-28/airasia-flight-loses-contact-with-air-traffic-control/5990424">alteration to the flight route</a> due to severe weather. While the precise cause of the crash is not yet clear (with the aircraft’s black box still to be retrieved), it seems likely that weather played a role. </p>
<p>AirAsia CEO Tony Fernandes later <a href="http://www.smh.com.au/world/airasia-flight-qz8501-unique-weather-may-have-caused-plane-crash-says-ceo-20141230-12fs40.html">said</a> that “unique weather conditions” were present around the time of the crash. He reportedly stated that climate change may have played a part in more dangerous conditions for air travel, linking it to flooding in Malaysia and Thailand. </p>
<p>When we examine aviation hazards in the tropics we’re usually talking about a single thunderstorm or cluster of storms, and attributing the intensity of a single thunderstorm to a climate change signal is essentially impossible. </p>
<p>There was a large region of intense thunderstorm activity around where the AirAsia accident occurred. Floods in Malaysia and Thailand were certainly linked to that. However, large regions of intense thunderstorms are common in this region and at face value this event doesn’t appear to fall outside the range of natural variability. </p>
<p>So, what does the science tell us about flying and climate change?</p>
<h2>Stormy science</h2>
<p>Flight QZ8501 flew into a region of severe tropical storm activity. </p>
<p>Thunderstorms are a significant hazard for aircraft, producing turbulence through upward and downward motion in the main body of a storm. This upward wind can exceed 100 kilometres per hour straight upwards; the downward motion is usually a little weaker. Aircraft try to avoid these regions due to the extreme turbulence they can produce. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/68452/original/image-20150108-1982-10lmwqv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/68452/original/image-20150108-1982-10lmwqv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/68452/original/image-20150108-1982-10lmwqv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/68452/original/image-20150108-1982-10lmwqv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/68452/original/image-20150108-1982-10lmwqv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/68452/original/image-20150108-1982-10lmwqv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/68452/original/image-20150108-1982-10lmwqv.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">
<figcaption>
<span class="caption">Storms, such as this one moving across Port Phillip Bay, can create hazardous turbulence for aircraft.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/mrpbps/5225681529">mrpbps</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Other parts of thunderstorms can be hazardous too. Turbulence can occur <a href="http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-11-00062.1">well above thunderstorms</a>, due to breaking atmospheric waves. Just like waves break in the ocean, there are waves in the atmosphere that can break above and around thunderstorms. </p>
<p>These waves occur in the clear air around a storm. So the storm margins can be hazardous as well.</p>
<p>Another danger storms pose is icing, which <a href="http://www.news.com.au/travel/travel-updates/four-massive-objects-found-on-sea-bed-thought-to-be-lost-airasia-jets-fuselage/story-fnizu68q-1227173834505">some reports</a> have blamed for the crash. Ice can build up on the body of aircraft and engines can ingest large volumes of ice particles affecting their operation. </p>
<h2>Dodging storms</h2>
<p>In the tropics, thunderstorms are very deep and almost impossible to fly over, so aircraft have to fly around the thunderstorm region. In situations like that encountered by the AirAsia plane, with a very large region of thunderstorm activity, it’s very difficult or impossible to navigate around or through without experiencing some kind of turbulence. </p>
<p>A rule of thumb often used is to avoid storms by 20 nautical miles (about 37 km). Pilots use visual cues and on-board radar, in collaboration with air traffic control, to assist avoidance. </p>
<p>Often with a large region of storms, pilots fly through cloudy conditions but avoid the most hazardous “convective cells”.</p>
<h2>Out of the tropics</h2>
<p>In mid-latitude regions away from the tropics we get the more traditional definition of clear-air turbulence (or CAT). This is associated with jet streams - regions of strong horizontal wind. Those jet streams can cause turbulence above and below them due to wind shear, which is a change in wind speed with height. </p>
<p>Clear-air turbulence is a significant hazard because it’s invisible. But it often occurs in shallow layers, so changing height by a few thousand feet can sometimes help to avoid it. </p>
<p>The final type of turbulence is that which occurs over mountains, known as mountain-wave turbulence. It also occurs in clear air, through a similar process to thunderstorm waves. Air flowing over mountains can generate waves that can break. </p>
<p>Regions with large mountains, such as New Zealand, the Himalayas and the Rocky Mountains in the United States, are well known for being turbulent. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/68457/original/image-20150108-1982-mv74lp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/68457/original/image-20150108-1982-mv74lp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/68457/original/image-20150108-1982-mv74lp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/68457/original/image-20150108-1982-mv74lp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/68457/original/image-20150108-1982-mv74lp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/68457/original/image-20150108-1982-mv74lp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/68457/original/image-20150108-1982-mv74lp.jpg?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">A plane flies towards dark clouds in Auckland. New Zealand is known for its turbulent skies.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/russellstreet/5285292054">russellstreet</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>In 1992 a DC-8 cargo plane over the Colorado Rockies experienced extreme turbulence, losing one of its engines and part of its wing. Amazingly, this aircraft landed safely. You rarely hear of turbulence damaging aircraft, but it can occur.</p>
<h2>Is there a climate link?</h2>
<p>A provocative <a href="http://www.nature.com/nclimate/journal/v3/n7/full/nclimate1866.html">2013 paper</a> received a lot of media attention for drawing a link between climate change and turbulence. The researchers focused on model projections of future climate over the Atlantic Ocean and clear-air turbulence associated with the jet stream. They showed that there is a potential for an increase in turbulence along existing flight routes. </p>
<p>A lot of that increase was related to a poleward shift in the jet stream - something that is a robust signal in many climate models in both hemispheres. This shift put the jet streams closer to existing flight routes over the Atlantic Ocean. </p>
<p>The assumption made in ascertaining an increase in turbulence was that those routes would remain unchanged. But there’s a global trend in aviation for much more intelligent use of air space and much better hazard avoidance, so I anticipate that there would be quite rapid adaptation in the aviation industry to any trends in hazards.</p>
<p>Very few studies have examined aircraft turbulence in the tropics. Because of that essentially nothing has been done on what potential changes in tropical storms might mean for aviation. </p>
<p>There’s certainly been <a href="http://www.nature.com/ngeo/journal/v5/n10/full/ngeo1568.html">research</a> showing that precipitation extremes in the tropics should increase in a warmer world. But there’s only an indirect link between precipitation intensity and the hazards at high altitudes where aircraft fly. Any changes in the nature of the aviation hazards are still to be determined.</p><img src="https://counter.theconversation.com/content/36002/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Todd Lane receives funding from the Australian Research Council. He has received funding in the past from NASA and the U.S. Federal Aviation Administration. Todd is the current President of the Australian Meteorological Society (AMOS). This article does not represent the views of the Society.</span></em></p>On December 28, AirAsia flight QZ8501 crashed into the sea between Indonesia and Singapore. Shortly before the crash, the pilots requested an alteration to the flight route due to severe weather. While…Todd Lane, Associate Professor, School of Earth Sciences, The University of MelbourneLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/353272014-12-18T19:46:39Z2014-12-18T19:46:39ZAustralia faces a stormier future thanks to climate change<figure><img src="https://images.theconversation.com/files/67450/original/image-20141217-19707-1k6g87x.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">'Plates of the Outback' - A supercell thunderstorm near Urana, NSW drifts over the landscape.</span> <span class="attribution"><span class="source">John Allen</span>, <span class="license">Author provided</span></span></figcaption></figure><p>The <a href="http://www.abc.net.au/news/2014-12-04/brisbanites-still-cleaning-up-after-last-weeks-mega-storm/5944768">supercell</a> that hit Brisbane on November 27 this year caused more than A$500 million worth of damage, produced hail up to 7.5 cm in diameter, and lashed the city with winds of more than 140 km an hour. </p>
<p>In the news, we hear about tornadoes or supercells, and wonder if climate change is beginning to have an impact on these events. </p>
<p>In fact, the evidence suggests that while there has been no increase in severe storm activity in the past, we are likely to see stronger and more frequent storms in the future.</p>
<h2>The science of storms</h2>
<p>Growing up in Sydney’s western suburbs, I remember the summer thunderstorms appearing in the afternoon to the west, and wondering just why we see these castles in the sky. </p>
<p>Thunderstorms form when moisture and warmth near the Earth’s surface is overlapped by cooler air, causing an “updraft” of rising air. The more warm and moist the air, the stronger the <a href="http://en.wikipedia.org/wiki/Atmospheric_instability">thunderstorm’s updraft</a>. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67448/original/image-20141217-19873-18m13c8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67448/original/image-20141217-19873-18m13c8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67448/original/image-20141217-19873-18m13c8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=383&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67448/original/image-20141217-19873-18m13c8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=383&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67448/original/image-20141217-19873-18m13c8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=383&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67448/original/image-20141217-19873-18m13c8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=482&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67448/original/image-20141217-19873-18m13c8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=482&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67448/original/image-20141217-19873-18m13c8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=482&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An example of a supercell thunderstorm updraft near Chingapook, Victoria. The red lines show where warm moist air moves towards the storm, and rises through the cooler dry air roughly outlined by the blue line. Wind shear pushes the precipitation away from the updraft, and allows the storm to rotate clockwise, producing a supercell.</span>
<span class="attribution"><span class="source">John Allen</span></span>
</figcaption>
</figure>
<p>Thunderstorm clouds are like the bubbles you see in a saucepan of water on the stove, where the heated water rises through cooler water above. Most of the time, a thunderstorm fills the sky for an hour, rises, rains and then disappears as if it was never there. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/67275/original/image-20141215-5257-hn02sd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67275/original/image-20141215-5257-hn02sd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/67275/original/image-20141215-5257-hn02sd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=468&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67275/original/image-20141215-5257-hn02sd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=468&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67275/original/image-20141215-5257-hn02sd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=468&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67275/original/image-20141215-5257-hn02sd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=589&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67275/original/image-20141215-5257-hn02sd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=589&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67275/original/image-20141215-5257-hn02sd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=589&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Locations with reported severe thunderstorms for Australia for the period 2003-2010. 1550 events represent observations of tornadoes (red), hail (blue) and wind (green).</span>
<span class="attribution"><span class="source">John Allen</span></span>
</figcaption>
</figure>
<p>But in certain situations, these storms can become “severe”, <a href="http://www.bom.gov.au/vic/sevwx/about.shtml">producing hail in excess of 2 cm, wind gusts above 90 km per hour and sometimes tornadoes</a>. To form, severe thunderstorms typically need some degree of changing wind speed and direction at different levels of the atmosphere — known as wind shear. </p>
<p>If you’ve ever looked up at the sky and seen clouds moving in different directions, that is wind shear. Wind shear organises thunderstorms, moving rain away from the updraft, and allowing the storm to grow outside the normal lifetime of a thunderstorm, becoming stronger. The strongest of these organised storms are known as supercells and produce most hail larger than 5 cm, as well as tornadoes.</p>
<h2>Severe storms widespread in Australia</h2>
<p>Every year, Australia sees many severe thunderstorms, but we only hear about the few that hit populated areas, as someone needs to be present to observe the effects of a storm. </p>
<p>In reality, severe thunderstorms are found over the entire continent, but the intersection between tropical moisture and stronger wind shear means that they are most commonly found over the east coast and interior, stretching from Rockhampton to Melbourne. </p>
<p>To estimate their frequency, we can use a combination of potential updraft strength and wind shear to give an idea of how many days conditions are right for severe thunderstorm development. Using this approach, we can estimate Brisbane gets around 25 favourable days, Sydney 20 and Melbourne 10 days per year.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67348/original/image-20141216-14157-t7fevu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67348/original/image-20141216-14157-t7fevu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67348/original/image-20141216-14157-t7fevu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=309&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67348/original/image-20141216-14157-t7fevu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=309&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67348/original/image-20141216-14157-t7fevu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=309&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67348/original/image-20141216-14157-t7fevu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=388&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67348/original/image-20141216-14157-t7fevu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=388&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67348/original/image-20141216-14157-t7fevu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=388&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 Need of Rescue’ - A squall line moves along the Queensland coast toward Nowra.</span>
<span class="attribution"><span class="source">John Allen</span></span>
</figcaption>
</figure>
<h2>Has 2014 been particularly stormy?</h2>
<p>Overall, the frequency of severe storms in 2014 was about average, or even slightly below. Perhaps we are just forgetting some of those days when the storms weren’t as extreme, or those which missed populated areas. </p>
<p>The Brisbane supercell hailstorm of November 27 has a damage bill rising above <a href="http://www.abc.net.au/news/2014-12-04/brisbanites-still-cleaning-up-after-last-weeks-mega-storm/5944768">A$500 million</a>, but is far from unprecedented in either hail size or damage (<a href="http://hardenup.org/umbraco/customContent/media/600_Brisbane_SevereStorm_1985.pdf">in 1985 a similar event caused A$1.7 billion in equivalent damage</a>). </p>
<p>Similar hail events have often befallen Melbourne (<a href="http://www.theage.com.au/victoria/storm-brings-chaos-to-melbourne-20100306-ppm4.html">2010</a>, <a href="http://www.abc.net.au/news/2011-12-26/christmas-day-hail-storm-hits-melbourne/3747854">2011</a>), Perth (<a href="http://www.abc.net.au/news/2010-03-22/perth-reeling-from-freak-storm/375436">2010</a>), and Sydney (<a href="http://www.bom.gov.au/nsw/sevwx/0708summ.shtml">2007</a>). </p>
<p>In terms of damaging winds, estimated gusts (around 140 km per hour) were not as strong as the 1985 storm (around 185 km per hour), but similar to the storm that affected the Brisbane suburb <a href="http://en.wikipedia.org/wiki/The_Gap,_Queensland">The Gap</a> in 2008. </p>
<p>If we just look at days favourable to severe thunderstorms, there is little indication that there is an increasing frequency of severe thunderstorms outside of natural variability since 1979.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67350/original/image-20141216-14157-1jdcmeq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67350/original/image-20141216-14157-1jdcmeq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67350/original/image-20141216-14157-1jdcmeq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=243&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67350/original/image-20141216-14157-1jdcmeq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=243&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67350/original/image-20141216-14157-1jdcmeq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=243&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67350/original/image-20141216-14157-1jdcmeq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=305&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67350/original/image-20141216-14157-1jdcmeq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=305&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67350/original/image-20141216-14157-1jdcmeq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=305&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 Shelf’ - A shelf cloud associated with a squall line stretches across the Victorian plains near Mitiamo.</span>
<span class="attribution"><span class="source">John Allen</span></span>
</figcaption>
</figure>
<h2>Will severe storms become more common?</h2>
<p>In a warming climate, results for <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00426.1">Australia</a>, the <a href="http://news.nationalgeographic.com/news/2013/09/130924-global-warming-climate-change-thunderstorms-tornadoes/">United States</a> and Europe have shown that the the surface air becomes warmer and moisture increases, making updrafts stronger, while the wind shear available to organise storms appear to decrease. </p>
<p>This battle between the elements seems to end with the strength of updrafts winning, and results in more days with stronger severe thunderstorms. Over the east coast, projected increases by the end of the 21st century for Melbourne, Sydney and Brisbane range between 114% and 160% of present levels. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67349/original/image-20141216-14132-1gyxxe7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67349/original/image-20141216-14132-1gyxxe7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67349/original/image-20141216-14132-1gyxxe7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67349/original/image-20141216-14132-1gyxxe7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67349/original/image-20141216-14132-1gyxxe7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67349/original/image-20141216-14132-1gyxxe7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67349/original/image-20141216-14132-1gyxxe7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67349/original/image-20141216-14132-1gyxxe7.jpg?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"></a>
<figcaption>
<span class="caption">‘Crawler of the Dish’ - Lightning crawls through the anvil from a storm near Parkes, NSW while a semi-trailer drives down the road.</span>
<span class="attribution"><span class="source">John Allen</span></span>
</figcaption>
</figure>
<p>Are we certain though? Several factors remain unexplained in a warming climate. </p>
<p>If the air above the surface warms as well, then it is possible that warming the surface won’t result in as many thunderstorms, but they will be stronger. </p>
<p>If we don’t get as many patterns which pull the conditions favourable to thunderstorms together, then maybe the frequency won’t change or will simply shift the season. </p>
<p>It is important to remember that even as the climate changes, our poor knowledge of past events is insufficient to say with any degree of certainty that a severe thunderstorm is beyond what was possible before. </p>
<p>What this change does mean is an increasing likelihood that we will see severe thunderstorms more often, and the question remains as to whether Australia as a nation is prepared to respond.</p><img src="https://counter.theconversation.com/content/35327/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>John Allen is a member of both the Australian Meteorological and Oceanographic Society and the American Meteorological Society. </span></em></p>The supercell that hit Brisbane on November 27 this year caused more than A$500 million worth of damage, produced hail up to 7.5 cm in diameter, and lashed the city with winds of more than 140 km an hour…John Allen, Postdoctoral Research Scientist, Severe Thunderstorms, Tornadoes, Convection & Climate Predictability, Columbia UniversityLicensed as Creative Commons – attribution, no derivatives.