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What’s cranking up the heat across south-eastern Australia?

Across south-eastern Australia this morning, people are waking up to forecasts of scorching heat for the week ahead. Players and spectators heading to the Australian Open should prepare for some baking…

Monday’s heatwave forecast - with even worse heat predicted for the south-east this week. http://www.bom.gov.au/australia/heatwave/

Across south-eastern Australia this morning, people are waking up to forecasts of scorching heat for the week ahead. Players and spectators heading to the Australian Open should prepare for some baking hot days at the tennis: 35°C today, rising to 41°C on Tuesday, with temperatures in the high 30s or low 40s expected to linger until the weekend.

Coming after a relatively mild summer weekend, many of us will be wondering why it’s got so hot, so quickly.

That was the question my colleagues and I asked ourselves a year ago, when we began looking at the causes of severe heat waves. In particular, we wanted to know what made the 2009 summer heat wave - which set new records for the most days above 40°C in many parts of south-eastern Australia, and which killed hundreds of people - quite so deadly. Were there any hidden culprits behind the record-breaking spell of fierce heat?

What we discovered was that a seemingly unrelated tropical cyclone off the Western Australian coast contributed to making the south-eastern Australian heat wave worse.

And what’s about to happen with this week’s heat is a textbook example of what we found.

Watching wild weather in the west

Tropical Cyclone Dominic over the Western Australian coast, late January 2009. Wikimedia Commons/NASA

This week, a tropical low is forecast to intensify over northern Western Australia, and a trough will extend from north-west to south-east across the state. Whether or not a tropical cyclone develops, the effects of these low pressure systems will be felt as far away as Melbourne and Hobart.

Our recent research in the internationally peer-reviewed journal Geophysical Research Letters explains how tropical lows and tropical cyclones affect heat waves in south-eastern Australia.

In late January 2009, Tropical Cyclone Dominic hit the Western Australian coastline, causing minor structural damage and bringing down power lines in the small Pilbara town of Onslow. Flooding of a nearby river resulted in significant crop damage, and caused a train to derail near Kalgoorlie.

But as cyclones go, Dominic wasn’t so bad: at its peak, the cyclone only reached category 2 status, well below the most severe category 5 level.

Yet as our research showed, even at that level, the cyclone over in Western Australia still had powerful downstream effects for the extreme heat wave across South Australia, southern New South Wales, Victoria and Tasmania in late January and into early February 2009.

During this heat wave, Ambulance Victoria was swamped with a record number of emergency calls, while the Adelaide morgue ran out of room.

Melbourne’s iconic Nylex clock on the city’s hottest day on record: 7 February 2009, when the temperature climbed to 46.4 °C. Wikimedia Commons/Melburnian

Later, the Victorian Department of Health estimated that 374 “excess deaths” occurred in the week of January 26 to February 1 2009. Although it is not possible to directly attribute mortality solely to the heat wave, there was a clear spike above the normal death rate, highlighting the health risks of heat waves, particularly for elderly people.

So how did Tropical Cyclone Dominic increase the intensity of that heat wave? And how do tropical lows in Australia’s west - like the one we’re seeing again this week - affect the weather as far away as south-eastern Australia?

When the pressure’s on

It turns out that the position of the tropical cyclone, rather than its size or severity, is what really makes a difference.

It doesn’t even need to be a full-blown cyclone; as we’re currently seeing, even a tropical low can have a big impact on south-eastern Australia’s weather.

Heat waves in Victoria are associated with slow-moving high pressure systems, or anticyclones. These surface highs hang around over the Tasman Sea for several days, bringing hot northerly winds from the interior of the continent.

During heat waves in Victoria, there is also a similar anticyclone at higher levels in the atmosphere.

These upper level anticyclones form when very long, planetary-scale waves in the atmosphere (known as Rossby waves) break to the south of Australia.

Our recent research showed for the first time in Australia how those upper level anticyclones have been present in all of the most severe heat waves in Victoria over the past two decades.

How cyclones work

The circulation around tropical cyclones at low levels is cyclonic, as air spirals in a clockwise direction (in the Southern Hemisphere; it spirals the other way in the Northern Hemisphere) into the centre of the storm where the pressure is lowest.

At upper levels, the air flows out again from the centre, and its nature changes to anticyclonic, switching to rotate in an anti-clockwise direction.

The UK Met Office explains how cyclones and anticyclones work (note that in the Southern Hemisphere, the air flows in the opposite direction).

This outflowing air can intensify heat waves over Victoria in two ways. The first is when the outflow “nudges” the upper level jet stream, the band of strong westerly winds that circle the globe at mid-latitudes in both hemispheres.

When the outflowing air from the tropical cyclone nudges the jet stream south of western Australia, the disturbance generates more waves. This results in a stronger upper level anticyclone over Victoria.

The second way in which the intensification can occur is a direct result of the anticyclonic properties of the outflowing air. The outflowing air can be carried by the winds directly into the upper level anticyclone over Victoria.

The more intense the upper level anticyclone over Victoria, the more persistent it will be. This makes it more likely that a heat wave will form as higher temperatures continue for several days.

You can imagine this as being a bit like putting a pebble into a stream. The larger the pebble, the harder it will be for the water to shift it, and the more likely it is that the pebble will remain in place for a while as the water flows around it.

The cyclone effectively makes the pebble that is the anticyclone a little bit bigger, so that it stays stationary for longer.

Our improved understanding of how heat waves form should help weather forecasters better predict when extreme heat waves will hit Victoria.

It will also help in studies of how the intensity and duration of heat waves might change in the future due to climate change.

But in the short-term, when the heat is on at Rod Laver Arena this week: take a look way out west - and watch out for those cyclones.

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73 Comments sorted by

  1. Greg North

    Retired Engineer

    Often when there has been any sort of a hot spell down Melbourne way, the break can be exceedingly swift and dramatic, it not being unknown for a southerly/southwest change to bring up to a 20C temperature drop within an hour.
    Is it too early from your research Tess to assume that this is in part because as a cyclone downstream effect starts to weaken and less hot inland air is being pushed to the SE that the pebble effect will also give some acceleration to fronts coming through from the SW?

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  2. Phil Dolan

    Viticulturist

    A very simple explanation. It's always hot when the tennis is on, therefore tennis causes a heatwave.

    Except at Wimbledon where it causes rain.

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    1. In reply to Phil Dolan

      Comment removed by moderator.

    2. Jane Middlemist

      citizen

      In reply to Phil Dolan

      Now that you mention it, it's so obvious. You should alert the BoM.
      Just the breakthrough they need :)

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  3. Craig Myatt

    Industrial Designer / R&D

    That was a model of good science communication...and clearly an important issue to manage risks of mortality arising from climate change. Thanks...

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    1. John TW Hayton

      logged in via email @hayton.info

      In reply to Craig Myatt

      I'm not sure what your standard for good science communication is but this was not, in my opinion, a good example. I use The Economist as my standard for science writing for the general public.

      "The circulation around tropical cyclones at low levels is cyclonic", nudging jet streams - does that mean shifting their location? I'd suggest some english composition help is required. Having read the article I felt like I had to go back a couple of times to try and divine why cyclones in WA have such an effect in Victoria.

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    2. In reply to Craig Myatt

      Comment removed by moderator.

    3. David Semmens

      logged in via Twitter

      In reply to Craig Myatt

      I agree Craig. Very interesting and very well written. It's a complicated thing the atmosphere.

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    4. In reply to Craig Myatt

      Comment removed by moderator.

    5. In reply to James Hill

      Comment removed by moderator.

    6. Greg North

      Retired Engineer

      In reply to John TW Hayton

      " Having read the article I felt like I had to go back a couple of times to try and divine why cyclones in WA have such an effect in Victoria. "
      First John, there was:
      " What we discovered was that a seemingly unrelated tropical cyclone off the Western Australian coast contributed to making the south-eastern Australian heat wave worse.

      And what’s about to happen with this week’s heat is a textbook example of what we found.

      Watching wild weather in the west
      This week, a tropical low is forecast…

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    7. John Nicol

      logged in via email @bigpond.com

      In reply to Greg North

      Greg North

      The winds you suggest in the ESE or SE direction bearing down on Victoria, would be expected to be hot from coming from the inland over the desert regions which are obviously very hot at this time of the year. In "wind speak" they are Northe Westerlies.

      We have the same phenomenon in Queensland from hot dry Westerlies which come in more often in November/December but often persist into January. I guess these are part of the same system - I wasn't sure that one needed a PhD to understand that, but obviously you do.
      John Nicol.

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    8. David Arthur

      resistance gnome

      In reply to John Nicol

      "The winds you suggest in the ESE or SE direction bearing down on Victoria, would be expected to be hot from coming from the inland ..."

      With respect, Mr Nicol, I must be holding my atlas up the other way, because when Victoria's getting winds from the ESE or SE, they're generally coming from over the Tasman Sea.

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    9. John Nicol

      logged in via email @bigpond.com

      In reply to David Arthur

      David Arthur

      With even more respect, Mr Arthur, Greg North was defining/had defined, the wind directions to which he was referring as being TOWARDS the ESE and SE directions. If you had read my statement, I had added that in "wind speak" they were North Westerlies.
      John Nicol

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    10. David Arthur

      resistance gnome

      In reply to John Nicol

      Err, my mistake. It is apparent on a re-reading that the winds to which Mr North refers are moving in a south-easterly direction, ie from the north-west - what I might think to be a "north-westerly".

      It is apparent that my previous remark is founded in my misunderstanding the discussion between yourself and Mr North; to that extent, I'm not certain that my remark warrants any respect beyond "all due", and I shall try holding my atlas the other way up.

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    11. Greg North

      Retired Engineer

      In reply to John Nicol

      I think you can interpret what I stated with;
      " My simple interpretation is that a NW cyclone or low will cause winds heading roughly ESE to SE forming a trough if you like and on the coastal and closer to coastal moisture effect being lost there is then more hot dry air to be forced on ahead. "
      is much the same as what you are saying John, that is a wind heading in a SE direction is of course referred to as a Nwesterly, that being the direction from which the wind is flowing.
      Nope, no PhD needed.

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    12. John Nicol

      logged in via email @bigpond.com

      In reply to David Arthur

      David Arthur

      Thanks David. But let me be perfectly honest, I could easily have screwed up and that would not have been the first time either! In that case your comment would,have been helpful advice.

      Leave your atlas exactly where it is.
      Cheers, John Nicol

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  4. Comment removed by moderator.

    1. In reply to John Crest

      Comment removed by moderator.

    2. In reply to John Crest

      Comment removed by moderator.

    3. In reply to John Crest

      Comment removed by moderator.

    4. Justin Woods

      Ethicist and Public Intellectual

      In reply to Dianna Arthur

      I sent an email to Tony Jones at the BoM requesting details concerning their press release saying 2013 was .......

      I have not received a reply.

      There are no links to the data.

      A search of the website shows no links.

      It is interesting that most weather stations on the BoM site haven't been updated for 2013 - so how did they calculate their figures?

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    5. Greg North

      Retired Engineer

      In reply to Justin Woods

      " Given that oceans are 70% of the planet, and given the Pacific Ocean is the main factor in our weather, "
      Justin, it is true that the Pacific and weather patterns can have an effect on east coast weather just as the Southern Ocean will affect weather of southern Australia including the southern westerly parts.
      And then there is the Indian Ocean, Timor Sea etc. with the monsoonal deluges that are in turn partly fed off the Himalayas so some believe and that may be true though I've not studied…

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  5. Comment removed by moderator.

  6. Cory Zanoni

    Community Manager at The Conversation

    To quote user John Phillip:

    'John, it was a refreshing change to have an article on TC that actually explained the specific causality behind an 'event' rather than push some general explanation of "it's hot = AGW, it's cold = Nothing to do with AGW".'

    I agree with him.

    Let's keep comments on this article about the article. This is about the science and causality behind a specific event. That's all. Keep your posts on topic.

    (Yes, 'climate change' is mentioned but only in reference to how this study could help with our understanding of changes due to climate change in the future. That is not reason enough for conversation here to spiral out of control.)

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    1. john davies
      john davies is a Friend of The Conversation.

      retired engineer

      In reply to Cory Zanoni

      Unfortunately by the time I got to this so many comments have been moderated out that it doesn't make sense. e.g. You refer to comments by John Phillip that are no longer there.

      Having that, I very much agree with your comment about keeping on topic and not allowing conversation to spiral out of control, a major problem in the past. Your objective is laudable!

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  7. Comment removed by moderator.

  8. Justin Woods

    Ethicist and Public Intellectual

    "In particular, we wanted to know what made the 2009 summer heat wave - which set new records for the most days above 40°C in many parts of south-eastern Australia, and which killed hundreds of people - quite so deadly."

    Could you provide the link to the "hundreds of people killed"

    It is a well known fact that cold weather killers more old people, via pneumonia and other respitory problems, than hot weather through heat stroke.

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    1. Liz Minchin

      Queensland Editor at The Conversation

      In reply to Justin Woods

      Thanks Cory. Justin, there is more info in the article, inc this (link shows up in the text above):

      Later, the Victorian Department of Health estimated that 374 “excess deaths” occurred in the week of January 26 to February 1 2009. Although it is not possible to directly attribute mortality solely to the heat wave, there was a clear spike above the normal death rate, highlighting the health risks of heat waves, particularly for elderly people.

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    2. Ian Spencer

      Retired Principal

      In reply to Justin Woods

      Both in 1896 and 1939, the heatwaves claimed over 400 people each. Bourke for instance had +40C temps for 22 and 17 days respectively.
      These heatwaves were longer than 2009 but I don't know if cyclones were involved.
      Coincidentally, those heatwaves were the start of the Federation drought (1896-1902) and the Second World War drought (1939-44/45).

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    3. John Nicol

      logged in via email @bigpond.com

      In reply to Ian Spencer

      Ian Spencer

      I find it interesting that in an article such as this which should cover the broad aspects of any topic, coming from an academic, that these historic facts are not set out in an historic introduction. Unfortunately such omissions are all too common.

      I note the remarks about the new website, "scorcher" not being professionally monitored. However, I find it amazing that Yamba has a defined heatwave for 2014 where the temperature was between a fairly comfortable 30 - 32 C. I'd have thought any heatwave defined for Australia would have not kicked in below about 36 to 37 C (human blood heat) below which are not really subject to an extreme feeling of being "hot".
      John Nicol

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    4. Ian Spencer

      Retired Principal

      In reply to Ian Spencer

      John,
      Did they mean Yamba in SA (which takes its temps from Renmark)?

      http://www.weatherzone.com.au/sa/riverland/yamba

      By the way, the new released report re heatwaves starts at 1950, thus eliminating the data from previous droughts (though there was the 1958-1968 drought).

      Interestingly, Sydney had more +35C between 1921 and 1950 than between 1981 and 2010. No wonder they start at 1950.

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    5. John Nicol

      logged in via email @bigpond.com

      In reply to Ian Spencer

      Ian Spencer

      The one referred to is on the mid-North Coast of NSW. A nice little holiday spot.

      The site is at scorcher.org.au and one is presented with a map with patches and dots repesenting places where there have been recent "heat waves". The names of the towns are not marked on the map, but if you click on a dot, a new page comes up with that town and a plot of recent temperatures with a red thickening on the line representing the heat wave period. Yamba has been hotter on other days apparently but the heat did not last for the arbitrary three days.
      John Nicol

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    6. Ian Spencer

      Retired Principal

      In reply to John Nicol

      John
      Yes, see what you mean re Yamba. A couple of low 30s and a 28 does not constitute a heatwave when the average is 26.5C.
      And their claim as follows:-
      'The longest heatwave since 1910 was from 09-06-2005 to 14-06-2005' is not borne out when you look at the data.
      Yamba w/s 2005 daily temps.
      http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=122&p_display_type=dailyDataFile&p_startYear=2005&p_c=-673072347&p_stn_num=058012

      Thanks for pointing out the site.

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    7. Valerie Kay

      PhD candidate, public health

      In reply to John Nicol

      I think you may find that the heatwave is defined by the average temperature over the course of the day, not the maximum temperature. We can cope reasonably well with high maximums if we have cool nights. It is when the temperature also stays warm at night that we have trouble.

      This confusion between the average temperature and the maximum temperature seems to be common in these discussions. If The Conversation plans to have an article about weather and climate data, this would be one of the points it could clear up.

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    8. Ian Spencer

      Retired Principal

      In reply to Valerie Kay

      Valerie
      Yes, I note that the new definition (just released) is now defining a heatwave in those terms (more the Diurnal Temperature Range rather than the maximum).
      Another definition is that the max temp has to be in the ninth percentile for a total of 3 days (see Scorcher.org.au).
      The previous definition was 3 days of 5C or more above the average monthly mean (and I think that this definition has only been around for a few years).

      I'm not sure what constitutes a 'cold spell' now being experienced by some areas in the NT and WA.

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    9. John Nicol

      logged in via email @bigpond.com

      In reply to Ian Spencer

      Ian Spencer

      I don't think cold spells rate a mention when the exercise is aimed at making everything seem to be hot with `high temperatures which are of course "unprecedented".

      I note that quite a few of these very high temperatures are the "highest since 1931"! or 1896 or at some other previous date a long time ago. The records get broken by 0.1 degrees and it is well to remember that the global temperature and that of Australia provides a platform of about 0.3 C to begin with so it is easier fro an upsurge in temperature to go to a higher value. This fact is also never mentioned when talking about such phenomena as well as saying that 2010 for instance was the hottest year for a very long time.
      John Nicol

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    10. Ian Spencer

      Retired Principal

      In reply to John Nicol

      John
      The BOM states:-

      'The ACORN-SAT temperature dataset has been used for calculation of state and national temperature area averages in summaries from December 2012 onwards.'

      The ACORN system uses 112 stations Australian-wide with about 75 starting in 1910.
      Since its introduction we have had all these temp records broken - hottest day, months, seasons and now year.
      When I ran the temperarure data for NSW in Dec, 2012, 95 w/s were used to record a max mean temp anomaly of about 1.3C above average. However, the official reading was 1.64C above the average mean as ACORN only uses 26 w/s for NSW.
      It appears that using the ACORN data displays a higher reading than when using all the original stations.
      Thus we will see more records broken as the ACORN data has been adjusted (usually downwards) making the pre-1950s cooler.

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  9. Bob Constable

    logged in via Facebook

    Gee I've never seen so many "comment removed by moderator" on an article. This talk of heat waves must have made many people hot under the collar !

    Unfortunately understanding why it is going to be so hot shows that there is nothing we can do about it.

    I was wondering if there was a similar study on why the heatwave across NWS and QLD was occurring

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    1. David Arthur

      resistance gnome

      In reply to Bob Constable

      "Unfortunately understanding why it is going to be so hot shows that there is nothing we can do about it."

      On the other hand, we could adopt practices to minimise the likelihood of such events occurring hereafter.

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  10. Warwick Rowell

    Permaculturist at Rowell Consulting Services

    I found the article of interest, but I found the language a little loose. For instance, the use of waves in two different contexts is offputting. What is meant by waves breaking??

    I think there is room for a wonderful phase shift in education about meteorology, from the basic diagram on paper of surface level changes or cross sections I learned when I got my Private Pilot Licence in 1960.

    This article alludes to some of these basic and not so basic concepts for describing weather, but…

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    1. Tess Parker

      PhD candidate at Monash University

      In reply to Warwick Rowell

      Warwick, if you search for many of these terms on the web you will find a wealth of references, including books and journal articles. The interested reader should find a range of sources from basic texts to more advanced material.
      As to breaking waves, you can understand breaking to mean that the wave overturns, similar to ocean waves - although these waves are fundamentally different to waves in the ocean. It's a complex subject.

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    2. Ivan Smith

      Retired

      In reply to Tess Parker

      Having worked most of my life for ABC Radio as an operator across its many networks, the Weather Bureau's daily broadcasts by the duty announcer was interesting for me.
      The thing I got most out of the various terms was the sentence I devised, and it may have made some sense, was, "There is a deep upper low high over down under"!

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  11. John Nicol

    logged in via email @bigpond.com

    Tess parker

    I do not remember any reports of "hundreds" of people dying from a heatwave in 2009. I wonder if you could give me a reference to the details please. I know there was a heatwave then, as there has been before and since, but I was not aware until now that a significant number odf people died from it then or at any other time.
    Thanks,
    John Nicol

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    1. Michael J. I. Brown

      ARC Future Fellow and Senior Lecturer at Monash University

      In reply to John Nicol

      It is not hard to find relevant references via a google search. For example, the Australian Bureau of Statistics notes:

      "The last part of the heatwave coincided with the ‘Black Saturday’ bushfires in Victoria, which caused the loss of 173 lives and the destruction of over 3,500 structures. It was estimated (Victorian Department of Human Services, 2009) that 374 excess deaths occurred in Victoria during the first week of the heatwave (prior to any influence from the bushfires)."

      http://www.abs.gov.au/AUSSTATS/abs@.nsf/Lookup/1301.0Chapter1042009%E2%80%9310

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    1. Ivan Smith

      Retired

      In reply to Alice Kelly

      About a month ago, my daughter alerted me to this link and I find it fascinating. I have followed two cyclones - the WA one about that time, as we were booked to fly to Perth for a couple of weeks, (arriving in the 41 degree heat!). Didn't get the chance to follow Earth Wind, but wish I could have, due to problems with my Laptop.

      For a few days now, I have followed the Nth Qld cyclone as it approached and crossed the coast. To be able to check the various levels in the atmosphere, and stratosphere…

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  12. Mike Brisco

    Scientist at Flinders University of South Australia

    Kinda makes sense. For the past few years in SA - seemed we get more weather coming down from the north-west, ie right across the continent from Broome.

    Cyclone + anticyclone, both act to send air over the outback, where it gets heated - then and down to the SE. Kinda makes sense to me.

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  13. Warwick Rowell

    Permaculturist at Rowell Consulting Services

    Thanks for your response Tess.

    I liked the BoM site's explanation of the adiabatic cooling rate - without mentioning it!

    I wonder what would happen if we were no longer allowed to use surface pressure and charts of changes thereof to evaluate and discuss weather....

    Would satellite readings and photos and the historical records of their observations allow us to rebuild an adequate language around air mass movements and their temperatures?

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  14. Pat Moore

    gardener

    Thanks Tess, fascinating study subject and congratulations on your research findings. At least we're taking turns about across this sunbaking/burning continent with these frighteningly intense heatwaves. The uber-dry radiant heat from the extended period high temperature drought over western Queensland was dragged into untried towns on the coast last week. Thousands of roosting flying foxes dropped dead near Brisbane and 60% of Stradbroke Island has been blackened by a fortnight of fire. And…

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  15. Warwick Rowell

    Permaculturist at Rowell Consulting Services

    Alexander, thanks so much for that link. Really amazing to be able to look at current wind patterns all around the globe. You can see the cyclone off Tonga, the winds shifting into the Atlantic, the low that has caused problems in UK and western Europe.

    Here it is again for those of you who don't want to back up:

    http://earth.nullschool.net

    Another one:

    http://www.msnbc.com/sites/msnbc/files/map0106.jpg

    Seems to connect the 500mB jet stream diagram and the surface winds in the main article??

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  16. Warwick Rowell

    Permaculturist at Rowell Consulting Services

    David, in the olden days that would have been called a handsome retraction. Well done! :-)

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  17. James Hill

    Industrial Designer

    Well, I will try again by repeating that it would be good for the present federal administration to recognise the "Spike in mortality" caused by high temperature events, (and as highlighted in the article) instead of their dismissing this obvious danger to the mainly elderly in "Global Warming" as "Crap"!!!!
    Thankyou, and I will not ask why such a reasonable comment needed to be moderated.

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    1. Wade Macdonald

      Technician

      In reply to James Hill

      Errr, we do have an aging population as well James. That doesn't help stats in a warming climate.

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    2. James Hill

      Industrial Designer

      In reply to Wade Macdonald

      Well, getting rid of that demographic spike by ignoring any attempts to save those elderly lives will reduce the burden on the long-suffering taxpayer.
      Just sayin'.
      The climate crisis," Danger and Opportunity" in the pictographic Chinese translation, it has served "climate change is crap" Tony Abbott well, affording him the chance to become what he is.
      But spikes in population aside, many of the elderly are going to survive hotter days if efforts are made to keep then out of danger.
      Hey, but if there is no Danger?

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  18. Robert Balic

    Farmer

    What struck me about the claim that global warming was causing this heat wave is what happened in 1998 in Adelaide and Melbourne.

    November in 1997 was barely above the baseline of the UAH data, while early 1998 was 0.65 deg C above due to El Nino. It was back down to the baseline by December.

    From BOM, the Month - highest temp - highest temp recorded.

    Melbourne

    1997 Nov - 40.3 - 40.9
    1997 Dec - 33.7 - 43.7
    1998 Jan - 41.6 - 45.6
    1998 Feb - 42.3 - 46.4
    1998 Dec - 40.9 - 43.7

    Adelaide…

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