December 15, 2011 | Graham

Why it will be cooler this year.

What makes for those blisteringly hot Australian days isn’t so much climate change as geography. Sunlight that falls on trees and grass tends to be absorbed and used by plants and doesn’t heat the air nearly as much as sunlight that falls on desert.

So when we get an extremely hot summer’s day in Brisbane it’s because the westerly or north westerly wind is blowing in from the hot centre. In Victoria it’s the northerly summer winds that bring bushfires and 40 plus degree days just when they are playing the Australian Open tennis.

But we’ve had a lot of rain lately which means that more of the desert is covered in plants, which should make them less hot, and there is soil moisture to evaporate, which should also have a cooling effect. There is also more cloud in the sky which means more solar energy is reflected before it even reaches the earth.

So much for theory, what happens in reality?

The graph below is from the BOM average rainfall and temperature anomaly data. I’ve averaged them over 5 years to cut out some of the noise, and I’ve also multiplied the rainfall anomaly by 100 so that it plots better against the rainfall data.

Graph of rainfall and temperature anomalies for Australia since 1900

The theory appears to hold true with declines in temperature neatly matching increases in rainfall.

Other things that I noticed are that it has been much wetter since the 70s, than it was before, and that rainfall has increased in general at the same time that temperature has.

This graph suggests that rising temperatures may have been good for agriculture if there is a causal relationship between the temperature and the rainfall. It also suggests that we may be over-estimating the ability of our river systems to support irrigation if we judge them on the last 50 or so years of flow.

In any event, my forecast for temperature this year is that it will be significantly below where it was in the preceding years.

Posted by Graham at 7:28 am | Comments (8) |
Filed under: Uncategorized


  1. One of the important things in a business is ‘corporate memory’. Our family have lived on the land in Australia for more than two centuries, thus we have considerable family corporate memory of weather conditions as they determine our profitability. My father is 96 now and can still recall his father’s memories. His father was born in 1876, here in Stanthorpe where I now live. My father in turn has passed his memories on to his children. I have collected and recorded rainfall all my life. In this particular part of Australia we have a most unusual climate as we are situated in a belt of country between summer rain influences which come from the north and winter rain influences which come from the south.
    Our average yearly rainfall is 27inches, but in my lifetime of 71 years I have experienced a drought with only 11 inches for the year when we missed out on both rains and two years when I recorded 50 and 52 inches rain when we received both winter and summer rains. Australia recently experienced a 10 year drought which was the worst since the Federation drought that ended in 1911. It broke last year with huge floods across the country.
    Statistically the years 1900 to 1950 were hotter and dryer than the years 1951 to 2000.
    Presently we are enjoying the start of summer with pleasantly cool weather.

    Comment by Fay Helwig — December 15, 2011 @ 8:24 am

  2. Hi Fay, you mean the years in Stanthorpe between 1900 and 1950 were hotter and dryer, or the years nationally?

    Comment by Graham — December 15, 2011 @ 8:48 am

  3. Graham

    What’s the online link for the BOM data? It seems to show a steady increase in temperatures but it’s impossible to know for sure from this image.

    Is it your theory or someone else’s.

    Comment by Kevin Rennie — December 15, 2011 @ 11:54 am

  4. Hi Kevin,

    I’m not sure whose theory it is. I assume that it is widely known amongst hydrologists because I first became aware of the effects of rain on temperature listening to an interview with Stewart Franks about 12 months ago.

    You can download the same data from

    There is a steady increase in temperature on this graph, although it seems to really kick in around 1980. I haven’t tried running trend lines on it.

    But I just decided to try graphing to see exactly how water and temperature might interact this morning, and was surprised to see such a symmetrical relationship.

    As I say, I would have thought others had done this before. It explains some of, but obviously not all of, the high temperatures earlier this decade.

    Because a running average is a lagging indicator it will be interesting to see what happens in the next few years. All other things being equal the rainfall average should continue to go up, and the temperature average to go down.

    Comment by Graham — December 15, 2011 @ 1:19 pm

  5. Graham,
    The Stanthorpe Border Post newspaper published a chart in 2000 showing the recorded rainfall for the century, which showed less rainfall in the first fifty years than in the second fifty years. It would be interesting to match this local chart with a national chart.

    Comment by Fay Helwig — December 15, 2011 @ 8:29 pm

  6. I’m not sure about cause and effect. Heat over oceans also causes rain. Why not ask some climate scientists.

    Comment by Kevin Rennie — December 15, 2011 @ 9:10 pm

  7. Hi Fay, that’s what the graph shows for rainfall. I was wondering about the other half of your statement – that it was hotter in the first half of the century.

    Kevin, I think most rain is generated by water over oceans, although there are some recent papers suggesting that transpiration by forests also creates rain, so it is obviously still an area where things are not entirely settled. But certainly most of the rain you see on synoptic charts come from offshore.

    Although it’s not actually heat over the oceans but heat in the oceans that causes the evaporation, which mostly happens in the equatorial area with the heat being cycled down into the colder areas around the poles. Heat in the air is an effect of heat in the ocean, not the other way around.

    Comment by Graham — December 15, 2011 @ 10:46 pm

  8. For those interested the BOM has confirmed a fair bit of what is in this post Note in particular:

    “In 2011, the La Nina and heavy rainfall acted like an evaporative cooler for Australia,” said bureau climate change spokesman David Jones.

    “The year 2010 was relatively cool in recent historical context and 2011 was cooler again.”

    Comment by Graham — January 5, 2012 @ 8:46 am

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