Climate trends for thermal soaring

For pilots who soar at Lake Keepit or Mount Borah: relevant summer climate data for Manilla, NSW, since 1999.

Graph of some summer climate variables 1999 to 2015.

Variables relevant to thermal soaring

From my data I have selected three variables that are relevant to success in soaring flight using thermals. I have chosen to use values for summer: a total or average for the three months of December, January and February.
The variables are:

  • The number of hot days, when the maximum temperature was over 33°C;
  • The number of sunny days, when the cloud amount seen at 9 am was less than two octas;
  • The average daily temperature range in degrees celsius.

Changing values of the variables

The graph shows that each variable fluctuated wildly, with each summer very different from the last. These variables often moved in the same sense.
Two summers had high values of all three variables: 2006-07 and 2013-14. Two summers had low values of all three variables: 2007-08 and 2011-12. I would expect that longer and faster thermal soaring flights would have been achieved in the summers with high values, compared to those with low values.


I have fitted linear trend lines, and displayed their equations within the graph.
All three trend lines slope down. This suggests that summer thermal soaring conditions have been getting worse.
I have cited the values of “R-squared”, the Coefficient of Determination. All three R-squared values are abysmally low. Even the best is below 20%, which can be taken to mean that more than 80% of the variation has nothing to do with the trend line shown.
You could say that the trends are nonsense, but we are dealing with Climate Change here!

The future

In the spirit of Mark Twain, we can extend the trend lines forward to where they come to zero:

  • There will be no hot days above 33° by the summer of 2118;
  • There will be no sunny mornings with less than 2 octas of cloud by 2073;
  • Days will be no warmer than nights by 2423.

That last date seems too remote to worry about. However, the daily temperature range will be unacceptable when it gets down to 11°. That is the current summer value for Lasham, England, after all. According to the trend, the daily temperature range will be worse than at Lasham by 2117. That is the same year that the very last 33° day is expected.

Global Warming

You may be surprised that the linear trend lines fitted to this data set slope downwards. It seems to contradict Global Warming.
The number of hot days can decrease despite a trend to increasing maximum daily temperature. In any case, the rate of increase in summer daily maximum temperature in NSW (1910-2014) is very low: a Bureau of Meteorology graph shows it is only 0.004°/year. That is less than half the rate of increase of (mean annual) temperature in Australia as a whole: 0.010°/year. The reason I say that the number of hot days may not reflect the increase in daily maximum temperature relates to the other two variables: temperature range and cloud.
The trend for decrease in daily temperature range on my graph accords with Global Warming data and theory. In Australia as a whole the mean annual daily temperature range in the period 1910 to 2014 decreased at 0.002°/year, while in NSW summers it decreased very much faster: 0.011°/year. I get, for 1999 to 2016 at Manilla, a decrease of 0.036°/year. There is no contradiction there.
Cloud cover varies inversely with daily temperature range. When I observe decreasing temperature range, I expect to see more cloudy days and fewer sunny days, which I do.

The broad idea of Global Warming relates to what is known of the Ice Ages of the last million years. In the Last Glacial Period 21,000 years ago, the climate was about 4° colder than now, and very arid. Now, in an Interglacial Period, the climate is warmer and more humid. Global warming is expected to make the climate even warmer and more humid. Higher humidity will bring more cloud, just as appears on my graph.
More cloud in summer should prevent very hot days, just as my graph suggests, despite the (small) rate of increase in daily maximum temperature.

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