Heat waves in Mallacoota

In my recent weather report for January 2020 I commented on the number of heat-wave days in the month.  I thought it might be interesting to look back at the longer term picture of this type of weather.

To recap, the BoM defines a heat wave as:

3 or more consecutive days of temperatures significantly above average.  

I calculated the average maximum for each month and the standard deviation of the average and then defined the threshold for a heat wave as average +1SD.  I rounded these values down which makes a slightly lower threshold which compensates for the cases where there are 2 days above the threshold and the third is only 1 degree below.  This gave my temperature thresholds as >25^oC for November - March and I don't regard a temperature below 25^oC as worthy of being part of a heat wave.

The threshold values were:

In terms of perception of unpleasant weather the length of the heat wave is (IMHO) a significant contributor: a heat wave lasting 6 days is more than twice as unpleasant as a 3 day wave.  So rather than counting the number of heat waves count Heat Wave Days  which I define as a day that contributes to a heat wave.   Note that this can span the end of a month: a situation such as 30 Jan =30^oC;  31 Jan =30^oC; 1 Feb = 28^oC would count as a heat wave with 2 days in January and 1 in February.

Looking over the period of BoM records for Mallacoota (since 1994) the total number of HWD by month is shown below, which seems sensible.

 Looking at the total number of HWD for each Summer (eg 1994 represents the period November 1993 - March 1994) gives a jagged line which while appearing to have an upwards trend this is far below statistically significance.

I wouldn't want anyone to get excited about this statement as being an argument against climate change. 

1. It is a very short series;
2. The measure I have used may lack some rigour (eg is +1SD the correct measure to use in calculating the threshold?); and
3. The airport site often shows a much lower minimum than sites in town as the sea-breeze cuts in in the afternoon so may be biased to not record extreme temperatures.

However I think it is an interesting perspective.