ACORN2 is the latest BOM version of station data in Australia. You can plot the average temperature anomaly data for December using their trend tracker. Shown below the graph is the average temperature for the baseline period 1961-1990. The value is 27.2C. Ed Hawkins used the anomaly data and simply added on this average temperature to produce this plot.
- Different timescales for Tmax and Tmin
- Missing values are now simply left blank whereas in ACORN1 they were set to -9999.9
- Sometimes ( eg. Merriden) there are up to 17 days of consecutive data simply missing .
- The start dates for the two files are quite often different which means for example we have only a tmin and a missing tmax .
Having resolved all these problems I then calculated the anomalies for December and the area averaged temperatures for Australia. I got good agreement for the temperature anomalies.
However I got wildly different values for the Average December temperature. I got 23.8C as the area averaged temperature for Australia between for 1961-1990 instead of their 27.2C.
So obviously I must be wrong – or am I? I take the daily average temperature to be (Tmax+Tmin)/2 and then average this value in each of the 12 months between 1961 and 1990 and for each of the 112 stations. My values then agree almost perfectly with various travel/tourist website averages for expected monthly temperatures. For example my monthly average values for Sydney are:
22.965, 23.093, 21.857, 19.548, 16.531, 14.016, 13.068, 14.176, 16.347, 18.671, 20.297, 22.175
Compared with https://www.holiday-weather.com/sydney/averages/
My area averaged value for Australia uses a triangulation method so I thought that maybe I had screwed that up, so to check I recalculated everything using the CRU 5×5 grid method, and I basically got exactly the same result. So I think the difference is instead the following.
Everyone uses monthly average temperatures – GHCN, Berkeley, CRU etc to calculate anomalies whereas I am using the daily values. So my 1961-1990 averages are based on the 30 year average daily temperature for each station during any particular month. These are then spatially integrated to give the normal climatology. So why are they not the same as ACORN or GHCN?
My suspicion is that ACORN, GHCN, Berkeley etc. uses the monthly values over the 30 years period. So the minimum temperature is the lowest Average temperature and NOT the lowest Minimum (night-time) temperature for a given month. This could explain why they get higher values.
Finally here are my trends for December. The increased temperatures in December are mostly because the minimum (night-time) temperatures have been rising much faster. There is not much change in the average maximum temperatures up until 2018.
P.S. I have no time to check all this as I will soon be on a plane to Australia !