From the Bureau’s hastily published “Fast Facts”:
“This means that each one second temperature value is not an instantaneous measurement of the air temperature but an average of the previous 40 to 80 seconds.”
That is complete nonsense.
At the end of each minute, the following data are recorded:
- Lowest one second reading of the previous 60 seconds
- Highest one second reading of the previous 60 seconds
- Reading at the final second of the minute.
Firstly, 40 seconds is not one minute, the integration period recommended by the WMO in 2014 and by the Bureau’s own officers in 1997. Anything less than 60 seconds is not compliant.
Secondly, consider this plot, which is from actual 1 minute temperatures recorded at Hervey Bay Airport on 22 February 2017. (Data purchased by me from the Bureau).
Fig. 1:
Sunrise was at about 5:40 a.m. Temperatures do not increase until about 6:30 a.m. Note the strangely low temperature- the daily minimum- which was reported as occurring sometime in the 60 seconds before 06:00:00. The BOM would have us believe that each of the values in Figure 1, including the low of 23.2C, are “averages” of the previous 40 to 80 seconds.
Next consider what happens in that minute from 5:59 to 6:00, as per the following plot.
Fig. 2:
We don’t know in which seconds the high and low readings for that minute occurred, so I have shown them for each of 59 seconds. I have shown the 5:59 and 6:00 readings: both were 25.3C.
Consider how the value at 06:00 was obtained:
If by an “average” (however derived) of less than 60 seconds, the methodology is non-compliant.
If by an “average” of the previous 60 seconds, it must include values that contributed to the High of 25.4C and the Low of 23.2C.
If by an “average” of anything greater than 60 seconds, it must include values that contributed to both the Low and High values, and as well, values that contributed to the 5:59 reading- which is the same as the 06:00 reading.
Similar logic applies to the Low and High readings.
It follows that the intermediate instantaneous atmospheric temperatures that contributed to all three reported “average” values must have ranged from much higher than 25.4C to very much lower than 23.2C.
Look at Figure 1 again. The air temperature at Hervey Bay on 22 February must have spiked down very much lower than the 23.2C plotted.
Really?
In the early morning there is very little near ground turbulence so temperatures do not fluctuate from one minute to the next by very much. In How Temperature Is Measured in Australia Part 2 I showed that 91% of low temperatures vary from final second temperatures in the same minute by 0.2C or less. A difference of 2.1C is extraordinary. Fluctuations greater than that are difficult to believe.
However, in a comment at How Temperature Is Measured In Australia Part 1, Tony Banton, a retired meteorologist, says that the BOM explanation of cooler ground level air mixing upwards is correct. If we accept that explanation, we must then face the problem of “comparability”.
In 61 seconds, the Hervey Bay AWS has reported temperatures of 25.3, 25.4, 23.2, and finally 25.3 degrees. The BOM asserts that a liquid-in-glass thermometer will be able to respond as quickly and show similar temperatures- and remember, 23.2C was the morning’s official minimum.
My response: rubbish. The data for 22 February at Hervey Bay show that no averaging is used at all, and the Low Temperature of 22.3C 23.2C is an instantaneous one second recording from a rogue downwards spike, whatever the cause, whether a natural event or other (e.g. electrical) factor.
Temperatures reported by the BOM are not fit for purpose of accurate reporting of maxima and minima, identifying records, or identifying warming or cooling by comparison with historic liquid-in-glass data.
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