Jennifer Marohasy
There is more than one way to ruin a perfectly good historical temperature record. The Australian Bureau of Meteorology achieves this in multiple ways, primarily through industrial scale remodelling (also known as homogenisation – stripping away the natural warming and cooling cycles that correspond with periods of drought and flooding), and also by scratching historical hottest day records, then there is the setting of limits on how cold a temperature can now be recorded and also by replacing mercury thermometers with temperature probes that are purpose-built, as far as I can tell, to record hotter for the same weather.
The Australian Bureau of Meteorology (BoM) regularly claims new record hot days, and Australian scientist report that heat records are now 12 times more likely than cold ones. But how reliable – how verifiable – are the new records?
I have been trying for five years to verify the claim that the 23 September 2017 at Mildura was the hottest September day ever recorded in Victoria. According to media reporting at that time, it was the hottest September day all the way-back to 1889 when records first began. Except that back then, back in September 1889, maximum temperatures were recorded at Mildura with a mercury thermometer. Now they are recorded with a temperature probe that is more sensitive to fluctuations in temperature and can thus potentially record warmer for the same weather.
In the absence of any other influences, an instrument with a faster response time [temperature probe] will tend to record higher maximum and lower minimum temperatures than an instrument with a slower response time [mercury thermometer]. This is most clearly manifested as an increase in the mean diurnal range. At most locations, particularly in arid regions, it will also result in a slight increase in mean temperatures, as short-term fluctuations oftemperature are generally larger during the day than overnight.”
Research Report No. 032, by Blair Trewin, BoM, October 2018, page 21.
To standardise recordings from temperature probes with mercury thermometers, one-second readings from probes are normally averaged over one minute – or batches of ten second readings are averaged and then averaged again over one minute. That is the world-wide standard to ensure recordings from temperature probes are comparable with recordings from mercury thermometers. But the Australian Bureau of Meteorology do
not do this, instead they take one-second instantaneous readings and then enter the highest of these one-second spot readings for any given 24-hour period as the official maximum temperature for that day.
Many Australians check the ‘
Latest Weather Observations’ for their local weather station online at the Bureau website, but few realize that the values they see there represent the last one-second recording for any given half hour period.
For example, for the 23rd September 2017, the highest value for that day as shown on the observations page for Mildura was 37.2 °C, recorded at 12:00pm.
Yet 37.7 °C was entered into the data archive as the official maximum temperature for 23rd September 2017 at Mildura.
This represents a discrepancy of 0.5 °C.
This is because the Bureau uses the
highest one-second reading as the the maximum temperature for that day, while the
last (
not the highest or averaged) one-second reading for any 30-minute period is displayed on the ‘Latest Weather Observations’ page.
Maybe a truck exhaust nearby for one second.
Don`t worry BOM has it!
There is absolutely no averaging. None at all in direct contravention of international norms and standards.
This is confusing, most unconventional, and in fact ridiculous.
The World Meteorological Organisation (WMO) provides a clear definitions of daily maximum temperature. This temperature can be read directly from a mercury thermometer, but when using a temperature probe ‘instantaneous’ values must be averaged over one to ten minutes.
