by JoNova, November 20, 2019

After we were shocked at the latest ACORN changes to our Very Hot Days data, I asked Chris Gillham if we could see the effect of Bureau of Meteorology changes to the original raw data – and he replied it would be too time-consuming writing the code to calculate 40C+ days among the millions of daily temperatures from 112 weather stations across Australia since 1910. Then he did it anyway.

Wow. In 2011, the BoM’s ACORN 1 adjustments wiped out some of the “very hot days” recorded at weather stations in the early 1900s. These were records that had stood for a whole century. Then, quietly six years later, the ACORN 2 readjustments turned the statistical air conditioner on again and cooled people from 100 years in the future.

It’s all especially miraculous given that even the old World War I  data was recorded in official BoM-approved Stevenson screens. The BoM won’t consider pre 1910 data because it wasn’t standardized, but even when it is, they still have to “fix” it. And in the intervening years after 1910, the Urban Heat Islands have grown and electronic equipment that can record one-second-records have been introduced across the nation. With the old equipment, 40C+ extremes were harder to get than with today’s micro-minute spikes caused by gusts of hot air rolling off carparks and tarmacs.

What we see in the 60 longest running ACORN sites, all open in 1910, is that the raw temperature data had just as many “very hot days” in the World War I era as it does now. Oh boy.

No wonder the BOM was keen to move the “Very Hot Days” graphics and data and tuck them away in a remote page on their website.

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