Shifting temperature patterns in Perth
Sourced from The Climate of Western Australia 1876-1899 by Government Astronomer W. Ernest Cooke, below are monthly average maximum and minimum temperature comparions between Perth's first official weather recording station and the Perth Metro 9225 station at Mt Lawley.
However, due to different locations, elevations and thermometer shelters, Perth Botanic Gardens records are thought to overestimate temperatures by approximately 04C. The comparison charts above are also corrupted by Perth Metro 9225 being located about 12 kilometres inland from the original site of Perth Botanic Gardens which was near the Swan River.
Stevenson Screen thermometer shades were in use at Perth Observatory from 1897 and The Climate of Western Australia 1876-1899 provides an accurate record of temperatures to 1899.
The same locations can be compared by using Australian Bureau of Meteorology raw maximum and raw minimum temperature data for Perth Observatory between 1901 and 1930. Details about all three recording locations can be viewed at Perth temperature history.
It should be noted that "raw" BoM temperatures have been adjusted by the bureau. For example, it was noted in February 1935 by WA Divisional Meteorologist E.B. Curlewis that "During February 1921, the mean daily maximum temperature reading was 93.7 and this stands out as the hottest February on record". When converted, 93.7 Fahrenheit is 34.3 Celsius. However, the BoM lists February 1921 with a mean maximum of 33.7C.
The above charts compare three different locations and to check whether the shifts in temperature patterns were climatic rather than instrumental or environmental, the Perth Botanic Gardens maximum and minimum temperatures can be compared with Perth Airport.
Due to the possible thermometer bias of data from 1876 to 1899, temperatures should be compared from 1901 to 1930 at Perth Observatory and from 1944 to 2012 at Perth Airport.
To compare temperatures during the earliest and latest periods of the temperature record, minima and maxima at Perth Botanic Gardens from 1876 to 1899 can be compared against Perth Airport from 1981 to 2010.
To eliminate any pre-1900 thermometer bias, Perth Observatory from 1901 to 1930 can be compared to Perth Airport from 1981 to 2010.
With a seasonal shift in temperature patterns, it is worth considering seasonal rainfall patterns at the same Perth weather stations. Below is BoM raw rainfall data summed into quarterly segments per decade at Perth Botanic Gardens, Perth Regional Office, Perth Metro and Perth Airport stations:
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