climate change wa

Shifting temperature patterns in Perth

Perth's seasonal temperature patterns have shifted over the past 150 years with most warming between November and April, while winter/spring months have remained about the same temperature or even cooled.

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.

perth metro maximum temperatures

perth metro minimum temperatures

Maxima have increased mostly from April to August while minima have increased mostly from December to May. There is a period from July to November when either or both minima and maxima are in some months warmer from 1876 to 1899 than from 1994 to 2012.

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.

perth metro and botanic gardens temperatures

Again, the different locations show that most warming occurred during summer, autumn and spring months, with mean temperatures about the same or even warmer from July to September between 1897 and 1899 at Perth Observatory compared to Perth Metro since 1994.

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.

perth metro mean temperatures

Again, most warming occurred during the summer, autumn and spring months, with mean temperatures almost the same from June to September at Perth Observatory as at Perth Metro.

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.

perth airport maximum temperatures

perth airport minimum temperatures

Perth Botanic Gardens maxima from July to December between 1876 and 1899 again matched or were warmer than corresponding months at Perth Airport from 1944 to 2012. The same pattern was even more pronounced among minima.

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.

perth airport mean temperatures

In this instance, almost all warming has occurred from September to March with temperatures almost the same in the rest of the year.

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.

perth gardens and airport temperatures

perth botanic garden temperatures

Again, maxima were the same or even warmer from 1876 to 1899 than from 1981 to 2010, the shift even more pronounced among minima.

To eliminate any pre-1900 thermometer bias, Perth Observatory from 1901 to 1930 can be compared to Perth Airport from 1981 to 2010.

perth airport temperatures

Like all comparisons between different locations and regardless of thermometer bias, this demonstrates that most warming in Perth has occurred during the summer months with little if any warming from about June to September.

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:

perth metro rainfall

perth airport rainfall

Most of Perth's decline in annual rainfall has occurred since the 1940s in the autumn and winter months.


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