Was Perth's summer of 2010/2011 a record?

The Bureau of Meteorology has stated that Perth's summer of 2010/11 was the equal hottest on record with a mean daily maximum of 32C, matching 2009/10 and 1977/78.

Trouble is, the mean daily maximum at Perth Metro (9225) for December, January and February in 2010/2011 was 31.896666666 ... i.e. 31.9C.

.1C might seem like a small difference, but it's not when you're claiming a record.

How could the BoM get it wrong? It appears they've made the mistake of rounding the mean up for each separate summer month, and then averaging those three rounded numbers. The BoM's calculator would have shown the average at 31.966666C ... i.e. 32C when rounded up a fourth and final time.

To break it down ...

The December daily maximum average was 29.26451612C. The BoM has rounded this up to 29.3C.

The January daily maximum average was 32.49354838C. The BoM has rounded this up to 32.5C.

The February daily maximum average was 34.15C. The BoM has rounded this down to 34.1C in their daily weather observations but rounded it up to 34.2C in their Monthly mean maximum temperatures.

If you average 29.3, 32.5 and 34.2 (or 34.1), you'll get 32 (once you've rounded it up).

But that's still .1C above the actual average of all the daily summer maxima.

The BoM claims the summer of 2010/2011 equalled the summer record of 2009/2010. Hmmm ... the average of all the daily temperatures in the summer of 2009/2010 was 31.99333333C = 32C rounded up. 31.99333333C is .096 higher than the 31.896666666 of 2010/2011. Sticking to the BoM protocol for rounding up, it was .1C hotter in the summer of 2009/2010 than the summer of 2010/2011.

So how about the summer of 2010/2011 equalling the summer record of 1977/78? Unfortunately not. The average of all the daily temperatures that summer was 31.9722222222C = 32C rounded up. 31.9722222222C is .075C (.08C if you round it up) higher than the 31.896666666C of 2010/2011. Sticking to the BoM protocol for rounding up, it was .1C hotter in the summer of 1977/78 than the summer of 2010/2011.

So it turns out that the summer of 2010/11 was the third hottest on record:

2009/10 = 31.993333333C
1977/78 = 31.972222222C
2010/11 = 31.896666666C

View the daily summer temperatures 2010/2011 at Perth Metro (2995) here.

History

In fact, the claim of a record summer might be further argued if you consider the historic pre-1900 temperatures collected from instruments in the Perth Botanic Gardens by Government Astronomer W Ernest Cooke. The BoM doesn't recognise the accuracy of these historic records (PDF 29mb) but since they are not homogenized and are collected from instruments not affected by the Urban Heat Island of a concrete city that was subsequently built around them, they are at least worth considering.

They show the average maximum at the Perth Botanic Gardens in the summer of 1879/80 was 32.73C.

The BoM also claims that March 2011 was the hottest ever with a mean maximum of 31.9C, which was .1C higher than the previous hottest March in 1978. Because Perth Metro (2995) didn't begin recording until 1994, the BoM is referencing its 1978 data from the Perth Regional Office site (9034).

If the BoM had instead referenced the Perth Gardens site (9097), it would have found the March mean maximum in 1881 was 32.2C.

The BoM also claims the first three months of 2011 in Perth were the hottest ever with a mean maximum of 32.9C, which was .6C hotter than the previous first quarter, again in 1978. At the Perth Gardens site, the first three months of 1880 also had a mean maximum of 32.9C.

Perth Gardens and Perth Regional Office are both four kilometres from Perth Metro.

Although only two days managed to reach 40C (just) at Perth Metro over the entire 2010/11 summer, it was undoubtedly hot and persistently so - thus the (near) record temperatures.

Outside Perth, it should be noted that large swathes of WA recorded their highest-ever summer rainfall, the mean maximum outside the South West Land Division and the western Gascoyne was below the long-term average, and the mean minimum across WA was the sixth hottest on record. (BoM seasonal WA summary)

As always, it's worth looking back through the history books of Perth weather to put the 2010/11 summer in perspective:

Perth Metro
Dec 2010 mean temp - 22.95
Jan 2011 mean temp - 26
Feb 2011 mean temp - 27.55

Summer mean 25.5 C

The 2010/11 summer figures below are from Perth Metro and the pre-1900 figures are from The Climate of Western Australia: 1876-1899 (PDF 29mb).

Mean max in Dec 2010 - 29.3
Mean max in Dec 1882 - 31.11

Mean max in Jan 2011 - 32.5
Mean max in Jan 1880 - 36.56

Mean max in Feb 2011 - 34.2
Mean max in Feb 1876 - 34.28

A broader comparison can be made from mean temperatures across all available years, as listed below. Note that Perth Metro is 4km from Perth Botanic Gardens and 4km from Perth Regional Office (which is 2km from Perth Botanic Gardens). Perth Regional Office stopped recording in 1992 and Perth Metro began recording in 1994. Perth Metro records the "official" Perth temperatures.

Dec av mean max from 1876 to 1899 - 28.6 at Perth Botanic Gardens (Cooke)
Dec av mean max from 1876 to 1930 - 28.2 at Perth Botanic Gardens (9097)
Dec av mean max from 1897 to 1992 - 27.4 at Perth Regional Office (9034)
Dec av mean max from 1994 to 2010 - 28.8 at Perth Metro (9225)

Jan av mean max from 1876 to 1899 - 31.1 at Perth Botanic Gardens (Cooke)
Jan av mean max from 1876 to 1930 - 30.4 at Perth Botanic Gardens (9097)
Jan av mean max from 1897 to 1992 - 29.7 at Perth Regional Office (9034)
Jan av mean max from 1994 to 2011 - 31 at Perth Metro (9225)

Feb av mean max from 1876 to 1899 - 31.4 at Perth Botanic Gardens (Cooke)
Feb av mean max from 1876 to 1930 - 30.6 at Perth Botanic Gardens (9097)
Feb av mean max from 1897 to 1992 - 30 at Perth Regional Office (9034)
Feb av mean max from 1994 to 2011 - 31.5 at Perth Metro (9225)

Dec av mean min from 1876 to 1899 - 15.8 at Perth Botanic Gardens (Cooke)
Dec av mean min from 1876 to 1930 - 15.9 at Perth Botanic Gardens (9097)
Dec av mean min from 1897 to 1992 - 16.3 at Perth Regional Office (9034)
Dec av mean min from 1994 to 2010 - 16.2 at Perth Metro (9225)

Jan av mean min from 1876 to 1899 - 17 at Perth Botanic Gardens (Cooke)
Jan av mean min from 1876 to 1930 - 17.2 at Perth Botanic Gardens (9097)
Jan av mean min from 1897 to 1992 - 17.9 at Perth Regional Office (9034)
Jan av mean min from 1994 to 2011 - 18 at Perth Metro (9225)

Feb av mean min from 1876 to 1899 - 17.2 at Perth Botanic Gardens (Cooke)
Feb av mean min from 1876 to 1930 - 17.2 at Perth Botanic Gardens (9097)
Feb av mean min from 1897 to 1992 - 18.1 at Perth Regional Office (9034)
Feb av mean min from 1994 to 2011 - 18.2 at Perth Metro (9225)

Dec av mean from 1876 to 1899 - 22.2 at Perth Botanic Gardens (Cooke)
Dec av mean from 1876 to 1930 - 22.1 at Perth Botanic Gardens (9097)
Dec av mean from 1897 to 1992 - 21.9 at Perth Regional Office (9034)
Dec av mean from 1994 to 2010 - 22.5 at Perth Metro (9225)

Jan av mean from 1876 to 1899 - 24.1 at Perth Botanic Gardens (Cooke)
Jan av mean from 1876 to 1930 - 23.8 at Perth Botanic Gardens (9097)
Jan av mean from 1897 to 1992 - 23.8 at Perth Regional Office (9034)
Jan av mean from 1994 to 2011 - 24.5 at Perth Metro (9225)

Feb av mean from 1876 to 1899 - 24.3 at Perth Botanic Gardens (Cooke)
Feb av mean from 1876 to 1930 - 23.9 at Perth Botanic Gardens (9097)
Feb av mean from 1897 to 1992 - 24.1 at Perth Regional Office (9034)
Feb av mean from 1994 to 2011 - 24.9 at Perth Metro (9225)

Summer av mean 1876-1899 - 23.53 at Perth Botanic Gardens (Cooke)
Summer av mean 1875-1930 - 23.27 at Perth Botanic Gardens (9097)
Summer av mean 1897-1992 - 23.27 at Perth Regional Office (9034)
Summer av mean 1994-2011 - 23.96 at Perth Metro (9225)

Annual av mean max from 1876 to 1899 - 24.2 at Botanic Gardens (Cooke)
Annual av mean max from 1876 to 1930 - 23.8 at Botanic Gardens (9097)
Annual av mean max from 1897 to 1992 - 23.3 at Regional Office (9034)
Annual av mean max from 1994 to 2010 - 24.5 at Perth Metro (9225)

Annual av mean min from 1876 to 1899 - 12.3 at Botanic Gardens (Cooke)
Annual av mean min from 1876 to 1930 - 12.7 at Botanic Gardens (9097)
Annual av mean min from 1897 to 1992 - 13.3 at Regional Office (9034)
Annual av mean min from 1994 to 2010 - 12.6 at Perth Metro (9225)

Annual av mean from 1876 to 1899 - 18.3 at Botanic Gardens (Cooke)
Annual av mean from 1876 to 1930 - 18.3 at Botanic Gardens (9097)
Annual av mean from 1897 to 1992 - 18.3 at Regional Office (9034)
Annual av mean from 1994 to 2010 - 18.6 at Perth Metro (9225)

The reliability of temperature comparisons between Perth Metro (9225) and Perth Regional Office (9034), which are four kilometres apart, is questionable. The historic records from 1897 to 1992 at Perth Regional Office suggest that Perth's mean annual maximum was 1.2C cooler than the average mean recorded at Perth Metro since 1994.

However, the same comparison shows that the mean minimum was .7C less. It's difficult to believe that different local topography and weather conditions are not responsible for a significant increase in maxima simultaneous with a significant decrease in minima, no matter the homogenising by BoM to present a valid historic record of Perth temperatures.

The data suggests that if Perth's current temperatures are sourced from Perth Metro, for the sake of accuracy the city's meteorological records effectively only began in 1994. That being the case and as illustrated in the chart below, Perth temperatures have barely shifted over the past 16 years - if anything, the mean temperature trendline has dropped slightly because there was a greater reduction in minima than increase in maxima.

Spells of hot weather

The Dec 2010 mean max at Perth Metro was 29.3, compared to:

Perth Regional Office (9034)
29.3 in Dec 1906
30.4 in Dec 1909
30.3 in Dec 1915
30.3 in Dec 1927
31.7 in Dec 1931
30.3 in Dec 1958
30.3 in Dec 1972
30.7 in Dec 1977
29.3 in Dec 1979

Perth Metro (9225)
29.8 in Dec 1994
30.2 in Dec 1997
31.5 in Dec 1999
29.7 in Dec 2000
30.4 in Dec 2002
30 in Dec 2004
30.4 in Dec 2006
30.8 in Dec 2009

Perth Gardens (9097)
30.6 in Dec 1878
30.2 in Dec 1880
29.3 in Dec 1881
31.1 in Dec 1882
29.9 in Dec 1885
29.7 in Dec 1894
30.2 in Dec 1899
30.4 in Dec 1905
30.6 in Dec 1906
31.7 in Dec 1909
31.2 in Dec 1915
29.8 in Dec 1921
30.2 in Dec 1927

The Dec 2010 mean min at Perth Metro was 16.6, compared to:

Perth Regional Office (9034)
17.2 in Dec 1905
17.4 in Dec 1906
17.8 in Dec 1909
17.6 in Dec 1915
17.5 in Dec 1921
16.9 in Dec 1925
17.8 in Dec 1927
19.2 in Dec 1931
17.2 in Dec 1933
17.7 in Dec 1936
17.2 in Dec 1942
17.6 in Dec 1958
17.0 in Dec 1960
17.4 in Dec 1962
17.2 in Dec 1963
18.3 in Dec 1965
17.5 in Dec 1967
16.9 in Dec 1969
17.2 in Dec 1970
19.5 in Dec 1972
16.9 in Dec 1974
17.8 in Dec 1975
17.1 in Dec 1976
19.6 in Dec 1977
18.4 in Dec 1978
18.5 in Dec 1979
17.8 in Dec 1982
17.9 in Dec 1983
16.9 in Dec 1986
17.7 in Dec 1987
17.1 in Dec 1988
16.7 in Dec 1990
17.7 in Dec 1991

Perth Metro (9225)
17 in Dec 1994
16.9 in Dec 1997
16.7 in Dec 1998
17.8 in Dec 1999
16.7 in Dec 2000
17.4 in Dec 2002

Perth Gardens (Cooke and 9097)
16.7 in Dec 1878
16.7 in Dec 1882
16.9 in Dec 1885
16.8 in Dec 1888
16.6 in Dec 1893
16.8 in Dec 1894
16.7 in Dec 1904
17.8 in Dec 1905
17.8 in Dec 1906
18.0 in Dec 1909
16.7 in Dec 1912
16.9 in Dec 1914
17.6 in Dec 1915
17.7 in Dec 1921
16.9 in Dec 1925
17.6 in Dec 1927

The Jan 2011 mean max at Perth Metro was 32.5, compared to:

Perth Regional Office (9034)
32.5 in Jan 1956
32.5 in Jan 1961
34.4 in Jan 1962
33.2 in Jan 1974
33.3 in Jan 1978
32.5 in Jan 1981
32.8 in Jan 1992

Perth Metro (9225)
32.8 in Jan 2008
32.5 in Jan 2009
33.4 in Jan 2010

Perth Gardens ( Cooke and 9097)
34.7 in Jan 1878
36.6 in Jan 1880
33.1 in Jan 1896
32.7 in Jan 1918

The Jan 2011 mean min at Perth Metro was 19.5, compared to:

Perth Regional Office (9034)
19.6 in Jan 1920
19.7 in Jan 1934
20.0 in Jan 1961
20.7 in Jan 1962
19.5 in Jan 1965
19.6 in Jan 1973
19.7 in Jan 1974
20.9 in Jan 1978
20.4 in Jan 1980
20.0 in Jan 1983
20.1 in Jan 1986
19.7 in Jan 1989
20.6 in Jan 1992

Perth Metro (9225) Record high

Perth Gardens (9097)
19.6 in Jan 1915
19.6 in Jan 1920

The Feb 2011 mean max at Perth Metro was 34.1 (or 34.2, dependent upon how you round it), compared to:

Perth Regional Office (9034)
34 in Feb 1975
33.7 in Feb 1921

Perth Gardens (Cooke and 9097)
34.3 in Feb 1876
34.2 in Feb 1882
34.3 in Feb 1921

Perth Metro (9225)
34.6 in Feb 1996 at Perth Metro
34.2 in Feb 2011 at Perth Metro

The Feb 2011 mean min at Pert Metro was 21, compared to:

Perth Regional Office (9034)
21.1 in Feb 1921
21.6 in Feb 1975
22.7 in Feb 1985.

Perth Metro (9225)
Record high

Perth Gardens (9097)
21.2 in Feb 1921

The summer of 2010/2011 was hot, mainly because of the record number of consecutive February days above 30C. However, it wasn't the hottest ever Perth summer, despite a strong La Nina in eastern Australia pushing hot easterly trade winds further west than usual, weaker than average summer sea breezes and above average temperatures in the Indian Ocean (as claimed by climatologists but at odds with SEAFRAME stations off the WA coast).

Although there were an extraordinary number of days above 30C, there were also an extraordinary number of days below 40C. The claims of a record hot summer in Perth are questionable, particularly as it was a hot summer that never got very hot.

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