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Australian very hot days and hot season rainfall

Northern Australia Southern Australia Western Australia Eastern Australia South-west and south-east Australia


The average number of very hot days (40C+) each year since 1910 in Australia can be compared with annual rainfall averages, but days of 40C or more only happen from November to April and winter rainfall can distort the correlation between the two.

The charts on this page show the correlation between national, northern Australia, southern Australia, eastern Australia, south-west Australia and south-east Australia very hot days and average November to April rainfall since 1910 in those rainfall districts (as defined by the Bureau of Meteorology).

Both the number of very hot days and November to April rainfall are based on averages during each year, rather than totals among whichever number of weather stations are analysed in different regions.

Among all 112 weather stations in the Australian Climate Observation Reference Network (ACORN), 60 were open and recording maximum temperatures in 1910 - Adelaide, Albany, Alice Springs, Bathurst, Boulia, Bourke, Bridgetown, Broome, Bundaberg, Burketown, Cairns, Cape Leeuwin, Cape Moreton, Cape Otway, Carnarvon, Charleville, Charters Towers, Cobar, Darwin, Deniliquin, Eddystone Point, Esperance, Gabo Island, Gayndah, Georgetown, Geraldton, Halls Creek, Inverell, Kalgoorlie, Katanning, Kerang, Launceston, Longreach, Low Head, Mackay, Marble Bar, Marree, Melbourne, Mildura, Miles, Moree, Moruya Heads, Mount Gambier, Normanton, Palmerville, Perth, Port Lincoln, Port Macquarie, Richmond (Qld), Robe, Sale, Snowtown, Sydney, Tennant Creek, Tibooburra, Wagga Wagga, Walgett, Wandering, Wilsons Promontory and Yamba.

Below is the national chart showing correlations between very hot days (sourced to original RAW, not adjusted ACORN) at the 60 ACORN weather stations that were open in 1910 (eliminating the 52 that opened between 1910 and 1976 in locations that on average were warmer and bias the very hot day trend - see analysis), compared to average November to April rainfall at all bureau weather stations in Australia.

very hot days in 60 ACORN weather stations

Very hot day (40C+) annual averages:

1910-1963 13.81
1964-2017 12.60

Rainfall November to April averages:

1910-1963 51.30mm
1964-2017 59.37mm

Below are the total number of very hot days (40C+) at the 60 long-term ACORN weather stations, comparing 1910-1963 with 1964-2017 (first and second half both 54 years).

very hot day totals in 60 ACORN weather stations

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Northern Australia

Below is the northern Australia rainfall region chart showing correlations between very hot days (sourced to original RAW, not adjusted ACORN) at the 19 northern Australia ACORN weather stations that were open in 1910 (Alice Springs, Boulia, Broome, Bundaberg, Burketown, Cairns, Carnarvon, Charters Towers, Darwin, Gayndah, Georgetown, Halls Creek, Longreach, Mackay, Marble Bar, Normanton, Palmerville, Richmond (Qld) and Tennant Creek), compared to average November to April rainfall at all bureau weather stations in northern Australia.

very hot days in 19 northern ACORN weather stations

Very hot day (40C+) annual averages:

1910-1963 24.09
1964-2017 21.39

Rainfall November to April averages:

1910-1963 68.96mm
1964-2017 81.13mm

Below are the total number of very hot days (40C+) at the 19 long-term ACORN weather stations in northern Australia, comparing 1910-1963 with 1964-2017 (first and second half both 54 years).

very hot day totals in 19 northern Australia weather stations

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Southern Australia

Below is the southern Australia rainfall region chart showing correlations between very hot days (sourced to original RAW, not adjusted ACORN) at the 41 southern Australia ACORN weather stations that were open in 1910 (Adelaide, Albany, Bathurst, Bourke, Bridgetown, Cape Leeuwin, Cape Moreton, Cape Otway, Charleville, Cobar, Deniliquin, Eddystone Point, Esperance, Gabo Island, Geraldton, Inverell, Kalgoorlie, Katanning, Kerang, Launceston, Low Head, Marree, Melbourne, Mildura, Miles, Moree, Moruya Heads, Mount Gambier, Perth, Port Lincoln, Port Macquarie, Robe, Sale, Snowtown, Sydney, Tibooburra, Wagga Wagga, Walgett, Wandering, Wilsons Promontory and Yamba), compared to average November to April rainfall at all bureau weather stations in southern Australia.

very hot days in 41 southern ACORN weather stations

Very hot day (40C+) annual averages:

1910-1963 7.52
1964-2017 7.43

Rainfall November to April averages:

1910-1963 31.04mm
1964-2017 34.43mm

Below are the total number of very hot days (40C+) at the 41 long-term ACORN weather stations in southern Australia, comparing 1910-1963 with 1964-2017 (first and second half both 54 years).

very hot day totals in 41 southern Australia weather stations

See south-eastern Australia below

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Western Australia

Below is a chart of Western Australia (the entire state and not designated as a rainfall region by the bureau) showing correlations between very hot days (sourced to original RAW, not adjusted ACORN) at the 13 WA ACORN weather stations that were open in 1910 (Albany, Bridgetown, Broome, Cape Leeuwin, Carnarvon, Esperance, Geraldton, Halls Creek, Kalgoorlie, Katanning, Marble Bar, Perth and Wandering), compared to average November to April rainfall at all bureau weather stations in Western Australia.

very hot days in 13 WA ACORN weather stations

Very hot day (40C+) annual averages:

1910-1963 18.65
1964-2017 16.11

Rainfall November to April averages:

1910-1963 35.15mm
1964-2017 44.51mm

The chart below shows how the RAW very hot days above have been adjusted by ACORN 1 and ACORN 2.

hot days in 13 WA ACORN weather stations

RAW very hot day (40C+) annual averages:

1910-1963 18.65
1964-2017 16.11

ACORN 1 very hot day (40C+) annual averages:

1910-1963 17.17
1964-2017 16.43

ACORN 2 very hot day (40C+) annual averages:

1910-1963 14.38
1964-2017 14.56

Rainfall November to April averages:

1910-1963 35.15mm
1964-2017 44.51mm

See south-western Australia below

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Eastern Australia

What about eastern Australia where most of the population live with all the associated infrastructure?

The BoM provides annual data for specified rainfall regions, including eastern Australia which encompasses all of Queensland, NSW, Victoria and Tasmania.

Within those four states there are 36 ACORN stations that were open and taking observations in 1910 (Bathurst, Boulia, Bourke, Bundaberg, Burketown, Cairns, Cape Moreton, Cape Otway, Charleville, Charters Towers, Cobar, Deniliquin, Eddystone Point, Gabo Island, Gayndah, Georgetown, Inverell, Kerang, Launceston, Longreach, Low Head, Mackay, Melbourne, Mildura, Miles, Moruya Heads, Normanton, Palmerville, Port Macquarie, Richmond Qld, Sydney, Tibooburra, Wagga Wagga, Walgett, Wilsons Promontory, Yamba).

The animated chart below shows the annual average number of of very hot days at the 36 stations in the RAW, ACORN 1 and ACORN 2 daily temperature datasets …

very hot days in 36 eastern ACORN weather stations

The original RAW daily observations at the 36 long-term stations, all recorded in Stevenson screens, show very hot days occurred more frequently before the 1970s when climate change is said to have started to influence temperatures.

ACORN 1 significantly reduced these annual averages in the early years, and ACORN 2 has created an even greater increase in the number of very hot days across eastern Australia.

The 36 long-term stations compared above in RAW, ACORN 1 and ACORN 2 are all within the BoM’s climatologically distinct “eastern Australia” rainfall region.

The average rainfall data in November, December, January, February, March and April each year (the months when 40C+ days occur) can be compared with how many very hots days (defined by the bureau as 40C+) occurred annually at the 36 ACORN stations that were open and taking observations in 1910 within the eastern Australia rainfall region.

The chart below demonstrates that annual rainfall, which invariably requires cloud cover on the day and/or surrounding days, has a clear correlation with the yearly number of very hot day.

rainfall and very hot days in 36 eastern ACORN weather stations

RAW very hot day (40C+) annual averages:

1910-1963 12.02
1964-2017 10.65

ACORN 1 very hot day (40C+) annual averages:

1910-1963 10.68
1964-2017 10.84

ACORN 2 very hot day (40C+) annual averages:

1910-1963 9.66
1964-2017 10.08

Rainfall November to April averages:

1910-1963 69.17mm
1964-2017 72.26mm

The frequency of original RAW very hot days correlates with cloudy rainfall days and their trends since 1910, not with C02 or climate warming. ACORN adjustments reduce the natural correlation between cloudy rainfall days and the number of very hot days that occur in wet or dry years.

Most of the increase in rainfall and reduction in very hot days has been experienced in northern Australia. In south-east Australia, rainfall increased only slightly and average annual very hot days increased from 3.42 in 1910-1963 to 3.90 in 1964-2017.

ACORN 2 cooling adjustments to original very hot day observations have been negligible since the late 1990s, coinciding with the national installation of automatic weather stations that log one second electronic recordings of brief hot air instead of the delayed response time of the liquid thermometers they replaced.

All observations since 1910 have been in Stevenson screens, but most automatic weather stations are in smaller Stevenson screens that have greater heat conduction than their larger screen predecessors.

The adjusted reduction in historic very hot days implies early observations were incorrect every day, and are indicative of the adjustments to annual averages that support claims of a 1C mean temperature increase since the early 1900s.

Click here for a more detailed analysis of eastern Australia.

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South-west and south-east Australia

It should be noted that in most of Australia except the north where increased rainfall is overwhelming, there is a distinct increase in very hot days starting in the late 1990s.

This coincides with the introduction of automatic weather stations which have one second electronic readings that are more sensitive to brief bursts of hot air than the preceding liquid thermometers.

Below are annual very hot day frequency (sourced to original RAW, not adjusted ACORN) and the rainfall averages from November to April only at the nine ACORN weather stations in the south of Western Australia (Albany, Bridgetown, Cape Leeuwin, Esperance, Geraldton, Kalgoorlie, Katanning, Perth and Wandering), and excluding the other six months when very hot days don't occur.

very hot days in nine south-west ACORN weather stations

Very hot day (40C+) annual averages:

1910-1963 3.98
1964-2017 4.79

Rainfall November to April averages:

1910-1963 24.63mm
1964-2017 24.48mm

Below are very hot day (sourced to original RAW, not adjusted ACORN) and rainfall correlation trends in the south of WA from 1910 to 1999 :

very hot days in nine south-west ACORN weather stations

Very hot day (40C+) annual averages:

1910-1954 3.89
1955-1999 4.65

Rainfall November to April averages:

1910-1954 24.06mm
1955-1999 24.61mm

The south-west of Australia shows little if any correlation between rainfall and very hot days. This is partly influenced by the nine stations including Kalgoorlie, where rainfall has increased since 1910 and very hot days have declined, but is potentially because the region receives so little rainfall during November to April (average 24.5mm since 1910 compared to Australia's 55.3mm, southern Australia's 32.7mm and northern Australia's 75.0mm) that the frequency of cloudy rainfall days has little influence on the frequency of very hot days.

It is also possible though unprovable that because three of the nine stations (Albany, Geraldton and Perth) are major population centres, urban heat from surrounding infrastructure has caused some days to reach a maximum of 40C.

Below are annual very hot day frequency (sourced to original RAW, not adjusted ACORN) and the rainfall averages from November to April at the 20 ACORN weather stations open since 1910 in the south-east rainfall area of Australia (Adelaide, Bathurst, Cape Otway, Denilliquin, Eddystone Point, Gabo Island, Kerang, Launceston, Low Head, Melbourne, Mildura, Moruya Heads, Mt Gambier, Port Lincoln, Robe, Sale, Snowtown, Sydney, Wagga Wagga, Wilson Promontory), and excluding the other six months when very hot days don't occur.

very hot days in 20 south-east ACORN weather stations

Very hot day (40C+) annual averages:

1910-1963 3.42
1964-2017 3.90

Rainfall November to April averages:

1910-1963 44.68mm
1964-2017 45.34mm

Below are very hot days (sourced to original RAW, not adjusted ACORN) and rainfall correlation trends in Australia's south-east rainfall region from 1910 to 1999 :

very hot days in nine south-west ACORN weather stations

Very hot day (40C+) annual averages:

1910-1954 3.64
1955-1999 2.99

Rainfall November to April averages:

1910-1954 44.25mm
1955-1999 45.63mm

South-eastern Australia has the highest density of long-term ACORN stations open since 1910, and the charts above show that very hot days correlated with November to April rainfall from 1910 to 1999, but no longer correlated with November to April rainfall after 1999 when automatic weather stations with more rapid thermal reactions (one second readings) were broadly introduced.

For example and as per the chart above, south-eastern Australia had a then-record average 67.44mm of rain in November-April 1974 and no very hot days, but 69.32mm in 2010 with an average 4.5 very hot days and 74.16mm in 2011 with an average 1.82 very hot days. If this is attributed to climate warming, it implies that record rainfall has little or no bearing on how many days have a maximum of at least 40C.

It is also possible though unprovable that because six of the 20 stations (Sydney, Richmond NSW, Hobart, Melbourne, Laverton and Adelaide) are designated by the BoM as urban heat islands with anomalous temperature trends, urban heat from surrounding infrastructure has caused some days to reach a maximum of 40C.

The average number of very hot days in south-eastern Australia in 2011 was 1.82 in RAW and 2.00 in ACORN 2, again suggesting that daily temperature adjustments and/or the rapid heat sensitivity of small-screen automatic weather station are influencing trends to a greater extent than either cloudy rainfall days or CO2.

Click here for an Excel download containing the annual very hot day averages and average rainfall data used in the analysis above.

Note: The charts on this page are not produced by the Bureau of Meteorology but are based on the daily temperature and rainfall data provided by the bureau in its different datasets. Animation charts are free for use on any platform.




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