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ACORN 2 influence on Tasmania temperature trends

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In late 2018 the Australian Bureau of Meteorology provided access to the ACORN 2 daily temperatures (Australian Climate Observations Reference Network - Surface Air Temperatures), which is the bureau's dataset of homogenised or adjusted raw temperatures at 112 Australian weather stations dating back to 1910.

It is assumed that ACORN 2 will replace the ACORN 1 dataset (introduced in 2012) as the historic reference used for comparison of modern temperatures. These are usually presented to the public as temperature anomalies compared to a 1961-90 baseline at each weather station.

There are seven ACORN stations in Tasmania. This page analyses the daily temperatures at all seven weather stations but averages the three with a start year of 1910 to ensure a uniform timescale.

Daily temperatures in ACORN 1, ACORN 2 and RAW have been annualised, and below are charts showing the difference between the two homogenised datasets when compared to their original raw annual temperatures. This analysis only allows the inclusion of years within which there were no months with less than 15 days of recorded temperatures, as is the ACORN protocol.

Three Tasmanian stations begin in 1910 and end in 2017 in ACORN 1, ACORN 2 and RAW - Eddystone Point, Launceston and Low Head. The first charts below show maximum and minimum temperature trends at these three stations with associated data that, although not the same as all seven ACORN stations across Tasmania, suggest the new ACORN 2 dataset has cooled the states's ACORN 1 maximum by 0.07C and cooled its minimum by 0.05C comparing 1910-1963 with 1964-2017 (first and second halves).

Cape Bruny in Tasmania presents an interesting comparison of an Automatic Weather Station and a manual Stevenson screen both observing daily temperatures over 20 years. The Cape Bruny analysis raises questions about the warming influence of AWS temperature readings in all latitudes across Australia.

Excel files at the bottom of each station chart and summary can be downloaded to compare how the ACORN 2 algorithms influence daily, monthly and annual temperatures at all five sites since 1910.

acorn 2 tasmania maximum temperatures

1910-1963 - v1 16.71C / v2 16.80C / raw 16.77C

1964-2017 - v1 17.28C / v2 17.30C / raw 17.12C

v1 warmed 0.57C / v2 warmed 0.50C / raw warmed 0.45C

Average change per decade : v1 0.08C / v2 0.08C / raw 0.06C

Tasmania 2000-2017 - v1 17.66C / v2 17.67C / raw 17.55C

1961-90 - v1 17.12C / v2 - 17.14C / raw 16.88C

Download Excel spreadsheet for three Tasmanian weather stations in ACORN 1, ACORN 2 and RAW

acorn 2 tasmania minimum temperatures

1910-1963 - v1 8.18C / v2 8.32C / raw 8.75C

1964-2017 - v1 8.82C / v2 8.91C / raw 8.90C

v1 warmed 0.64C / v2 warmed 0.59C / raw warmed 0.15C

Average change per decade : v1 0.10C / v2 0.09C / raw 0.03C

Tasmania 2000-2017 - v1 9.21C / v2 9.21C / raw 9.21C

1961-90 - v1 8.69C / v2 - 8.84C / raw 8.85C

Download Excel spreadsheet for three Tasmanian weather stations in ACORN 1 and ACORN 2 and RAW

Station menu

Butlers Gorge, Cape Bruny, Eddystone Point, Grove, Hobart, Launceston, Low Head

Butlers Gorge

acorn 2 butlers gorge maximum temperatures

1945-1980 - v1 12.42C / v2 12.31C / CDO raw 12.66C

1981-2017 - v1 13.23C / v2 13.23C / CDO raw 13.22C

v1 warmed 0.81C / v2 warmed 0.92C / raw warmed 0.56C

Average change per decade : v1 0.23C / v2 0.29C / raw 0.20C

Butlers Gorge 2000-2017 - v1 13.46C / v2 13.47C / raw 13.41C (2018 was 13.85C)

1961-90 - v1 12.80C / v2 12.79C / raw 12.97C

Pre 1972 .0 F rounding v1 - 20.78% / v2 - 18.95% / raw - 87.75%

Post 1972 .0 C rounding v1 - 10.16% / v2 - 9.54% / raw - 30.67%

ACORN catalogue: The original site at Butlers Gorge (also numbered 096003) operated from 1944 at the Hydro-Electric Commission settlement. The site appears to have been unchanged during the 1944–1993 period. In the early years of observations a HEC-pattern screen (single-louvred and lower than a Bureau standard screen) was used; this was replaced with a Bureau standard screen some time between 1973 and 1976. Observations appear to have been made at non-standard times (earlier than 9 am) on occasions. The settlement was abandoned, with all buildings removed, by 1969. Observations continued until October 1993. The current site is an automatic weather station which was installed about 150 m east of the previous site. It commenced observations in May 2008. Data from Lake St Clair (096071) was used in the 1993–2008 period, with the overlaps at both ends of this period used to merge the datasets. Lake St Clair is approximately 20 km north of Butlers Gorge but the overlap data indicates that temperature differences between the two locations are minimal. The site is over marshy tussock grass about 200 m southeast of the visitors’ centre.

1998-2007 raw max averaged 13.37C and post AWS 2009-2017 averaged 13.53C

Butlers Gorge max Excel spreadsheets

acorn 2 butlers gorge minimum temperatures

1945-1980 - v1 3.31C / v2 3.09C / CDO raw 2.93C

1981-2017 - v1 3.17C / v2 3.18C / CDO raw 2.91C

v1 cooled 0.14C / v2 warmed 0.09C / raw cooled 0.02C

Average change per decade : v1 0.02C / v2 0.10C / raw 0.11C

Butlers Gorge 2000-2017 - v1 3.28C / v2 3.27C / raw 3.13C (2018 was 3.48C)

1961-90 - v1 3.31C / v2 3.26C / raw 2.91C

Pre 1972 .0 F rounding v1 - 15.86% / v2 - 20.65% / raw - 86.01%

Post 1972 .0 C rounding v1 - 9.75% / v2 - 10.16% / raw - 29.67%

1998-2007 raw min averaged 2.94C and post AWS 2009-2017 averaged 3.40C

Butlers Gorge min Excel spreadsheets

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Cape Bruny

acorn 2 cape bruny maximum temperatures

1923-1969 - v1 14.79C / v2 14.70C / CDO raw 14.48C

1970-2017 - v1 15.33C / v2 15.30C / CDO raw 15.33C

v1 warmed 0.54C / v2 warmed 0.60C / raw warmed 0.85C

Average change per decade : v1 0.12C / v2 0.14C / raw 0.15C

Cape Bruny 2000-2017 - v1 15.59C / v2 15.59C / raw 15.59C (2018 was 16.01C)

1961-90 - v1 15.15C / v2 15.19C / raw 15.14C

Pre 1972 .0 F rounding v1 - 31.96% / v2 - 19.03% / raw - 74.42%

Post 1972 .0 C rounding v1 - 20.55% / v2 - 15.74% / raw - 20.52%

ACORN catalogue: The site appears to have been close to its current location before a move in April 1939. It also moved on 13 November 1961, and 45 m east in March 1969. An automatic weather station (094198) has been operating since 1997 at Cape Bruny. This is expected eventually to supersede the current site. It is about 50 m west of the current site, close to the 1961–1969 location of the manual site, but is in a more exposed location on top of the ridge. This exposure has clearly reduced rainfall readings but impacts on temperature are less clear.

94010 1987-1996 average max 15.57C vs 94198 AWS 1998-2007 (50 metres west) average max 16.02C

Cape Bruny max Excel spreadsheets

acorn 2 cape bruny minimum temperatures

1923-1969 - v1 8.29C / v2 8.04C / CDO raw 8.45C

1970-2017 - v1 9.26C / v2 9.05C / CDO raw 9.25C

v1 warmed 0.97C / v2 warmed 1.01C / raw warmed 0.80C

Average change per decade : v1 0.16C / v2 0.19C / raw 0.13C

Cape Bruny 2000-2017 - v1 9.49C / v2 9.38C / raw 9.48C (2018 was 9.92C)

1961-90 - v1 8.93C / v2 8.68C / raw 9.05C

Pre 1972 .0 F rounding v1 - 20.38% / v2 - 25.55% / raw - 74.04%

Post 1972 .0 C rounding v1 - 15.13% / v2 - 12.08% / raw - 22.04%

94010 1987-1996 average min 9.47C vs 94198 AWS 1998-2007 (50 metres west) average max 9.28C

Cape Bruny min Excel spreadsheets

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Eddystone Point

acorn 2 eddystone point maximum temperatures

1910-1963 - v1 17.13C / v2 17.29C / CDO raw 16.66C

1964-2017 - v1 17.79C / v2 17.87C / CDO raw 17.28C

v1 warmed 0.66C / v2 warmed 0.58C / raw warmed 0.68C

Average change per decade : v1 0.08C / v2 0.06C / raw 0.13C

Eddystone Point 2000-2017 - v1 18.19C / v2 18.20C / raw 18.19C (2018 was 18.36C)

1961-90 - v1 17.56C / v2 17.67C / raw 16.76C

Pre 1972 .0 F rounding v1 - 17.58% / v2 - 17.77% / raw - 67.46%

Post 1972 .0 C rounding v1 - 22.64% / v2 - 16.04% / raw - 30.53%

ACORN catalogue: There is little documentation on the site prior to the 1990s, but it is believed that there were no significant site moves between 1908 and 1996. An automatic weather station was installed in late 1996, with an associated site move about 70 m southeast. Parallel observations were carried out with the former site (using the station number 092115), but some observations from the new site were incorrectly recorded against the comparison number, making it impracticable to use the parallel observations to assess the impact of the move.

Assuming the AWS was operational from 1996, 1986-1995 raw max averaged 17.09C and post AWS 1997-2006 averaged 17.88C

Eddystone Point max Excel spreadsheets

acorn 2 eddystone point minimum temperatures

1910-1963 - v1 10.07C / v2 9.95C / CDO raw 9.79C

1964-2017 - v1 10.56C / v2 10.51C / CDO raw 10.51C

v1 warmed 0.51C / v2 warmed 0.56C / raw warmed 0.72C

Average change per decade : v1 0.07C / v2 0.08C / raw 0.10C

Eddystone Point 2000-2017 - v1 10.72C / v2 10.72C / raw 10.72C (2018 was 10.86C))

1961-90 - v1 10.47C / v2 10.38C / raw 10.41C

Pre 1972 .0 F rounding v1 - 18.19% / v2 - 16.66% / raw - 67.62%

Post 1972 .0 C rounding v1 - 22.85% / v2 - 24.31% / raw - 33.13%

Assuming the AWS was operational from 1996, 1986-1995 raw min averaged 10.37C and post AWS 1997-2006 averaged 10.60C

Eddystone Point min Excel spreadsheets

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Grove

acorn 2 grove maximum temperatures

1955-1985 - v1 17.25C / v2 17.39C / CDO raw 16.86C

1986-2017 - v1 17.57C / v2 17.55C / CDO raw 17.50C

v1 warmed 0.32C / v2 warmed 0.16C / raw warmed 0.64C

Average change per decade : v1 0.10C / v2 0.07C / raw 0.17C

Grove 2000-2017 - v1 17.63C / v2 17.63C / raw 17.64C (2018 was 17.84C)

1961-90 - v1 17.36C / v2 17.49C / raw 16.99C

Pre 1972 .0 F rounding v1 - 16.49% / v2 - 17.33% / raw - 44.03%

Post 1972 .0 C rounding v1 - 9.68% / v2 - 9.68% / raw - 12.26%

ACORN catalogue: The original site (094069) was also on the Research Station grounds and began operating in 1952. New buildings had been built approximately 15 m away from the site at some point between 1966 and 1985, most likely after 1980. The instruments were relocated 100 m west on 17 January 1990 in anticipation of a new building which was never constructed. The site closed in 2010. The current automatic weather station was established 190 m southwest of the previous site, and has been operating since June 2004.

94069 overlap with 94220 16 June 2004 to 9 February 2010 : 94069 17.43C / 94220 17.42C

1994-2003 raw max averaged 17.46C and post AWS 2005-2014 averaged 17.62C

Grove max Excel spreadsheets

acorn 2 grove minimum temperatures

1955-1985 - v1 5.51C / v2 5.19C / CDO raw 5.78C

1986-2017 - v1 5.64C / v2 5.56C / CDO raw 5.85C

v1 warmed 0.13C / v2 warmed 0.37C / raw warmed 0.07C

Average change per decade : v1 0.11C / v2 0.17C / raw 0.05C

Grove 2000-2017 - v1 5.76C / v2 5.75C / raw 5.92C (2018 was 6.37C)

1961-90 - v1 5.63C / v2 5.31C / raw 5.91C

Pre 1972 .0 F rounding v1 - 16.41% / v2 - 18.63% / raw - 49.03%

Post 1972 .0 C rounding v1 - 9.54% / v2 - 9.71% / raw - 13.13%

94069 overlap with 94220 17 June 2004 to 10 February 2010 : 94069 6.03C / 94220 5.59C

1994-2003 raw max averaged 5.84C and post AWS 2005-2014 averaged 5.76C

Grove min Excel spreadsheets

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Hobart

acorn 2 hobart maximum temperatures

1918-1967 - v1 17.25C / v2 16.90C / CDO raw 16.69C

1986-2017 - v1 17.79C / v2 17.58C / CDO raw 17.35C

v1 warmed 0.54C / v2 warmed 0.68C / raw warmed 0.66C

Average change per decade : v1 0.07C / v2 0.11C / raw 0.13C

Hobart 2000-2017 - v1 17.96C / v2 17.63C / raw 17.75C (2018 was 18.19C)

1961-90 - v1 17.62C / v2 17.27C / raw 17.06C

Pre 1972 .0 F rounding v1 - 20.09% / v2 - 18.89% / raw - 27.46%

Post 1972 .0 C rounding v1 - 10.53% / v2 - 10.08% / raw - 11.76%

Hobart has annual max temps back to 1883: 1883-1891 + 1894 + 1896-1907 16.79C

ACORN catalogue: The site has been on the same property since its establishment in 1882. A new building was built in the area in 1966 and the screen was moved approximately 5 m to be clear of this. An automatic weather station was installed on 29 June 1992, and became the primary instrument on 1 November 1996.

1986-1995 raw max averaged 17.25C and post AWS 1997-2006 averaged 17.33C

Hobart max Excel spreadsheets

acorn 2 hobart minimum temperatures

1918-1967 - v1 8.63C / v2 8.59C / CDO raw 8.13C

1986-2017 - v1 8.86C / v2 8.89C / CDO raw 8.86C

v1 warmed 0.23C / v2 warmed 0.30C / raw warmed 0.73C

Average change per decade : v1 0.03C / v2 0.04C / raw 0.09C

Hobart 2000-2017 - v1 9.07C / v2 9.09C / raw 9.07C (2018 was 9.50C)

1961-90 - v1 8.74C / v2 8.76C / raw 8.65C

Pre 1972 .0 F rounding v1 - 21.16% / v2 - 16.74% / raw - 31.73%

Post 1972 .0 C rounding v1 - 12.52% / v2 - 9.83% / raw - 12.54%

Hobart has annual min temps back to 1883: 1883-1891 + 1894 + 1896-1907 7.80C

1986-1995 raw min averaged 8.72C and post AWS 1997-2006 averaged 8.90C

Hobart min Excel spreadsheets

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Launceston

acorn 2 launceston maximum temperatures

1910-1963 - v1 17.46C / v2 17.35C / CDO raw 17.92C

1964-2017 - v1 17.82C / v2 17.80C / CDO raw 17.26C

v1 warmed 0.36C / v2 warmed 0.45C / raw cooled 0.66C

Average change per decade : v1 0.06C / v2 0.08C / raw -0.04C

Launceston 2000-2017 - v1 18.15C / v2 18.13C / raw 17.80C (2018 was 18.15C)

1961-90 - v1 17.67C / v2 17.63C / raw 16.99C

Pre 1972 .0 F rounding v1 - 16.39% / v2 - 17.82% / raw - 34.68%

Post 1972 .0 C rounding v1 - 10.84% / v2 - 9.76% / raw - 12.13%

Launceston 91049 has annual max temps back to 1901: 1901-1902 + 1904-1909 17.78C

91049 overlap with 91104 1 April 1939 to 30 December 1959 : 91049 18.18C / 91104 16.61C

91104 overlaps with 91311 29 July 2004 to 16 June 2009 : 91104 17.59C / 91311 18.27C

ACORN catalogue: The original site (091049) was at several locations around the centre of Launceston. There was a site move in December 1929 and further site moves on 29 March 1941, 29 September 1942 and 26 February 1948. Temperature observations continued at this location until 1959 but they were not used in ACORN-SAT after 1947. The first airport site (091104) was close to the terminal building and operated from 1939. It moved at some point in 1968–1969 to a location on the western side of the runway and north of the terminal. An automatic weather station was installed at this location on 19 May 1992 and became the primary instrument on 1 November 1996. The site continued until June 2009. The current site (091311) began operating at its current location 1.4 km southeast of the previous site on 12 July 2004. The airport is 160 m higher in elevation than the town, and is also further from the ocean. This results in markedly lower temperatures than those in the town, especially overnight.

1986-1995 raw max averaged 17.05 and post AWS 1997-2006 averaged 17.43C

Launceston max Excel spreadsheets

acorn 2 launceston minimum temperatures

1910-1963 - v1 4.71C / v2 5.14C / CDO raw 7.01C

1964-2017 - v1 5.92C / v2 5.96C / CDO raw 6.27C

v1 warmed 1.21C / v2 warmed 0.82C / raw cooled 0.74C

Average change per decade : v1 0.17C / v2 0.12C / raw -0.12C

Launceston 2000-2017 - v1 6.31C / v2 6.31C / raw 6.31C (2018 was 6.30C)

1961-90 - v1 5.66C / v2 5.73C / raw 6.32C

Pre 1972 .0 F rounding v1 - 16.58% / v2 - 18.99% / raw - 40.52%

Post 1972 .0 C rounding v1 - 11.68% / v2 - 10.19% / raw - 13.03%

Launceston 91049 has annual min temps back to 1898: 1898 + 1902 + 1906-1909 6.20C

91049 overlap with 91104 1 April 1939 to 31 December 1959 : 91049 6.68C / 91104 5.95C

91104 overlaps with 91311 30 July 2004 to 17 June 2009 : 91104 6.39C / 91311 6.28C

1986-1995 raw min averaged 6.34C and post AWS 1997-2006 averaged 6.09C

Launceston min Excel spreadsheets

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Low Head

acorn 2 low head maximum temperatures

1910-1963 - v1 15.59C / v2 15.81C / CDO raw 15.74C

1964-2017 - v1 16.34C / v2 16.34C / CDO raw 16.84C

v1 warmed 0.75C / v2 warmed 0.53C / raw warmed 0.90C

Average change per decade : v1 -0.18C / v2 -0.24C / raw -0.29C

Low Head 2000-2017 - v1 16.67C / v2 16.67C / raw 16.67C (2018 was 16.92C)

1961-90 - v1 16.19C / v2 16.18C / raw 16.90C

Pre 1972 .0 F rounding v1 - 19.29% / v2 - 19.67% / raw - 65.03%

Post 1972 .0 C rounding v1 - 11.74% / v2 - 10.16% / raw - 25.49%

Low Head 91057 has annual max temps back to 1895: 1895-1899 + 1901-1909 = 15.61C

91057 overlap with 91293 8 January 1998 to 5 December 2001 : 91057 17.58C / 91293 16.65C

ACORN catalogue: The original site (091057) was in the grounds of the Low Head lighthouse, which is on a small hill south of the headland. Prior to June 1937 the screen did not strictly meet Stevenson screen specifications, and was painted brown for a time around 1933, a period when the standard of observations was poor for a few years. Comparison of documentation from 1933 and 1940 indicate either a site move or new buildings during that time, with the latter being more likely. The site moved by 8 m in April 1949. At some point between 1968 and 1986 a gravel driveway was built in close proximity to the screen (the data suggest this occurred in about 1970). The site moved by 25 m on 15 August 1988. The automatic weather station (091293) began operating on 8 January 1998 at a site 200 m north of the previous location.

Assuming the AWS was operational from 1998, 1988-1997 raw max averaged 17.13C and post AWS 1999-2008 averaged 16.59C

Low Head max Excel spreadsheets

acorn 2 low head minimum temperatures

1910-1963 - v1 9.80C / v2 9.76C / CDO raw 9.50C

1964-2017 - v1 10.20C / v2 10.34C / CDO raw 10.01C

v1 warmed 0.40C / v2 warmed 0.58C / raw warmed 0.51C

Average change per decade : v1 0.07C / v2 0.09C / raw 0.11C

Low Head 2000-2017 - v1 10.58C / v2 10.58C / raw 10.59C (2018 was 11.00C)

1961-90 - v1 9.88C / v2 10.11C / raw 9.71C

Pre 1972 .0 F rounding v1 - 16.67% / v2 - 15.92% / raw - 63.05%

Post 1972 .0 C rounding v1 - 19.96% / v2 - 10.08% / raw - 29.29%

Low Head 91057 has annual min temps back to 1895: 1895-1899 + 1901-1909 = 8.58C

91057 overlap with 91293 9 January 1998 to 12 December 2001 : 91057 10.07C / 91293 10.61C

Assuming the AWS was operational from 1998, 1988-1997 raw min averaged 9.85C and post AWS 1999-2008 averaged 10.46C

Low Head min Excel spreadsheets

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