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Is Perth's record heat a thermometer error?

There is evidence that record hot months, seasons and years recorded in Perth since 2011 are due to or influenced by a change in temperature probe at the Perth Metro 9225 weather station in Mt Lawley.

Climate researcher Warwick Hughes has drawn attention on his blog to a fascinating discrepancy between maximum temperature recordings at Perth Metro and Perth Airport 9021, a weather station that acts as an effective proxy since its opening in 1945.

His findings highlight the unreliability of recording equipment and raise questions about the accuracy of Perth maximum temperatures.

Perth Metro recorded a record 21 days above 37C in the 2012/13 summer, as well as its hottest ever summer and its equal hottest year in 2012.

Warwick has charted the difference in recorded maxima between Perth Regional Office 9034, Perth Metro and Perth Airport back to 1945 and found that a consistent difference between temperatures collapsed in 2011, coinciding with an equipment change at Perth Metro:

perth metro thermometers

Warwick found there was a fairly stable temperature difference with airport maxima throughout the record of Perth Regional Office from 1945 to 1992, although there was a significant increase from the late 1960s.

Perth Regional Office moved from Mt Eliza in West Perth to Wellington St in East Perth in 1967. Since the airport weather station is stable beyond movement within the confines of the airport itself, this supports evidence of a bias in the Perth temperature record from 1967 to 1992 because of that move, primarily due to Urban Heat Influence in the CBD and a lower weather station elevation (read more).

The weather station moved north away from the CBD and inland from the Swan River to Mt Lawley in 1994, establishing Perth Metro 9225. The average maximum temperature difference with Perth Airport dropped immediately to slightly above its level before 1967.

This changed in 2011 when for the first time ever the maximum temperature difference between Perth Metro and Perth Airport dropped to almost zero.

The change in average difference can be seen in timeline charts of average maxima at both stations since 1994, providing evidence that one or both weather stations experienced an artificial rather than natural change to its temperature recordings in 2011.

Below are the average annual maxima charted for both stations since 1994:

bom thermometers in perth

Below are the average monthly maxima charted for both stations since 1994:

perth thermometers

After 18 years during which Perth Airport consistently had higher maxima, particularly in summer, the two stations began to record equal average temperatures in 2011.

For the first time ever in 2012, Perth Metro's annual average maximum was slightly warmer than Perth Airport.

To analyse the cause of this unprecedented shift in correlating average temperatures, records at the two stations were compared between Perth Metro from 1 January 1994 to 31 December 2013.

During the 6,928 days from 1 January 1994 to 31 December 2012 (excluding 10 days due to missing BoM temperature records), 374 days or one full year had exactly the same maximum at both the airport and Perth Metro.

On 1,474 days, Perth Metro was hotter and on 5,090 days Perth Airport was hotter.

The same day temperature differences are tabulated below, showing the number of days when Perth Metro was warmer or cooler than Perth Airport in degrees.

perth temperature differences

Differences up to 7C are cause for concern. The 7C was on 26 May 1998, with the surrounding days being:

25 May Metro 28.0C, Airport 27.9C
26 May Metro 26.3C, Airport 19.3
27 May Metro 21.0C, Airport 21.0C

For Perth Airport's 6C difference, it was 6 Jan 2003 with the surrounding days being:

5 Jan Metro 26.9C, Airport 28.8C
6 Jan Metro 29.6C, Airport 35.6C
7 Jan Metro 34.0C, Airport 34.5C

It is possible that a very unusual weather anomaly caused the 26 May 1998 aberration. On 5 Jan 2003, there was a 1.9C difference between maxima and Perth Metro and Perth Airport. Nevertheless, differences of this magnitude suggest recording errors.

Perth Metro has an elevation of 25 metres and is 8.5km north-north-west of Perth Airport, which has an elevation of 15 metres. The airport is 10m lower and a bit further inland so it would be expected to be slightly warmer than Mt Lawley on average.

However, the differences point to uncorrected raw temperature errors and both days are unadjusted in Perth ACORN.

Below are time charts of the temperature differences, listed above, of 1C and 2C or greater.

perth hot temperatures

perth extreme temperatures

The chart below shows how Perth Airport was consistently warmer until 2011, since when the difference between maxima at the two stations has become more stable:

perth temperature variations

The frequency and severity of temperature differences during the 1990s raise further questions about the accuracy of Perth temperature recordings at that time. If such anomalies are natural, they should be consistent throughout the record.

The maximum temperature differences are seasonal, as illustrated by discrepancies above 1C charted below.

perth temperature changes

A similar pattern is seen in averaged maximum temperature differences for each month:

perth airport temperature history

It is worth noting that in the 61 days from 1 January to 2 February 2013, there were 54 days with a temperature difference - Perth Metro warmer on 10 days (largest 1.2C), the airport warmer on 44 days and warmer by 1C+ on 20 days (largest 1.8C).

When you monthly categorise all 6,928 days with a different maximum temperature, there is little difference even though Januarys have had about 30 days more than average, mostly at the expense of Februarys:

perth historic temperature changes

It is apparent that something happened in 2011 to make one of the stations either cool down or warm up relative to the other. Which one? Charts of both stations showing their number of summer days above 37C might give a clue:

perth airport temperatures

perth metro temperatures

The BoM claims Perth Metro had an equal record hot 2012 (25.67103825C), correct when rounded but slightly cooler than 2011 (25.7083333333C).

Perth Airport cooled from 2011 to 2012 (by 0.41C) but temperatures recorded by the new thermometer equipment at Perth Metro dropped by just 0.04C.

Below collates and averages temperature separations from 1994 to 2012:

bom thermometers errors in perth

perth airport temperatures

Will a rounded 0.6C be the average extra warmth at Perth Metro in coming years since the new air temperature probe was installed in July 2011?

Note the last three years with Perth Metro 0.37C hotter in 2012 than 2010 and Perth Airport 0.36 cooler. The table below shows the difference in annual maxima from preceding years, hotter or cooler, and the separation between those differences.

bom temperature errors in perth

Record years, seasons and months have been claimed in Perth since the air temperature probe was installed at Perth Metro in July 2011.

The unprecedented shift in temperature differences and correlating average maximum temperatures at the two weather stations are very unlikely due to natural influences.

Their immediate coincidence with the installation of a new Perth Metro temperature probe raises questions about the accuracy of these claimed heat records.

Minimum temperatures

For minima, during the 6,936 days from 1 January 1994 to 31 December 2012 (excluding six days due to missing BoM temperature records), 367 days or one full year had exactly the same maximum at both the airport and Perth Metro.

On 4,130 nights, Perth Metro minima were warmer and on 2,439 nights Perth Airport was warmer.

A similar convergence of minimum temperatures at Perth Metro and Perth Airport is not apparent since 2011. However, it is noteworthy that Perth Airport nights were slightly cooler than at Perth Metro until the year 2000, when either the airport temperatures rose or Perth Metro temperatures dropped immediately and permanently by about .5C.

perth minimum temperature changes

A likely explanation for the significant temperature shift starting 1997 is movement of the Perth Airport weather station about one kilometre to the north in that year, illustrated in the photo graphic below courtesy of Warwick Hughes:

perth airport weather station

The sudden change in minimum temperature differences between Perth Metro and Perth Airport from 1997 can be seen in average annual minima recorded at both weather stations.

perth temperature comparison

The sharp increase in minima in 2011 is apparent at both weather stations and can be assumed natural.

Nevertheless and as with maxima but in a different way, the very rapid alteration in minimum temperature differences and averages in 1997 strongly suggests an artificial rather than natural influence corrupting Perth's temperature record.

Excel data file with daily maximum temperatures for Perth Metro 9225 and Perth Airport 9021

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