Metrication influence on
Australian temperature trends

Did instrument and/or observation changes associated with the 1972 metrication of weather stations have any influence on Australia's long-term temperature trends?

This question was addressed in Techniques involved in developing the Australian Climate Observations Reference Network - Surface Air Temperatures (ACORN-SAT) dataset (p70) published in 2012 by the Australian Bureau of Meteorology and the CSIRO.

Three dataset comparisons were tested using sea surface temperatures, upper air observations at ACORN-SAT locations with mean 850 hPa temperatures, and minima and maxima at locations with temperatures at 1500 and 1600 respectively.

bom metrication comparison

The decision not to adjust ACORN-SAT for a suspected artificial 0.1C mean temperature increase was essentially based on the premise that major La Nina events, with record rainfall and cloud cover, may have caused a natural warming variation during the early 1970s.

Metrication on 1 September 1972 resulted in equipment changes at all weather stations and a significant reduction in the proportion of station observers who frequently recorded Fahrenheit thermometer readings as whole numbers without a fraction (e.g. 84 instead of 84.4, 95 instead of 94.7, 73 instead of 73.7).

The implications of pre-metric temperature rounding are referenced in A historical annual temperature dataset for Australia (Torok and Nicholls, BoM Research Centre, Melbourne, 1996):

torok temperature record australia

Australian averaged annual rainfall - long-term 1910-2019 average 458.5mm (temperature estimates based on RAW observations at all 112 ACORN stations)

1968-71 - 453.5mm / average maximum 24.56C / average minimum 12.91C
1973-76 - 631.6mm / average maximum 24.67C / average minimum 13.22C

1962-71 - 430.0mm / average maximum 24.66C / average minimum 12.94C
1973-82 - 532.6mm / average maximum 25.00C / average minimum 13.23C

1910-26 - 444.9mm / average maximum 24.79C / average minimum 13.22C
2003-19 - 488.1mm / average maximum 25.86C / average minimum 13.63C

Record rainfall and cloud cover would normally be expected to result in a warming of overnight minima as heat is trapped by clouds, preventing its release into the upper atmosphere and space.

Conversely, record rainfall and cloud cover would normally be expected to result in a cooling of daytime maxima as clouds increase the albedo reflection of sunlight and the evaporation of moisture dissipates the energy of heat.

Heavy rainfall and abundant clouds would not normally be expected to increase mean temperatures. Questions should be asked of the assumption that wet weather, rather than an artificial influence, caused the 1972 temperature increase.

Below is an extract from Recent apparent changes in relationships between the El Nino - southern oscillation and Australian rainfall and temperature, published in 1996 by Nicholls et al. This study identifies the 1972 temperature increase but tries to attribute it to a sudden differential warming of the Indian Ocean relative to the Pacific Ocean, which is as credible as more rainfall and clouds causing warmer temperatures.

nicholls rainfall and temperature

Analysis

It is incongruous for record rainfall and cloud cover to result in minima increasing but maxima not decreasing. This analysis tests unadjusted raw temperatures at 110 stations in the ACORN-SAT dataset, including their relationship to annual rounding proportions, to determine whether it is appropriate to attribute the 0.1C mean warming in 1972 to natural rather than artificial causes.

The ACORN-SAT stations Learmonth and Rabbit Flat are excluded because they opened in 1970 and 1976, invalidating their inclusion to a comparison of 1968-71 and 1973-76 averaged temperatures.

Daily raw minimum and maximum temperature data were downloaded from the Bureau of Meteorology's Climate Data Online website and converted from Celsius to Fahrenheit, a script calculating annual and monthly temperature averages (monthly with minimum 15 days of recordings), as well as annual percentage proportions of temperature rounding based on a .91><.09 formula to determine if any day's reading was a rounded Fahrenheit number (see benchmark accuracy).

Minima should rise due to increased rainfall cloud cover. Key points:

At 110 ACORN-SAT stations, average minima increased 0.32C from 12.94C in 1968-71 to 13.26C in 1973-76. Average rounding proportions fell from 44.62% to 21.20%.

At 110 ACORN-SAT stations, average minima increased 0.33C from 12.94C in 1962-71 to 13.27C in 1973-82. Average rounding proportions fell from 50.90% to 21.27%.

At 110 ACORN-SAT stations, average minima increased 0.21C from 13.28C in 1910-26 to 13.49C in 2000-16. Average rounding proportions fell from 56.68% to 13.61%.

minimum temperature trends

The minimum temperature trends charted above might be misleading as there were 58 stations open in 1910 and the remaining 54 were progressively added to ACORN-SAT over the decades before all 112 were operating by the mid 1970s.

At the 58 ACORN-SAT stations that were all open in 1910, average minima increased 0.30C from 13.38C in 1968-71 to 13.68C in 1973-76. Average rounding proportions fell from 47.40% to 22.33%.

At the 58 ACORN-SAT stations that were all open in 1910, average minima increased 0.29C from 13.39C in 1962-71 to 13.68C in 1973-82. Average rounding proportions fell from 55.22% to 22.24%.

At the 58 ACORN-SAT stations that were all open in 1910, average minima increased 0.53C from 13.39C in 1910-26 to 13.92C in 2000-16. Average rounding proportions fell from 56.72% to 14.47%.

minimum temperature trends

The bureau excludes eight of the 112 stations from estimated national temperature trends because they are artificially warmed by urban heat (Sydney, Melbourne, Adelaide, Hobart, Townsville, Rockhampton, Richmond NSW, Laverton).

At the 104 ACORN-SAT stations used by the bureau to estimate trends, excluding city and major regional centres influenced by urban heat, average minima increased 0.33C from 12.92C in 1968-71 to 13.25C in 1973-76. Average rounding proportions fell from 45.43% to 21.65%.

At the 104 ACORN-SAT stations used by the bureau to estimate trends, excluding city and major regional centres influenced by urban heat, average minima increased 0.37C from 12.92C in 1962-71 to 13.29C in 1973-82. Average rounding proportions fell from 52.44% to 21.69%.

At the 104 ACORN-SAT stations used by the bureau to estimate trends, excluding city and major regional centres influenced by urban heat, average minima increased 0.12C from 13.39C in 1910-26 to 13.51C in 2000-16. Average rounding proportions fell from 58.39% to 13.65%.

minimum temperature trends

In addition to the bureau's exclusion of eight urban-warmed locations, the figures and chart below also exclude Learmonth and Rabbit Flat to allow a more precise estimation of pre and post 1972 metrication temperatures.

At the 102 ACORN-SAT stations used by the bureau to estimate trends, excluding Learmonth and Rabbit Flat, average minima increased 0.31C from 12.96C in 1968-71 to 13.27C in 1973-76. Average rounding proportions fell from 45.57% to 21.76%.

At the 102 ACORN-SAT stations used by the bureau to estimate trends, excluding Learmonth and Rabbit Flat, average minima increased 0.33C from 12.96C in 1962-71 to 13.29C in 1973-82. Average rounding proportions fell from 52.49% to 21.83%.

At the 102 ACORN-SAT stations used by the bureau to estimate trends, excluding Learmonth and Rabbit Flat, average minima increased 0.11C from 13.39C in 1910-26 to 13.50C in 2000-16. Average rounding proportions fell from 58.39% to 13.54%.

minimum temperature trends

Maxima should decrease due to increased rainfall cloud cover. Key points:

At 110 ACORN-SAT stations, average maxima was static from 24.70C in 1968-71 to 24.70C in 1973-76. Average rounding proportions fell from 42.76% to 19.20%.

At 110 ACORN-SAT stations, average maxima increased 0.26C from 24.71C in 1962-71 to 24.97C in 1973-82. Average rounding proportions fell from 48.20% to 19.52%.

At 110 ACORN-SAT stations, average maxima increased 0.68C from 24.89C in 1910-26 to 25.57C in 2000-16. Average rounding proportions fell from 54.31% to 13.85%.

maximum temperature trends

The maximum temperature trends charted above might be misleading as there were 58 stations open in 1910 and the remaining 54 were progressively added to ACORN-SAT over the decades before all 112 were operating by the mid 1970s.

At the 58 ACORN-SAT stations that were all open in 1910, average maxima increased 0.03C from 24.93C in 1968-71 to 24.96C in 1973-76. Average rounding proportions fell from 43.15% in 1968-71 to 19.94% in 1973-76.

At the 58 ACORN-SAT stations that were all open in 1910, average maxima increased 0.19C from 24.99C in 1962-71 to 25.18C in 1973-82. Average rounding proportions fell from 51.05% to 19.90%.

At the 58 ACORN-SAT stations that were all open in 1910, average maxima increased 0.66C from 25.11C in 1910-26 to 25.77C in 2000-16. Average rounding proportions fell from 53.69% to 14.35%.

maximum temperature trends

At the 104 ACORN-SAT stations used by the bureau to estimate trends, excluding city and major regional centres influenced by urban heat, average maxima increased 0.02C from 24.90C in 1968-71 to 24.92C in 1973-76. Average rounding proportions fell from 43.19% to 19.55%.

At the 104 ACORN-SAT stations used by the bureau to estimate trends, excluding city and major regional centres influenced by urban heat, average maxima increased 0.30C from 24.92C in 1962-71 to 25.22C in 1973-82. Average rounding proportions fell from 49.38% to 19.81%.

At the 104 ACORN-SAT stations used by the bureau to estimate trends, excluding city and major regional centres influenced by urban heat, average maxima increased 0.54C from 25.28C in 1910-26 to 25.82C in 2000-16. Average rounding proportions fell from 56.19% to 13.73%.

maximum temperature trends

At the 102 ACORN-SAT stations used by the bureau to estimate trends, excluding Learmonth and Rabbit Flat, average maxima was static from 24.87C in 1968-71 to 24.87C in 1973-76. Average rounding proportions fell from 43.75% to 19.70%.

At the 102 ACORN-SAT stations used by the bureau to estimate trends, excluding Learmonth and Rabbit Flat, average maxima increased 0.26C from 24.89C in 1962-71 to 25.15C in 1973-82. Average rounding proportions fell from 49.73% to 20.05%.

At the 102 ACORN-SAT stations used by the bureau to estimate trends, excluding Learmonth and Rabbit Flat, average maxima increased 0.60C from 25.14C in 1910-26 to 25.74C in 2000-16. Average rounding proportions fell from 55.93% to 13.77%.

maximum temperature trends

Discussion

Average national raw maxima were static from 1968-1971 to 1973-76 among 110 ACORN stations but an increase is masked by the record cloud cover and rainfall that cooled daytime temperature readings in the early 1970s.

The averaged 0.32C increase in national average minima from 1968-71 to 1973-76 was partly caused by nighttime cloud cover trapping heat, but the increase is substantial. As an example of a similar record rainfall period, 2005-08 rainfall averaged 467.7mm annually and in 2009-12 averaged 587.7mm, but average annual minima decreased from 13.52C to 13.49C between those four year periods (and maxima cooled from 25.73C to 25.50C).

During a strong La Nina period that increased nighttime minima by 0.32C averaged across Australia, the static daytime maxima suggests an artificial rather than natural influence on temperature recordings. The Pacific ENSO shift said to have caused a global temperature increase didn't start until early 1977 (source).

The immediate and significant decline in temperature rounding after 1972 metrication may be indicative of a majority of regional weather station observers previously choosing to evenly round or in some cases truncate down the temperature scale they could see on their Fahrenheit thermometers.

An example of truncation might be an early morning farmer (some with poor eyesight, others rushing to herd their dairy cattle) who sees a temperature reading considerably above 67F but decides to record 67F because it seems more accurate and honest than writing 68F, creating a pre-metric cooling bias. The proportion of observers who regularly truncated their Fahrenheit temperature recordings is unknown but anything above 50% immediately creates a bias.

This analysis demonstrates that temperature rounding was widespread at 69.9% for minima and 67.4% for maxima when averaged at all 112 ACORN stations from 1910 to 1971, and was sufficiently common to produce a major discontinuity.

From 1968-71 to 1973-76 at 110 stations, there was a mean temperature increase (0.00 + 0.32 / 2) of 0.16C, which is higher than the bureau’s estimate of a mean rise of 0.1C and represents 16% of the claimed 1.0C increase in ACORN-SAT mean temperatures from 1910 to 2016.

In 1996, Simon Torok et al published Recent apparent changes in relationships between the El Nino - southern oscillation and Australian rainfall and temperature which analysed an apparent maximum temperature increase around 1972/73 of about 0.3C comparing 1910-1972 with 1973-1992, but downplayed the possible impact of changed metric instrumentation and instead attributed the temperature/rainfall/SOI correlation shift to increased Indian Ocean temperatures that may have occurred at about the same time that Celsius thermometers replaced Fahrenheit thermometers.

The paper did not analyse the minimum temperature/rainfall/SOI correlation. The 0.3C maximum temperature shift calculated in the paper is similar to the 0.26C shift calculated above among the 102 non-urban ACORN stations (excluding Learmonth and Rabbit Flat) from 1962-1971 to 1973-1982.

The analysis on this page also finds a 0.33C increase in minimum temperature calculated above among the 102 non-urban ACORN stations (excluding Learmonth and Rabbit Flat) from 1962-1971 to 1973-1982.

The Torok et al paper recognises the increase in Australian rainfall since the early 1970s but asserts that Indian Ocean temperatures may have caused the maximum temperature increase, which was similar to the minimum temperature increase, and confirms the calculations on this page but with a dubious explanation that it was because of increased rainfall and ocean temperatures rather than the metrication change that happened at the same time.

It is difficult to determine whether the prominence of observation rounding before 1972 had a significant influence on average temperatures.

However, the two charts below are based on the unique rounding percentage median for each of 110 ACORN locations, which is used for each station to average temperatures above or below that particular median in both RAW min and max.

rounding minimum temperature influence

rounding maximum temperature influence

Conclusion

The averaged failure of Australian maxima to decline during years of record rainfall and cloud cover suggests an artificial metrication influence that should be adjusted in Australia's homogenised ACORN-SAT temperature set.

Note: The spike in temperature rounding from 1997 to 2006 within all charts above is due to Automatic Weather Stations being installed at many ACORN-SAT sites, a significant number with faulty readings that for several years rounded all temperatures evenly up and down. Maxima in particular had a sudden upward shift to a warmer plateau since the bureau's AWS rounding repairs concluded in 2006. This may be the influence of AWS and the installation of smaller Stevenson screens since the late 1990s.

ACORN-SAT Observation Practices (p11-13) details when 106 weather stations had small Stevenson screens installed. The data show 29 were installed from 1900 to 1989 and 77 from 1990 to 2009. Among the 77, the average year of installation was 1998: 1990 - 1, 1991 - 1, 1992 - 4, 1993 - 2, 1994 - 4, 1995 - 5, 1996 - 7, 1997 - 15, 1998 - 6, 1999 - 5, 2000 - 10, 2001 - 6, 2002 - 2, 2003 - 2, 2004 - 3, 2005 - 1, 2007 - 1 , 2009 - 1.

The chart below shows that when 40% of all small Stevenson screens were installed from 1997 to 2001, Australian average rainfall increased significantly but maximum temperatures remained warmer than their pre-1997 averages when there was 22% less rainfall. When rainfall returned to long-term average levels from 2002 to 2009, maxima averaged 0.62C warmer than 1990-96. This suggests high rainfall partly masked an artificial warming influence caused by the widespread installation of small Stevenson screens (large Stevenson screen interior 0.23m3, small Stevenson screen interior 0.06m3).

small stevenson influence

Note: Annual temperature rounding estimates in this analysis are benchmarked against Extreme Temperature Events in Australia, a 2001 PhD thesis by ACORN-SAT architect Blair Trewin from the Bureau of Meteorology. His study estimates that among 94 of the ACORN-SAT stations, the average proportion of Fahrenheit temperatures rounded by observers from 1957 to 1971 was 51.5%. Relevant charts with rounded Fahrenheit percentages at most Australian weather stations can be viewed here.

The C>F conversion script used in this analysis estimates the rounding proportion at the same 94 stations from 1957 to 1971 was 51.3%. This demonstrates that the averaged comparison of annual rounding proportions is accurate. A comparison of the 1957-71 benchmarks and this analysis calculation of rounding for all stations can be viewed here. Some benchmarked stations have identical rounding proportions to this study's calculations. Others have higher or lower proportions, dependant on each station observer's personal habit of adding .1 or .9 to their Fahrenheit temperature readings.

Decadal trends

A comparison of rounding proportions, temperatures and rainfall can also be viewed in a decadal context among long-term weather stations that were taking observations in 1910:

historic australian decadal minimum temperature trends