Paper summary: the world’s longest known series of parallel temperature data: Adelaide, 1887–1947

Parallel weather observations are crucial for combining historical weather observations with modern records, so we can accurately see what our climate was like back in time.

Most datasets go for two or three years, but in Adelaide, Australia, Government Astronomer and Meteorologist Charles Todd and his team took measurements in two thermometer stands for almost 60 years! This makes the Adelaide dataset the longest known record of its kind in the world.

The monthly mean values of these records were found and analysed in the 1990s, but the daily observations remained lost to modern research until recently, when they were unearthed by volunteers in the South Australian Bureau of Meteorology office. This remarkable team imaged and digitised the data, turning the chicken scratchings into a dataset we could explore.

A page of the journals of Glaisher stand observations for Adelaide. The Stevenson screen observations have already been digitised and are available at the Bureau of Meteorology. You can see all of the images at the ACRE database page from the Australian Meteorological Association Citizen Science Unit.

By examining the differences in the parallel data on a daily scale, we found the following key results:

1) The maximum temperatures recorded in both screens became more different the hotter it got, BUT not us much as we expected when it gets really hot. This means we can use the historical data with a bit more reliability when looking at historical heatwaves.

Differences in maximum temperatures (Glaisher stand – Stevenson screen) by month, and by 5ºC bins from 1887 to 1947. Image adapted from Ashcroft et al. 2021.
The differences in maximum temperature during the 1939 heatwave in Adelaide, still considered to be one of the most extreme events experienced in the city. The Glaisher stand observations are warmer than those from the Stevenson Screen, but it’s not a linear relationship between absolute temperature and difference, not when you get to these extreme temperature values. Newspaper clipping from ABC, graph adapted from Ashcroft et al. 2021.

2) The minimum temperatures were only slightly different year round.

Differences in minimum temperature (Glaisher – Stevenson screen) by month and 5ºC bin from January 1887 to April 1938, when a change in observation times made the minimum series from Glaisher stand and Stevenson screen meaningless to compare. Comparing the maximum and minimum temperature plots shows the minimum temperature difference to be similar regardless of the absolute temperature. Image adapted from Ashcroft et al. 2021.

3) The relationships we found meant we could more accurately combine old temperature records (taken in a Glaisher stand since 1859) with recent observations to build a 160 year temperature record for Adelaide.

4) The recent period is warm in both maximum and minimum temperature, and the pre-1872 maximum temperatures are also warmer than average stand out as particularly warm. We looked into the pre-1870 maximum temps to see how they related to variations in rainfall, and found the warming couldn’t be completely explained by the dry conditions experienced at that time. This means there is probably some data quality issues remaining.

Annual mean maximum and minimum temperatures (?C) in the merged Adelaide series, 1859 to 2019. Pre-1872 maximum temperature data are shown with a dashed line and in a lighter colour, as it is likely that inhomogeneities remain in this part of the record. Clear positive trends can be seen in both variables from the mid-20th century. Image adapted from Ashcroft et al. 2021.

Almost 160 years in the making, the paper exploring all of this in more detail is available in the International Journal of Climatology (feel free to email me if you don’t have access). You can also explore the data, as well as images of the original records.

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