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