Analysis of Maximum Temperature trends Canberra Airport Observations from 1991 to 2025

[Gavin O'Brien]

Hi everyone,

I have spent many hours compiling the Maximum and Minimum Temperatures recoded at Canberra Airport Met Station from 1991 to last year by grouping the daily Maximum /Minimum Temperatures in set of 5 degrees from -10 degrees (not observed) to >40 degrees. The resultant trend is startling! I have completed an analysis of the Maximum temperatures and hope to complete the Minimum ones soon.

Whilst the most days exceeding 40 degrees are in January. Before 2013 there was no occurrences dating back to 1991 (I am yet to see where there are occurrences, if any, going back to 1939.) 

The heatwave events (> 40 degrees) are interesting. Summer, 2013(1) 2014(1),2017(1) 2018(1) in 2019, an astonishing 5 days and the most recent; Summer 2020 with 2 days. Over the study period there were just 2 days recorded in December 2018.

The number of days when the temperature ranged between 35-40 degrees show a pronounced peak from the years 2011 to 2014 and again in the standout year of 2019. The summer 2013 has the most occurrences with 31 days. In contrast he year 2022 a wet, cool La Nina Year had none. December 2025 recorded 6 days exceeding 35 degrees with the 19th peaking at 39.8 degrees 

I have also graphed the number of days each year when the Maximum Temperature was between 5 and 9 degrees, typical winter temperatures for Canberra. The coldest years were back in the 1990's ,1995 being a standout with 35 days failing to reach 5-9 degrees, most likely a response to the Mount Pinatubo (Philippines) eruption in June 1991.By contrast the years 2011,2012,2013, and 2014 had no days when the temperature failed to reach 5-9 degrees!

The monthly breakdown for number of days exceeding 40 degrees is.

December -2019=2days

January    - 2013=1 day

                   2014=1 day

                   2017=1 day

                   2018=1 day

                  2019=5 days

                  2020=2 days

February  -2017=2 days

                  2020=1 day

When graphs are prepared, the increasing trends become obvious.

While there was a grouping of 35 to 40 degree days between 2011 and 2014 and 2017 to 2019 and last year 2025 as an outlier, most years range between 5 and 10 days with 4 years recording no days.

The days with Maximum Temperatures between 5 to 9 degrees indicates a sharp decrease in occurrences. there are 4 years with no days between 2011 and 2014. 

The tend with hot days, (range 35 to 40 degrees) is not as pronounced in its upwards trend as the cooler days graph, affected as it is by the peak around 2013 and the cooler spell from 2020 to 2024.

Anecdotal evidence from studies I have read, indicated that cool season maximums are rising at a faster rate than summer ones, evidenced at my site by a sharp decrease in frosts, indicating warmer minimum temperatures. I will confirm this observation when I complete the other study.

In the longer term I will survey the trends extending back to 1939, when the Canberra Airport site was established. The current site dates back to 2010, so there may be a bias in results when compared with the 1939 site.

There is no doubt in my mind that these trends are real, as the airport site has not been impacted by urban expansion or major changes in the surrounding area since 1939.

Cheers,

Gavin