Record Breaking Temperature recorded at Cambridge University Botanic Garden: One year On

Just over a year ago, on Thursday 25th July 2019, a temperature reading of 38.7°C was recorded at Cambridge University's Botanic Garden. It was the highest temperature ever officially recorded in the UK, which made international headline news. Sally Petitt, Head of Horticulture, explains how weather is recorded at the Garden and the significance of these exceptional readings.

Activities within the Garden are varied and constantly evolving, but there is one daily task which has been part of our routine for over 100 years, and that is taking meteorological readings.

Today this task is carried out by Katie Martyr, Experimental Section Assistant, who succeeds a long line of dedicated weather observers, including John Kapor (Supervisor, Systematics). The role entails recording a range of readings including maximum and minimum temperatures, ground temperatures, rainfall, cloud cover, and wind speed and direction. Readings are taken at our weather station located in the Experimental Plots, at 9.00am Greenwich Mean Time (so 10.00am in summer), and capture data from the preceding 24 hours.

The station is monitored by the Met Office to ensure that it meets their requirements, and that there have been no changes to the site, such as new buildings or tree growth, which can influence microclimates. The ways of taking readings are relatively low tech, with most observations made using standard thermometers, measuring cylinders and visual observations. The maximum temperature is charted electronically on a thermometer housed in a louvred Stevenson Screen to ensure consistency of measurements. Readings from this thermometer are charted on to a computerised data logger. The reading is then taken by looking for the peak temperature in the preceding 24-hour recording period. All data is logged at the Garden and also with the Met Office which uploads the information on to their publicly accessible Weather Observers Website (WOW). It is this thermometer which took the record 38.7°C reading on 25 July.

In late July, forecasts indicated exceptionally high temperatures. During the week beginning 22 July, the temperature began to rise, and on Thursday 25 July it was evident that the temperature was abnormally high. Plants were wilting and being scorched, glasshouses and ticket offices had to be closed as they were too hot to bear, and staff were flagging - with some resorting to standing in buckets of cold water while potting up plants in the shade!

As usual, the readings for 25 July were not read until 10.00am on 26 July, when Katie excitedly measured a temperature of 38.7°C. This was noted and uploaded onto WOW. It was only later that afternoon the Met Office indicated that this might be a record temperature for the UK. We then waited for the Met Office to check our station before confirming that this was officially a new record. On 29 July they visited the site to calibrate the maximum thermometer computer, and to verify that the weather station and that none of the equipment had been tampered with. It was not until that afternoon that the Met Office declared that we had officially recorded a new maximum temperature for the UK of 38.7°C.

Inevitably this created a great deal of media interest and headlines. From our perspective, the real significance of this chance occurrence was not in reaching this temperature, but in knowing that our continued monitoring of weather here at Cambridge University Botanic Garden was valuable in defining this heatwave.  The University holds readings from our weather station dating back to 1904, though Met Office records show that we have been gathering valuable and continuous data for them since 1891. It is the historical significance of this data which is of real value to the Met Office and climate researchers as they are able to use this to establish how weather patterns are changing over time.

It also acts as a caution about what is happening to our environment, the inevitable consequences for the plants and animals around us, and the impact on our collections. If these new weather patterns become ‘the norm’, then we may need to adapt what we grow in the future. Mediterranean plants do well in our local conditions and there may be scope with changing weather to extend their representation. Conversely, we may also need to review whether we continue to bring in more moisture-loving, temperate species. 

Written by Sally Petitt, Head of Horticulture at CUBG