Are mid-20°C temperatures in May unusual?

The UK’s hottest day of 2022 so far was recorded at Heathrow with 27.5°C today (17 May 2022), leading to interest in how often we record these kinds of temperatures in the UK during May. Mike Kendon, from the Met Office National Climate Information Centre, puts this latest temperature recording into context.

As we get closer to summer, we often see more excitement about higher temperatures and the chance of heatwaves. Although we’re a way off an official heatwave, there has been interest in the UK reaching mid-20°C in May.

Today’s highest temperatures were confined to the southeast of England, with only 15 stations recording 25°C or above. The temperature of 27.5°C at Heathrow is around 10°C higher than the May 1991-2020 long-term average daily maximum temperature but nevertheless falls far short of the UK’s all-time May record of 32.8°C.

The Met Office observational record shows that 25°C is not a particularly unusual temperature for this time of year.

Looking at individual station data for the first recording of 25°C or above, interestingly, despite large annual variability, there is a clear shift in the date when 25°C or higher is first occurring in the UK moving earlier in the calendar. Based on the period 1961-1990, the average date is 19 May compared to 6 May for the period 1991-2020. Meaning that the first day of the year when 25°C or higher is recorded in the UK has moved forward by almost a fortnight between the two 30-year periods.

The earliest known occurrence of temperatures in excess of 25°C in the calendar year is 29 March with recordings in 1965 and 1968.

Inevitably with climate change in everyone’s mind, there is often the question of if higher temperatures are because of human influence. The statistics above reflect maximum temperatures at specific locations. By averaging the temperature across the whole of the UK we can also get a measure of widespread warm days, for example by measuring how often the UK average is above 20°C at this time of year.[1]

In the 1961-1990 averaging period there were 35 May days with a UK area-average daily maximum temperature of 20°C or above. In the 1991-2020 averaging period there were 76 May days with a UK area-average daily max temperature of 20°C or above. This means that the number of warm days in May has more than doubled in 30 years by this measure. 

When examining the link to climate change it is important to not only look at extremes though. The UK average mean temperature for May has increased from 9.8°C to 10.6°C in 30 years (1961-1990 compared to 1991-2020). As our climate warms, we expect to see a corresponding increase in the number of hot days.

[1] Data: HadUK-Grid

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