Monday 25th May
Sunday saw the relentless strong sunshine push the temperature to a peak of 30.6C at 17.10, which was a significant 13.3C above my long-term May average. Not surprisingly, the extreme heat was slow to dissipate overnight with the thermometer not dropping below 12.1C at 05.23 early Monday, just after sunrise in Marlborough at 05.03. The low was 5.1C above average. The UV level, not surprisingly, was in the ‘High’ category again, in fact for the fifth successive day. It was the hottest day since the 12th August 2025 when 32.8C was recorded.
The sun shone strongly after sunrise on Monday morning that had boosted the temperature to 21.2C by 08.00, already 3.9C above the average maximum temperature to May.
In winter there can be a wind chill factor with low temperatures and strong winds. Currently the weather conditions are producing the opposite meteorological state. At 08.30 the temperature was 24.3C but the THSW Index meant it felt more like 25.2C outside. The index takes account of data for temperature, humidity, sun and wind strength.
We are now experiencing heatwave conditions with the peak on Sunday being equal to or above the threshold temperature of 27C for Wiltshire, and will certainly reach that again on Monday and Tuesday. Analysing the data I noted that the average temperature for May of 17.3C, was exceeded between 08.00 and 23.30. Also of note was that the thermometer was equal to or above 30C between 15.45 and 19.00.
Data shows that the heatwave threshold of 27C was reached at 12.32 on Sunday but three hours earlier on Monday at 09.31.
A heatwave is described as three consecutive days with temperatures rising above the heatwave threshold. The original heatwave thresholds were based on the reference climate period of 1981-2010 and were intended to be flexible and revised in the light of climate change, therefore the heatwave thresholds were revised in 2022, using the most recent 1991-2020 climate averaging period, with the local threshold temperature rising from 26C to 27C. The review saw six counties move from 27C to 28C, one from 26C to 27C and one from 25C to 26C, the areas changed were in the south of the UK and the Midlands.
The anticyclone is almost overhead the UK with a barometric pressure of 1031.6mb at 08.00, and continuing to rise slowly. It position will produce a light breeze from the southeast today and tomorrow that will continue the flow of hot air from the Continent before a significant small change in direction on Wednesday to come from the east, that will bring a cooler airstream from the North Sea and result in temperatures dropping a few degrees.