A maximum of 25.6C at 15.22 on Monday was 5.0C above my 40-year average and the hottest day since 9th September 2023 (31.0C). This high was 1.4C short of the heatwave threshold. The low of 10.8C at 05.17 on Tuesday was just above average for June at +0.6C.
Tuesday revealed a splendid start to a new day with strong sunshine after sunrise that quickly dissipated the radiation fog by 06.15, which had formed in the early hours along the River Og valley.
Update at 16.15: day maximum of 27.6C logged at 13.37. Tomorrow should be similar but not Thursday so hot spell will not be classed as a heatwave.
Further update: Late afternoon surge with strong sunshine saw the thermometer climb back up to a high of 28.5C at 17.17
Why do heatwaves happen?
Heatwaves are most common in summer when high pressure develops across an area. High pressure systems are slow moving and can persist over an area for a prolonged period of time, such as days or weeks. They can occur in the UK due to the location of the jet stream, which is usually to the north of the UK in the summer. This can allow high pressure to develop over the UK resulting in persistent dry and settled weather.ilat
In England & Wales on Sunday, the Met Office issued a 4-day Heat Health Alert from 08.00 Monday 24th, a system which places each Local Authority region into one of four levels.
In March 2022 the Met Office announced that the official heatwave thresholds were being updated ahead of the summer, resulting in eight counties having their threshold increased by 1C. The heatwave threshold for Wiltshire was raised from 26C to 27C and must be equalled or exceeded for three consecutive days to be classed as a heatwave.
The original thresholds were derived from the distribution of July maximum temperatures in a 1981-2010 reference climate period and the change in threshold results from the Met Office update to a 1991-2020 reference climate.