Tuesday 23rd June
The heat steadily built during the morning and afternoon on Monday, that resulted in a maximum of 30.6C at 16.37. This made it the hottest day since 27th May during the last heatwave and a significant 9.8C above my long-term average. However, just after 17.00 cloud was observed edging in from the west and the radar showed a very intense area of thunderstorms and rainfall heading our way. As has happened on several occasions with shower activity arriving from the west, the bulk of the storm passed just to the north of Marlborough, towards Swindon, a repeat of the situation of last Saturday. A few heavy spots were observed just after 18.40, with more steady rain arriving justifier 19.20 that produced a total of 3.0mm. The past night was mild and humid with the thermometer not sinking below 18.0C at 04.06, which made it the warmest night this year and 7.8C above average.
I have previously mentioned that during summer we experience the opposite of wind chill, which occurs in winter, when outside it feels hotter than that indicated on a radiation shielded thermometer. At 14.00 I observed that with an indicated air temperature of 29.3C it actually felt more like 34.4C outside on the skin using data on humidity, solar radiation, wind strength and air temperature.
Tuesday began with the sun initially hidden behind cloud on the eastern horizon but by 06.30 had broken through and begun to shine strongly. However, thin high and variable cloud arrived around 08.00 that then limited the rise in temperature but by that time the thermometer had already risen to 22.7C, which was 2C above the June average maximum.
The Met Office have again issued an Amber Heat warning that will rise to Red on Wednesday and Thursday as the heat builds and the high humidity continues.
The anticyclone is slow relocating a little further southeast that will see the wind initially come from that direction wafting more heat from the Continent, but then slowly veer into the south and southwest by tonight, but still brisk.
Why the heat feels more intense
This period of heat is expected to feel especially oppressive due to high humidity levels.
Humidity plays a key role in how we experience heat. When moisture levels in the air are high, it becomes more difficult for the body to cool itself through sweating. This increases the risk of heat stress and associated health impacts.
Why thunderstorms develop in hot weather
Alongside the heat, there is also a risk of thunderstorms developing, particularly across central and southern areas.
Hot weather increases the amount of energy available in the atmosphere. As warm air rises, it cools and condenses, forming towering clouds that can develop into thunderstorms. When combined with moisture and atmospheric instability, this can lead to heavy downpours, lightning and gusty winds.
These storms can form quickly and be highly localised, bringing sudden changes in conditions and, at times, intense rainfall.