Saturday 23rd May
The sun was intense during Friday morning that resulted in the thermometer rising to a peak of 26.8C at 14.36 before hovering round that temperature until around 18.00, then very, very slowly it began to fall away. Cloud began to drift across from the west from around 23.00 that limited the loss of warmth into the atmosphere resulting in a very mild night with a minimum of 15.1C logged at 06.13. The high was a significant 9.5C above the May average whilst the low was also a significant 8.6C above average.
Saturday arrived with thin high cloud, being the residual cloud from a weather front that crossed overnight. The sun at first was muted but soon began to shine strongly that resulted in a temperature of 18.1C at 08.00, already 0.7C above the May average maximum.
The Azores high is continuing to build with a barometric pressure of 1026.8mb at 08.00, the highest for a month. The anticyclone will dominate our weather for much of next week, however, as it edges further north and east the wind direction will vary daily, to come from the east by mid-week. The position of the high pressure will mean we will be in a pool of hot air from far south, Iberia and North Africa, that will see temperatures possibly reach heatwave figures for Sunday into Tuesday. The heatwave threshold for Marlborough is 27C, more details below.
What is a heatwave?
A heatwave is an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity.
What is the definition for a UK heatwave?
A UK heatwave threshold is met when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. The threshold varies by UK county.
The Met Office heatwave threshold has been updated ahead of summer 2022. The initial heatwave thresholds were calculated based on the 1981-2010 climatology of daily maximum temperature at the mid-point of the meteorological summer (15 July). The revised thresholds will use the 1991-2020 averaging period introduced in January 2022. The geographical differences reflect the differences in climate across the UK.
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.