Sunday 24th May
Saturday gave us many hours of sunshine that was slightly muted thanks to the very thin, high cloud. This resulted in a slightly lower maximum than on Friday with a peak of 25.9C, being 8.6C above average. The warmth very quickly fell away late evening to register a very cool night, under clear skies, that produced a minimum of 8.2C at 05.08 early Sunday, just after sunrise in Marlborough at 05.04. The low was 1.2C above the average. There was obviously a wide diurnal range of temperature between day and night with a difference of 17.7C.
Sunday arrived with strong sunshine, even though there was again thin, high cloud, that after sunrise had lifted the temperature to 18.5C by 08.00, the warmest start to a new day this month.
The Azores anticyclone is almost overhead the UK today, its centre is just off the east coast with a barometric pressure here this morning of 1031.3mb, the highest pressure for two months. The centre of the high pressure will pirouette around the UK well into next week, continuing the spell of dry and hot weather. As it slowly changes position the wind will vary in direction. Today it will start from the east, rotate clockwise, this afternoon it is likely to come from the west and around midnight have veered further to come from the north, however, at all times it will be very light.
The maximum of 26.8C on Friday was just below the heatwave threshold of 27C for Wiltshire, however, with the ground absorbing more warmth each day, today is likely to see a maximum of at least 27C, which is likely to continue for two more days before the high edges a little downwards during the second half of the week. Thus we might have an official heatwave over the next three days.
At least one place in the UK broke the 30C barrier yesterday. I have trawled through my records to discover the history of years in Marlborough when a maximum of at least 30C was logged at my station. For each of the ten year spans I discovered that in the 1990’s there were three years, the 2000’s had five years, the 2010’s 6 years and now in the 2020’s every year of the last five has seen at least one day that saw a maximum of 30C. A very distinct pattern of an increase of heat over that 30 period of 30 years.
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.
The central map below, shows the initial heatwave thresholds for the UK, with the right hand map showing the updated thresholds for 2022. As a demonstration of our changing climate, we have also included the left hand map that shows what the thresholds would have been using the 1961-1990 climatology.
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.