A cold front on Sunday spoilt the day

The cold front that slowly descended from the north on Sunday, and lingered over the area, brought 3.8mm of rain and reduced sunshine to 4.4 hours. As a result it was a cool day with the thermometer 1.5C below the August average with a maximum of 19.6C. The wind was very light and variable coming from most points of the compass during the day.

Overnight was also cool with a minimum of 11.1C, the coolest night since 1st July and 0.6C below average.

Monday produced a cloudy start to the day with again very little air movement.

July 2021 Review

Although the summer month of July started well, with two dry days and daytime temperatures around average, it all changed on the 3rd as a depression crossed the country bringing rain and cooler temperatures. As one depression left our islands and another intense depression passed our way on the 5th and 6th bringing stronger winds than of late and heavy rain. During the evening of the 5th and overnight 10.1mm of rainfall was recorded. A second wet spell four days, brought on a south-westerly air stream from the 10th to the 13th, produced 11.7mm of additional rainfall.

However, it all changed again from the 14th as an Azores high pressure system began to take hold, initially ridging across the UK and then building to bring dry, hot days on a light north-easterly breeze. We enjoyed, if rather hot, nine consecutive dry days of real summer weather as the thermometer soared. The heat built steadily over several days with maxima in the high 20s and four days in excess of 31C. On the 18th the thermometer peaked at 31.7C making it the hottest day since 12th August 2020 when 34.1C was recorded. These days were also notable for the very light breeze that often did not gust into double figures.

It was during this period that the Meteorological Office issued its first ever ‘Extreme Heat Warning’. Such warnings highlight the negative impact of very hot weather on people’s health, daily lives and infrastructure.

As the anticyclone relocated into the northern North Sea and filled, pressure began to fall again combined with a depression that arrived from the Atlantic. This saw peak temperatures drop 10C or more and brought rain on the 23rd and 24th. However, although my garden would have liked a greater drink of welcome rain, we were fortunate not to suffer violent thunderstorms that brought flooding further east and lightning strikes that set fire to a house in Andover.

Another disturbed period towards the end of the month brought modest falls of rain, 4.4mm and 4.0mm on the 27th and 28th respectively.

On the 29th Storm Evert arrived in the western approaches being the first named summer storm of the year bringing unseasonably strong winds and heavy rain to the south coast. The wind strength increased, but not dangerously and brought sporadic rain but not heavy.

The month was mild being 1.2C above the 37-year average with the mean maximum 0.7C above average and mean minimum 1.6C above average.

The rainfall for July amounted to 52.8mm, which was 6.8mm below the long-term average. Although we had below average rainfall we missed on the exceptionally damaging rainfall and thunderstorms that other parts of the country experienced. However, this summer month we only enjoyed 13 dry days when the average is 18. The extremes were the exceptionally dry July of 1999 when just 10.1mm was recorded and the excessively wet July of 2007 with 127.2mm.

The diurnal range, the difference between day and night temperatures, has shown a rising trend for the maximum variation that in the 1980’s averaged 18.5C and now has risen on average to 20.5C.

Some fifteen or so years ago I asked a regional weather forecaster why no reference was made to the Jet Stream in the national forecasts that have such an important influence on our weather. I understood that it was believed to be not helpful at that time to include it for general public information.

The past couple of years have seen regular references to the effect of the Jet Stream referred to in daily forecasts.

The jet stream flows high overhead and causes changes in the wind and pressure at that level. This affects things nearer the surface, such as areas of high and low pressure, and therefore helps shape the weather we see. As the Jet Stream speed varies it can make the arrival of depressions more difficult to predict.

There are two Jet Streams that circulate our world, one in the northern hemisphere and one to the south of the Equator. The very strong rivers of wind travel at 70 to 200mph being 100-200 km in width and at great height, 30,000ft or more. These bands of air circulate the globe west to east, the northern Jet Stream starts in North Africa and ends over the UK and Continent.

No knowledge of this phenomenon was know before 1944, when it was discovered accidentally in 1944 as American B29 crews found they were travelling much faster than thought possible. As a result they were overshooting their calculated bombing targets.

The Jet Stream is formed when cold air from the Arctic meets warm tropical air, the greater the temperature difference the greater the speed and more intense storms. It can be straight but can meander and loop, much like a river, along which storms develop.

Put simply, if the Jet Stream is to the south of the UK we are in the cooler air but when it moves to the north of the UK we are positioned in warmer air.

The long-term position of Jet Stream appears to be changing as it looks to be moving northwards. If true, experts predict dramatic changes in our weather with warmer, wetter winters and hotter summers that feature more intense rain.