Thursday 23rd April
There were four significant features, meteorologically, on Wednesday that dominated our weather, wind strength and direction, sunshine, evaporation and humidity levels. The persistent strong wind, gusting to 24mph, meant that it felt cool out in the wind, however, the virtual wall to wall sunshine pushed the maximum temperature above average with a peak of 16.8C at 14.38, before thin cloud drifted across after 15.00, limited any further rise. The high was 2.5C above average. The past night was not quite so cold with a minimum of 4.6C at 04.32. This made it the warmest night since the 18th and was 0.7C above the April average.
As occurred on previous nights, cloud drifted in from the North Sea in the early hours that meant another cloudy start to the new day on Thursday. By 08.00, there was brightness as the sun struggled to breakthrough the thin, high cloud and had lifted the temperature to 6.4C, with the first glimpse of sun observed at 08.27.
The third significant feature from Wednesday was the increased equivalent loss of rainfall due to evaporation into the atmosphere from ground sources and plant life. The daily loss was 3.8mm, the highest since 25th August during one of the very hot days last year. There is no indication of rainfall in the forecast well into next week, which will mean April will be a very dry month with just 15.4mm of rainfall to date, which at the moment is 41mm below my 42-year average. The equivalent loss of rainfall this month now totals 55mm.
The fourth significant feature on Wednesday was due to the continuing strong, drying wind and lack of rain. The humidity level during recent afternoons has been falling low, day but day, and recently has been exceptionally low. The minimum on the last three days has been 46%, 53% and yesterday 36.6% at 15.25.
The high pressure that was centred near Iceland is very slowly relocating a little further south, which will mean subtle differences in our weather up to the weekend and into next week, as it becomes almost stationary over the UK. The fine, dry and sunny weather is likely to continue up to Sunday and possibly into next week with the easterly wind still present but falling much lighter. The forecast surface pressure charts indicate that a high pressure will develop near the Azores and attempt to link up with the UK high pressure system. The result will more fine, sunny and dry weather. The barometric pressure at 08.00 read 1029.6mb, the highest this month and still rising steadily.
A press article from the Met Office helps understand the dry air.
Why the air is so dry
One of the most striking features of the current setup is how dry the air has become. This is largely down to the direction from which our air is arriving.
Rather than coming from the Atlantic, where air typically picks up moisture, winds are drawing air in from the east and north-east. Much of this air has travelled across the near continent or even descended from higher levels over Scandinavia. As it sinks towards the surface, it undergoes compression, warming as it does so through a process known as adiabatic heating.
The effects of very low humidity
Very dry air can feel pleasant, but it does have some noticeable effects. One is an increased likelihood of static electricity. With little moisture in the air, static charge is less able to disperse, making static shocks more likely when touching household objects, car doors, or metal surfaces.
While this is more commonly experienced in winter, the unusually dry conditions this week mean it may occur more frequently than normal for the time of year.
Dry air also allows temperatures to rise more readily during the day under sunshine, while clear skies at night can allow temperatures to fall further, increasing the day-night temperature range.