The new month began with depressed temperatures and minimal rainfall. The weather fronts that passed over produced very light, brief rain of drizzle on several occasions. Under a clear sky during the night of the 5th/6th the thermometer dropped to -1.3C producing a brief but moderate air frost.
A change in wind direction from the 6th saw the breeze come from a southeasterly direction that resulted in temperatures rising thanks to the warmer air being wafted in from a very warm Continent. The warmth reached a peak of 24.1C on the 8th making it 9.8C above the early April average. This was the third highest April maximum I have recorded since my station started in 1984, after 26.2C in 2011 and 26.5C in 2018.
The following day brought matters back to normality with a drop of almost 10C.
The night of the 14th/15th brought a significant change from previous nights as it was the warmest since early November. The thermometer dropped very slowly after its peak to around 11C at 18.00 and hovered around there all evening until a low of 11.1C was logged for perhaps half an hour then edged upwards a fraction to remain around 11.7C all night. There was no fall off in temperature as is usual overnight, due to the thick, low cloud and the steady drift of a light wind from the south bringing the warmer air stream. This was the result of a warm front that crossed the UK in the early hours.
The centre of the high pressure built over the weekend of the 18th and 19th and then drifted slowly northwards over the following few days to settle between Iceland and Scandinavia. This resulted in the wind veering into the north on the 19th and then towards the Northeast and east from the 20th. This brought a much cooler air stream originating from around Iceland before travelling over Scandinavia around the eastern flank of the anticyclone.
A significant feature during the third week was low humidity due to the continuing strong, drying wind and lack of rain. The humidity level during recent afternoons had fallen low, day but day, and recently had been exceptionally low. The minimum on the three days from the 20th was 46%, 53% and 36.6% respectively at 15.25. During the afternoon of the 25th a minimum of just 23.9% was logged.
The very low humidity was due to the minimal rainfall at the beginning of the month and a drying, easterly wind under increasing warmth during the third week and 21 totally dry days. An equivalent loss of rainfall due to evaporation from ground sources exceeded 4mm almost every day from the 23rd to the end of the month.
The total rainfall for April was just 15.6mm making it the sixth driest April I have recorded since 1984, being just 24% of my 42-year record or minus 43mm. That is only part of the story as there was an equivalent loss of rainfall, due to evaporation from ground sources and plant life, of 85mm. It was no wonder the gardens were very dry!
The total rainfall for the period January to April was 346mm due to the very wet January and February. There was a very dry year for that period in 1997 when just 137mm was recorded and, by contrast, in 2014 a massive total of 495mm was recorded.
The mean temperature for April was 1.3C above my long-term average. Analysing that figure, I found that the mean daytime temperature was +2.0C whilst the mean nighttime temperature was +0.5C.
A total of three air frosts were recorded when the 42-year average is 5. Looking through the records, I found that there were 21 air frosts in the cold April of 2021 and zero air frosts in the two years of 2011 and 2018.
Double Double at Cirencester’s Barn Theatre
