Another sharp frost for three hours overnight

The wind on Saturday was light and variable from SSE to NW. The pool of cold air was still in position so the maximum of 11.4C was well below average, in fact almost 6C below the 37-year average. A light shower just after 13.30 amounted to 0.7mm.

The thermometer dropped below zero for over three hours overnight with a minimum of -1.1C at 05.26 on Sunday, which was 8C below average.

Sunday saw the sun in evidence shortly after dawn that lifted the temperature to 5.7C at 08.00. The wind continues to be light.

April 2021 Review

The month began with cool north-easterly winds that meant daytime and night time temperatures were below average, but the days were dry. However, by the 6th the weather turned from Spring back to Winter as an Arctic blast arrived brought on strong northerly winds.

All six days saw the wind strong, gusting regularly over 20mph with gusts to 27mph and 28mph on the 1st and 6th respectively. Wind chill became a factor when outside due to the wind strength, low temperatures and low humidity. Often it felt at least 2C colder outside than indicated on the thermometer.

Whilst mentioning wind chill I have discovered that there is no official definition of that feature, and the way it is measured varies around the globe. In the UK, a system called the Joint Action Group for Temperature Indices is used. The ‘feels like’ temperature measures the expected air temperature, relative humidity and the strength of the wind at 1.5 metres (human height) along with a formula that provides an understanding of how heat is lost from a person’s bare face at a walking speed of 3 metres per hour during cold and windy days.

The air from the Arctic was also unusually dry. The humidity in April at 08.00, when my measurements are taken, is usually around 90%. At that time on the 5th, 6th and 7th humidity of 67%, 73% and 79% was measured. During the daytime of the 5th and 6th it dropped as low as 26% and 44% respectively.

Another aspect of winter was observed on the 5th as ice pellets fell from the sky formed when snowflakes start to melt as they drop from a cloud, then fall through sub-freezing air, where they refreeze into grain-like particles. They are generally smaller than hailstones and bounce when they hit the ground. During the afternoon of the 6th a number of showers occurred that initially were very fine snow grains floating in the air but late afternoon two showers produced light, small snowflakes.

Snow in April is not unheard of as the last year when it was observed at my station (that started in 1984) was in was 2017. However, three separate days in April with snow observed is rare and has been recorded at this station previously on just one occasion, which was in 1998.

An intense anticyclone settled over the UK from 11th that trapped beneath it a pool of very cold air as a result of the Arctic blast of the previous week. Maximum temperatures were well below average with just 5.9C on the 10th being 8.4C below the 37-year average. There were several very cold nights when a hard frost set in with -3.9C and -2.9C in the early mornings of the 13th and 15th respectively.

Mid-April was a difficult time for gardeners with cold winds producing wind chill during several days and night frosts, five in succession from the 11th to 15th, as a result the ground remained cold. My soil thermometer positioned at a depth of 5cm registered just 3.4C at 08.00 on the 15th. Any tender plants not protected outside would have been severely frosted and killed off. These first fifteen days in April produced just one day and one night when the maximum temperature was above average.

By this halfway point in the month (15th) just 7.4mm of precipitation had been recorded when the 37-year average rainfall for April stood at 58.5mm.

It became slightly warmer on the 18th as the thermometer crept above the average maximum for only the second day in the month, but only slightly warmer (+1.3C).

Welcome rain fell mainly overnight of the 27th/28th that broke the dry spell of 15 consecutive days without rainfall.

The period from the 11th to the 20th brought 10 continuous nights with an air frost. During the course of the month we experienced 16 nights with an air frost when the average number of air frosts in April over the past 37 years is 5.2, so April 2021 was exceptional. Looking at my records since this station opened in 1984, I note that the greatest number of air frosts that occurred during the month of April was 10 in 1984, 1997 and 2013. It must be noted that the balmy month of April 2018 did not see even one air frost.

The exceptional number of frosty nights was due to the high pressure settling close to the country for long periods with its position producing winds from the north, northeast and east. The anticyclone also meant many clear nights so there was no blanket of cloud to provide a duvet to minimise warmth escaping from the ground into the atmosphere. There was an additional component of the weather that influenced this record breaking month, going back at least 60 years according the Metrological Office, and that was the lack of rainfall. As the month progressed the soil became ever drier that allowed for more radiative cooling.

Daylight hours of the 28th were unpleasant as the thermometer did not rise above 9.1C being 5.2C below the average maximum with wind chill in the morning making it feel up to 2C cooler outside. Sporadic rain also fell in the morning with more continuous light rain for much of the afternoon as the slow-moving depression rotated over the area.

The mean temperature for April of 6.2C was 2.6C below the 37-year average of 8.8C. Analysing the detail I find that the average maximum was 1.2C below average, but not surprising with the numerous frosty nights, the average minimum was a significant 4.1C below average and a record for this station set up in 1984. The Meteorological Office stated that it was the lowest average minimum for April since 1922 and the third lowest since records began in 1884. The contrasting years for April were an average of 5.75C in 1986 and 12.24C in April 2011.

As humans we have to adapt to varying temperatures by day and night. It is interesting to note that the diurnal range (variation between maximum and minimum) for April has been slowly increasing from an average of almost 18C in the 1980’s to approximately 19.5C in the 2020’s. April 2021 brought extremes with a range of just 2.8C between day and night on the 10th and 21.0C on the 19th (17.8C by day and -3.2C by night).

The breakdown of wind direction for each day will not be a surprise after such a cold month as the wind came from the north to east quadrant for 22 days and from the much warmer quadrant of west to south on just 5 days.

Another notable feature of the past month was the long period of 15 days from 12th to 26th without rainfall. In total there were 24 days without measurable rainfall, the average over 37 years was 15 days.

Precipitation for April totalled 25.7mm, which was 32.8mm below the average. The wettest day occurred on the 28th with 8.6mm. Set against this minimal rainfall was the fact that the equivalent rainfall of 67.2mm was lost through evaporation from ground sources and plant life.
It was announced on Earth Day (22nd April) that the Meteorological Office and Microsoft are planning a world-leading computer that will take weather and climate forecasting to the next level and help the UK stay safe and thrive. This new supercomputer – expected to be the world’s most advanced dedicated to weather and climate – will be in the top 25 supercomputers in the world and be twice as powerful as any other in the UK. The data it generates will be used to provide more accurate warnings of severe weather, helping to build resilience and protect the UK population, businesses and infrastructure from the impacts of increasingly extreme storms, floods and snow. That is reassuring!