Author: Eric Gilbert

Although the breeze from the northwest was minimal on Thursday with a maximum gust of 7mph, and we enjoyed 4.2 hours of welcome sunshine, the thermometer struggled to reach a maximum of 3.8C being 4.2C below average. By 16.49 the steadily falling temperature had reached freezing.

Wednesday night was very cold but the past night produced a severe frost and saw the thermometer steadily fall from its peak at just after midday to a minimum of -6.7C at 03.18 on Friday morning. This was the coldest night since 3rd February 2019 when an extreme low for this country of -11.3C was recorded.

The thermometer steadily rose after 03.18 as cloud arrived with the humidity rising so that by 08.00 on Friday the temperature was -1.7C under total cloud cover.

The frost has now permeated deep into the ground so that the soil thermometer at a depth of 5cm read -0.2C at 08.00.

December 2020 Review

Although the month began with high pressure and a dry day with 4.4 hours of sunshine, the following week saw a low-pressure system settle over the country and persist. The slack pressure gradient meant days with light winds, gloomy conditions and little sunshine. The early morning of the 4th brought heavy rain that amounted to 10.0mm. The sixth saw very low cloud that at dawn meant visibility was restricted to 500m but as the morning wore on the fog intensified and visibility dropped to 200m.

Throughout this period temperatures, not surprisingly, were depressed, especially when the wind backed into the northeast and then north. The thermometer only reached 2.1C on the 7th being 6.2C below average and the coldest day since 1st February 2019.

Yet again our weather turned upside down during the middle of the month as an intense and large area of low pressure developed in the North Atlantic that brought warm, moist air on south and southwesterly winds with the thermometer rising to its highest in December with a maximum of 12.9C on the 21st being 4.8C above average.

According to the World Meteorological Organization climate change continued its relentless march in 2020, which is on track to be one of the three warmest years on record. The ten-year period 2011-2020 will be the warmest decade on record, with the warmest six years all being since 2015. The WMO state that “Record warm years have usually coincided with a strong El Niño event, as was the case in 2016. We are now experiencing a La Niña, which has a cooling effect on global temperatures, but has not been sufficient to put a brake on this year’s heat”. The average global temperature in 2020 is set to be about 1.2 °C above the pre-industrial (1850-1900) level
Ocean heat is at record levels and more than 80% of the global ocean experienced a marine heatwave at some time in 2020. It has been recently estimated that round 93% of the extra energy accumulating from climate change is stored in the oceans..
The year 2020 has been the warmest year I have recorded since the station began in 1984 with the mean temperature 1.06C above the 36-year average.
Storm Bella was named on the 24th as a deep depression in the North Atlantic that brought wind and rain. The anemometer recorded a gust of 38mph on the 26th, which was the windiest day since 1st November. The depression also brought considerable rainfall with 17.7mm recorded on the 26th being the wettest day since 14th November (26.1mm). As the depression approached the UK winds came from the north, south then west on three successive days.

Up until the last week of December the month had been mild and damp with the mean temperature 1C above the 36-year average. However, on the 28th a cold air mass began to arrive from the north with a maximum of 2.1C and very light, wet snow just before dawn. Another light fall that just about covered the ground fell after dawn on the 29th. The coldest night of the month was logged during the early hours of the 31st when the thermometer dropped to -3.6C being 5.9C below the December average minimum.

The monthly rainfall amounted to 109.1mm being 17.3mm above the 36-year average. There were two particularly wet days on the 13th and 26th with 16.6mm and 17.7mm respectively. Although the mean temperature was well above average for much of the month the very cold spell at the end resulted in a mean temperature 0.09C above average.

There were three days with morning fog and snow was observed on two days, 28th and 29th.

A maximum of only 3.3C on Wednesday, which was 4.7C below average and a minimum overnight of -3.6C being 5.9C below average was due to the run of very cold air from the north to northwest air stream. Fortunately there was little wind strength during the past twenty-four hours, a maximum gust of just 10mph was logged, therefore there was minimal windchill.

The minimum was reached at 00.49 early on Thursday morning and recovered a degree or two during the following hours as cloud and fog formed, the latter limiting visibility to around 300m at 08.00 on Thursday. It was the coldest night since 28th November, just, when the thermometer fell to-3.7C.

Wednesday was dry, only the ninth this month, with the rainfall total standing at 109.1mm, which is 17.3mm above the 36-year average. We have now experienced four successive Decembers when the monthly rainfall exceed 100m (average is 91.8mm).

Winter was fully established on Tuesday with snowfall just after 08.15 and again, a little heavier, at 15.40. In between there was a little sunshine, logged at 0.74 hours, that lifted the thermometer to a maximum of 3.9C, which was 4.1C below average. Although the thermometer dropped 2C in the afternoon snow, there was sufficient warmth in the atmosphere for any lying snow to melt.

The thermometer hovered around 1C for much of the evening but after midnight fell again to a minimum of -0.4C at 02.20 producing a frost, before rising again to reach 0.6C at 08.00 on Wednesday.

The barometric pressure has been slowly rising over the past two days, after the deep depression eased away, so that on Wednesday morning there was minimal cloud and the possibility of some sunshine with the wind very light, sometimes completely still.

The effect of the colder weather can be seen from the soil thermometer at a depth of 5cm with successive readings of 5.9C, 1.3C, 1.2C and 0.9C over the last four days at 08.00.

Under the influence of the cold air mass brought on a northerly breeze that gusted to 18mph, the thermometer struggled to reach 2.1C at 12.01 on Monday being almost 6C below average. After the very early morning light fall of snow no more fell during the daylight hours.

Overnight the wind fell out completely for long periods with the thermometer dropping to -1.2C at 04.54 before lifting to -0.3C at 08.00 on Tuesday.

A few light flakes of snow were observed at first light on Tuesday, not enough to cover the ground. However more light snowflakes began to fall just after 08.15 from a narrow band of precipitation travelling in on a very light northwesterly air movement.

The barometric pressure at 08.00 on Monday was the lowest since November 2010 with a minimum pressure of 969.9mb as the centre of the deep depression was overhead Southern England. By Tuesday a rise of almost 20mb, as the depression eased away and began to fill slowly, gave a current reading of 987.5mb, still very low.

Although the barometric pressure had been sliding for twenty-four hours and was quite low on Sunday we enjoyed 4.8 hours of sunshine, predominantly in the morning. The thermometer eased upwards to reach a maximum of 6.2C in the brisk westerly breeze that was 1.8C below the average.

The thermometer hovered around 1.5C late evening but just after midnight began to fall steadily to reach a low of -0.3C at 04.48 producing a short lived frost.

The first precipitation from a narrow weather front brought snow that started just before 06.00, with a modest layer in evidence, that soon turned to wet snow. Some pavements were icy after the cold night from the first rain and sleet to fall. It was mostly seen on gardens and lawns rather than hard surfaces. However, this quickly turned to rain and any evidence of snow melted as the thermometer rose to 0.7C at 08.00 although snow could be seen at 09.00 on the slightly higher fields rising to Savernake Forest.