Royston (Iceni) Weather Station
Daily Weather Observations - December 2000
December 2000 Weather Review
Temperatures, rainfall and sunshine were all above average in December.
Despite December ending on a very cold note, the persistent mildness of the first fortnight ensured that the monthly mean temperature was once again above normal, increasing the number of months with an above average mean temperature to 38 out of the last 48 months (i.e. since 1st January 1997).
The first air frost of the Autumn/Winter season did not occur until a very late 26th December, but this then ushered in a very cold spell which lasted to the end of the month, during which time air frost was recorded every day and the temperature remained below 0°C for a total of 110.2 hours. The maximum temperature achieved on 29th was –1.1°C, this being the first "ice day" in Royston since 10th January 1997.
The lowest minimum and grass minimum temperatures of the month were –5.1°C and –8.5°C respectively, both recorded on 29th. A total of 9 ground frosts was recorded in December.
In contrast to these low temperatures a night minimum temperature as high as 12.3°C was recorded on 12th. The highest temperature of the month was not much higher than this at 13.5°C on both 11th and 12th. The mean daily maximum temperature was 0.6°C above the 25 Year (1973/1997) Royston average at 7.7°C, whilst the mean night minimum temperature of 4.3°C was 1.3°C above average.
The mean earth temperature at 30 centimetres depth was 6.7°C. The earth temperature reached a minimum of 3.3°C on 31st during the cold spell, but would have fallen lower without the snow cover acting as an effective insulator. The earth temperature peaked at 9.3°C on 13th.
The most significant snowfall in Royston for almost 5 years occurred during the early morning of 28th when a relatively narrow band of snow crossed the country from the west. This snowfall gave a level depth of 8 centimetres of snow at the 0900 GMT observation hour on 28th. This snow depth reduced only slightly before a thaw accompanied by rainfall set in on 31st.
December rounded off a very wet year (details below) in Royston with above average rainfall, but the month was not as wet as any of the 3 Autumn months. Total rainfall in December was 66.7 millimetres, which is 126% of the 35 Year (1961/1995) average. The wettest day was 7th with 12.1 millimetres rainfall. A more significant rainfall event occurred on 23rd/24th but in this instance the 17.3 millimetres rainfall was spread over two rainfall days.
The number of "rain days" (0.2 millimetres or more) in the month was 20, whilst the number of days with 1.0 millimetres or more was 13. Snowfall was recorded on 2 days, and snow was lying (ground more than 50% covered at 0900 GMT) on 4 days. Rainfall duration in December was 96.0 hours.
The first half of December was relatively sunny, and by 18th the cumulative total of sunshine was only 2 hours short of the average for the whole month. Just when a very sunny December appeared in prospect a period of 6 consecutive days from 19th/24th occurred without a single ray of sunshine! The following 3 days could only manage 2.2 hours sunshine between them, but fairly sunny days from 28th/30th during the cold and snowy spell increased the sunshine total for the month to 63.6 hours, which is 135% of the average.
The fact that December finally registered a handsome excess of sunshine is perhaps surprising given that a total of 11 days in the month registered no sunshine at all. The sunniest day of the month was 3rd with 6.8 hours sunshine, and the next sunniest 15th with 6.5 hours.
One of the most significant aspects of the December weather in Royston was undoubtedly the gale which occurred in the early hours of 13th. At 0205 GMT a wind gust of 78 mph was recorded, this being the highest such wind speed recorded since the anemometer was installed at Royston (Iceni) Weather Station 3 years ago.
The mean wind speed for the whole of December was 5.9 mph. The highest daily (00/00 GMT) mean wind speed was 16.5 mph on 13th, whilst the lowest daily mean wind speed was 0.3 mph on 23rd.
As regards the daily wind directions as observed at 0900 GMT these were NORTH 1, NE 2, E 2, SE 6, S 6, SW 7, W 5, NW 0 and CALM 2.
As in November, the monthly mean MSL Air Pressure was very low, the mean of 1004 mB being the lowest in December since 1981 (but was equalled in 1993). The absolute highest and lowest air pressure readings were 1018 mB on 21st and 979 mB on 8th respectively.
The mean relative humidity in December was 90.9%. The highest daily (00/00 GMT) mean relative humidity value was 98.6% on 22nd and the lowest mean relative humidity value was 76.9% on 13th. The absolute lowest relative humidity reading recorded was 63% at 0215 GMT on 13th (during the gale!).
Fog (visibility below 1000 metres) was recorded at 0900 GMT on 2 days in December, 17th and 22nd. On 17th, the fog was thick with visibility falling below 200 metres at 0900 GMT.
To conclude this report, the year 2000 was very wet, and both warmer and sunnier than average, the latter element by only a modest amount, however. The mean daily maximum temperature for the whole year was 14.5°C, which is 0.7°C above average. The mean daily minimum temperature was 1.1°C above average at 7.6°C. Total rainfall was 821.0 millimetres (141 % of average), making 2000 the wettest year in Royston since 1958. Additionally, only 1937 and 1903 were wetter in the Royston rainfall record going back to 1853. The number of "rain days" in the year was 193. Total sunshine in the year 2000 was 1562.2 hours, which is 105% of normal.
Richard Barker
Richard Barker
Met. Observer, Royston (Iceni) Weather Station
4th January 2001
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