Royston (Iceni) Weather Station
Daily Weather Observations - December 2001
December 2001 Weather Review
The December 2001 weather in Royston was cold, dry and very sunny.
The month was mostly anticyclonic with high pressure positioned either over Europe or the eastern Atlantic blocking the progress of Atlantic depressions eastwards, and maintaining mostly cold and dry conditions over the British Isles.
The temperature exceeded 10°C on only 3 days (1st, 4th and 5th) in the month and this lack of mild days was a contributory factor in the mean daily maximum temperature being 1.3°C below average at 5.8°C.
The mean daily minimum temperature was 1.1°C, which is 1.9°C below average. It was the coldest December in Royston since 1996.
The highest temperature recorded in the month was 12.9°C on 5th, whilst the lowest temperature recorded was –4.1°C on 15th. The lowest grass minimum temperature recorded was –7.6°C on 31st.
Totals of 13 air frosts and 21 ground frosts occurred in the month. The number of ground frosts was the highest recorded in Royston since the memorable (for very cold and snowy conditions) December of 1981.
The mean earth temperature at 30 centimetres depth as measured daily at 0900 GMT was 5.1°C. This element showed a steady decline throughout the month from a maximum of 9.5°C on 1st to a minimum of 2.8°C on 31st.
The anticyclones which dominated the December weather kept rain bearing fronts at bay for much of the month with the result that it was a rather dry month. Although the number of "rainfall days" (0.2 millimetres or more) was as high as 14, daily rainfall values were generally small. In fact, the wettest day of the month, 4th, produced only 5.0 millimetres rainfall.
Total rainfall in December was 22.7 millimetres, which is 43% of the 1971/2000 average. It was the driest December in Royston since 1991. Rainfall duration during the month was 33.6 hours.
December was mostly snow free in Royston, but small amounts of snow did fall on 22nd, 25th, 29th and 30th. A few flakes of snow/sleet in the morning of 25th mean Christmas Day in Royston will be counted as a day with snowfall, but this certainly did not make it a White Christmas! The light falls of snow on 22nd and 30th gave a light ground cover of snow at 0900 GMT on the following days 23rd and 31st , but depths were negligible on both occasions.
Perhaps the most notable feature of the December weather was the exceptional amount of sunshine. Although 7 days in the month were completely sunless, including a 4 day period of ‘anticyclonic gloom’ from 10th to 13th, the sunshine total for the month still managed to reach 88.8 hours. This value is 189% of the average sunshine for December.
Ten days in December registered 5 hours sunshine or greater. Of these, 8th, 23rd and 31st were the sunniest, each registering 6.6 hours sunshine. A fact worthy of mention is that the December sunshine value was 11.5 hours higher than in November, a month which itself experienced an above average sunshine total (by 16%)!
Winds were very light on quite a few days in December, and on no less than 9 days at the 0900 GMT observation hour it was completely calm with no discernible wind direction. The analysis of daily wind directions at 0900 GMT shows the following frequencies: NORTH 1 day, NE 2, E 0, SE 1, S 3, SW3, W 9, NW 3, and CALM 9.
The mean wind speed in December was 4.1 mph. The highest daily (00/00 GMT) mean wind speed was 12.6 mph on 24th, whilst the lowest daily mean wind speed was (to nearest 0.1 mph) 0.0 mph on 10th and 18th. The maximum wind gust speed recorded in the month was 46 mph, on both 5th and 28th.
The mean air pressure (reduced to mean sea-level) in the month was 1024.9 mB. This is the highest mean air pressure value recorded in December since 1991. The highest air pressure reading in December was 1042 mB on 14th, and the lowest 998 mB on 25th.
The mean relative humidity value in the month was 91.0%. The highest daily (00/00 GMT) mean relative humidity was 99.9% on 11th, whilst the lowest daily mean relative humidity was 72.6% on 28th. The absolute lowest relative humidity reading in December was 56% at 1235 hours on 28th.
A few facts and figures for the year 2001 conclude this report. The mean daily maximum temperature for the whole year was 14.3°C, which is 0.5°C above the 25 Year (1973/1997) average. The mean daily minimum temperature was 6.9°C (+0.4°C). Even with the above average temperatures it was nonetheless the coolest year since 1996.
Total rainfall in 2001 was 752.3 millimetres, which is 129% of the 1971/2000 average. Although 2001 was a wet year it was not as wet as in the year 2000 which had 67.8 millimetres more rainfall. The total sunshine in 2001 was 1652.8 hours, which is 111% of average.
Richard Barker
Richard Barker
Met. Observer, Royston (Iceni) Weather Station
4th January 2002
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