Royston (Iceni) Weather Station
December 1999 Weather Review
The year finished with an exceptionally sunny December but also a wet one; the mean temperature was close to the average, however.
The sunshine total in December of 81.2 hours was some 70% above normal and only 0.8 hours less than in November. More than 5 hours sunshine was recorded on no less than 8 days, with a maximum duration of 6.4 hours on both 9th and 19th. The high sunshine total was achieved despite 7 days in the month being completely sunless.
With regard to the month’s rainfall it was the wettest December in Royston since 1993. Measurable rainfall (0.2 millimetres or more) fell on 21 days and the rainfall total was 73.2 millimetres (2.88 inches), which is 139% of the 35 Year (1961/1995) average for Royston. The wettest day of the month was 22nd with 12.9 millimetres (0.51 inches) rainfall, closely followed by 24th with 12.7 millimetres. Rainfall duration for the month was 69.8 hours.
The first snowfall of the season occurred in the late evening of 14th giving a light snow cover of 0.5 cms depth at the 09 GMT observation hour on 15th. Although a little further snowfall followed on 15th the snow cover was not sustained and had largely disappeared by the next day. A further light snowfall occurred in the early morning of 21st but this quickly thawed.
As mentioned at the beginning of this report the mean temperature in December was about average, but this concealed temperatures which were slightly higher than average by day and slightly lower than average by night. The mean day maximum temperature was 7.4°C (45.3°F) which is 0.3°C above the 25 Year (1973/1997) Royston average. The mean night minimum temperature was 0.4°C below the average at 2.6°C (36.7°F).
Although these maxima and minima are traditionally referred to in ‘day’ and ‘night’ terms it is worth noting that the highest temperature does not always occur during the day or the lowest temperature at night, particularly in the Winter! They are 24 hour (09/09GMT) readings taken at 09 GMT, with the maximum temperature being credited to the previous day and the minimum temperature being credited to the day of the reading.
The highest temperature recorded in December was 12.0°C (53.6°F) on both 3rd and 24th. This is the lowest maximum temperature recorded in December since 1981 (but equalled in 1996). At the other end of the scale the maximum temperature on 19th was only 0.6°C.
After several near misses the first air frost of the Autumn/Winter season finally occurred on 14th but then only with a minimum temperature of –0.1°C! Six further air frosts occurred by the end of the month, however, and the lowest temperature recorded was –4.5°C (23.9°F) on 20th. This was the lowest temperature recorded in Royston for nearly 2 years, since 1st February 1998, in fact. Air frost duration in December was 70.3 hours.
The lowest grass minimum temperature recorded was –8.0°C (17.6°F), also on 20th. A total of 13 ground frosts occurred in December.
The mean earth temperature at 30 cms (1 foot) depth as measured daily at 09 GMT was 5.0°C (41.0°F). The highest earth temperature recorded was 7.9°C (45.5°F) on 1st and the lowest 2.7°C (36.9°F) on 22nd.
The mean wind speed in December was 7.5 mph; the highest and lowest daily (00/00h) mean wind speeds were 14.8 mph on 6th and 0.3 mph on 18th respectively. The maximum wind gust speed was 55 mph on both 3rd and 23rd.
Daily wind directions at 09 GMT were N 1, NE 0, E 0, SE 0, S 4, SW 13, W 9, NW 2, CALM 2.
Mean sea-level air pressure was some 9 mb below average at 1006.2 mb. The highest air pressure reading observed was 1028 mb on 20th and the lowest 976 mb on 24th and 25th. The mean relative humidity throughout the month was 87.6%, ranging from a highest daily mean of 98.0% on 31st to a lowest daily mean of 75.8% on 3rd.
Mean cloud cover as observed daily at 09 GMT was 56%.
With regard to the year 1999 the main talking point will undoubtedly be the mean temperature. With the possibility of global warming being very much in the news, Royston has enjoyed its warmest year since 1990 when the mean temperature was higher than in 1999 by only the narrowest margin (less than 0.1°C). These two years have been the warmest in Royston since at least 1972. The mean temperature in 1999 was 11.4°C (52.5°F), which is 1.2°C above average. The 7 warmest years (those years with a mean temperature of 11.0°C or above) in the last 27 years in Royston have all occurred in the last 11 years, giving a clear indication of the rise in temperatures in the last decade.
1999 was a very sunny year in Royston with a sunshine total of 1705.3 hours which is approximately 215 hours above average. With the wet December the annual rainfall crept just above the average (by 3%), registering a total of 601.3 millimetres (23.67 inches).
Richard Barker
Richard Barker
Met. Observer, Royston (Iceni) Weather Station
5th January 2000
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