Royston (Iceni) Weather Station

Daily Weather Observations - December 2008

A very sunny and dry December was experienced in Royston, but it was colder than average.

The mean temperature in December was 1.1°C below average, which resulted in the month being the coldest December since 2001. The last week of the month was particularly cold with night air frosts and daytime temperatures struggling to reach more than 1-2°C.

The mean daily maximum temperature in December was 6.1°C, which is 1.0°C below the 30 Year (1976/2005) average. The mean night minimum temperature was similarly below average (-1.2°C) at 1.8°C.

The highest day maximum temperature in the month was 13.1°C on 20^th, whilst the lowest night minimum temperature was –3.3°C on 30^th. The lowest grass minimum temperature was –7.5°C, also on 30^th. Totals of 12 air frosts and 18 ground frosts were recorded in December, these being the highest such totals in any December since 2001.

On 31^st the lowest day maximum temperature of the month occurred, this being only 0.6°C. In contrast the highest night minimum temperature in December was 10.1°C on 20^th.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 5.3°C. The highest and lowest earth temperature values in the month were 7.4°C and 3.7°C on 24^th and 12^th respectively.

December was a dry month, particularly from 14^th onwards when rainfall to the end of the month totalled only 0.6 millimetre. The month’s rainfall total was 22.2 millimetres, which is 42% of the 1971/2000 average. It was the driest December in Royston since 1991.

The number of "rain days" (0.2 millimetres or more) was 11, with rainfall exceeding 1.0 millimetre on 6 of these days. The wettest day in December was 13^th with 9.9 millimetres rainfall. Rainfall duration in the month was 26.0 hours. After the snowfalls of October and November this year, December was snow free.

December commenced on a very sunny note and the 30 Year (1971/2000) normal for the month had been exceeded by 11^th. The rest of the month was not quite as sunny and contained all of the completely sunless (7) days of the month. However, total sunshine for the month of 88.5 hours still showed an 86% excess over the average.

The sunniest day of the month was 10^th with 6.8 hours sunshine, closely followed by 7^th with the same numerical value to 1 decimal point, but with 2 minutes less sunshine. It was the sunniest December in Royston since 2005.

The analysis of daily wind directions as observed at 0900 GMT reveals that winds with southerly and westerly components dominated the month, which is somewhat surprising given that December was a colder than average month. However, cold air from the Continent can arrive on southerly winds, and cold polar air on north-westerly winds, and this is basically what produced the colder spells in December. The wind direction analysis shows NORTH 1 day, NE 2, E 3, SE 0, S 5, SW5, W7, NW 6 and CALM 2.

The mean wind speed in December was 4.6 mph, and the maximum wind gust speed recorded was 46 mph on 19^th. The highest daily (00/00 GMT) mean wind speed was 11.5 mph on 20^th, with a maximum gust speed of 44 mph on that day. The following day, 21^st, was also windy with equivalent figures of 11.0 mph and 39 mph respectively. The lowest daily mean wind speed was 0.1 mph on 30^th.

The mean MSL air pressure for the month was 1015.6 mB, which is 1.5 mB above the 30 Year (1976/2005) average. The highest and lowest air pressure values in December were 1038 mB and 975 mB on 26^th and 4^th respectively.

The mean relative humidity in December was 94.9%, with a highest daily (00/00 GMT) mean relative humidity of 99.5% on 14^th and 15^th, and a lowest mean value of 87.3% on 19^th. The absolute lowest relative humidity reading in the month was 54% at 1350 GMT on 19^th.

Fog (visibilty < 1000 metres @ 0900 GMT) was not observed on any day during December.

A few brief details for the Year 2008 conclude this report. The mean daily maximum temperature for 2008 was 14.6°C, which is 0.5°C above the 30 Year (1976/2005) average. The mean night minimum temperature over the whole year was 0.4°C above average at 7.2°C. The annual rainfall was 648.3 millimetres (111% of 1971/2000 average) and annual sunshine was 1687.6 hours (112% of 1971/2000 average). With the exception of 2003 and 2005 all years from 1998 to date have been wetter than normal. It was the coldest year since 2001.

Richard Barker

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

7^th January 2009

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