Royston (Iceni) Weather Station

Daily Weather Observations - December 2005

It was a dry and very sunny December in Royston, whilst temperatures were marginally below average.

The high amount of sunshine in what is on average the dullest month of the year was one of the most noteworthy features of the December weather. Despite 4 days without any sunshine at all the month’s sunshine total nonetheless reached 95.9 hours, which is 201% of the 1971/2000 average.

The sunniest day of the month was 17^th with 6.6 hours sunshine. More than 6 hours sunshine was recorded on 4 other days in the month, namely 4^th, 9^th, 11^th and 19^th. It was the sunniest December since at least 1998.

December was also a dry month with the only appreciable rainfall being recorded on 30^th, a 24 hour amount of 9.5 millimetres. Total rainfall in the month was 22.6 millimetres, which is 43% of the 1971/2000 average. The number of "rain days" (0.2 millimetres or more) was 12, whilst on 6 of these days rainfall equalled or exceeded 1 millimetre. It was the driest December since 1991.

The first snowfall of the season occurred on 27^th giving a light covering at first but was later topped up to 2 cms depth. Further snowfall occurred on 30^th but this was soon overtaken by advancing milder air and turned to rain. Snow was lying (ground more than half covered with snow at the 0900 GMT observation hour) on each of the 4 days 27^th/30^th.

The first half of December was virtually frost free, but the second half of the month was colder, particularly by night, and a cold spell occurred just before the end of the month. The lowest night minimum and grass minimum temperatures of the month were both recorded on 28^th, -4.2°C and –9.5°C respectively. This latter reading was the lowest grass minimum temperature in the whole year. Totals of 7 air frosts and 16 ground frosts were recorded in the month, these being the highest totals in December since 2001.

The highest temperature recorded in December was 10.5°C on both 15^th and 16^th. This is the lowest absolute maximum temperature in any December since 1981.

The lowest day maximum temperature in the month was 0.5°C on 28^th. The highest night minimum temperature was 7.9°C on 16^th. The mean daily maximum temperature in December was 7.1°C, which is 0.1°C below the 30 Year (1973/2002) average. The mean night minimum temperature was 0.4°C below average at 2.6°C. This is the second consecutive month with a below average mean temperature, the first time that such a sequence has occurred since October/November 1998. Overall it was the coldest December in Royston since 2001.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 5.1°C. The highest and lowest earth temperatures in the month were 6.4°C and 3.5°C on 3^rd/4^th and 20^th respectively.

The mean wind speed in December was 2.4 mph. The highest daily (00/00 GMT) mean wind speed was 8.0 mph on 16^th, whilst the lowest daily mean wind speed was 0.0 mph (to nearest 1 decimal place) on 28^th. The highest wind gust recorded in the month was 48 mph at 1135 GMT on 30^th.

Winds with a southerly or westerly component prevailed during the month as illustrated by the following analysis of wind directions at 0900 GMT daily: NORTH 4 days, NE 1, E 0, SE 1, S 7, SW6, W 6, NW 3 and CALM 3.

The mean MSL air pressure in December was 1017.1 mB, which is 3 mB above the 30 Year (1973/2002) average. The highest and lowest air pressure readings in December were 1041 mB and 976 mB on 12^th and 2^nd respectively.

The mean relative humidity in December was 86.0%, ranging from a highest daily (00/00 GMT) mean value of 98.5% on 29^th to a lowest daily mean value of 72.8% on 17^th. The absolute lowest relative humidity reading in the month was 48% at 1655 GMT on 16^th and 1255 GMT on 17^th.

No fog (visibility < 1000 metres @ 0900 GMT) was observed during December.

Looking at the Year 2005 as a whole it was a warmer than normal year, but it is interesting to note that the annual mean temperature declined slightly for the 3^rd successive year after a 30 Year ‘high’ of 11.5°C in 2002. However, the annual decreases in mean temperature over the last 3 years have been only by 0.1°C each year and the mean temperature this year was still almost 1°C above the 30 Year (1973/2002) normal. The 2005 mean daily maximum and minimum temperatures were 14.9°C (+0.9) and 7.4°C (+0.7) respectively, rainfall was 499.2 millimetres (86%), and sunshine was 1684.3 hours (112%). It was the driest year since 1997.

Met. Observer, Royston (Iceni) Weather Station

5^th January 2006

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