Royston (Iceni) Weather Station

Daily Weather Observations - December 2007

December was a sunny and dry month with temperatures marginally above average.

The most significant feature of the month’s temperature regime was the minimum temperature of –6.1°C on 20^th. This was the lowest temperature recorded in any month for almost 11 years, since 3^rd January 1997 (although this temperature was equalled on 18^th February 2003). At the other end of the scale the highest temperature of the month was 14.1°C on 6^th. This value has been exceeded in December in 7 years since 1972, but only 1 of these years was in the last 13.

The mean daily maximum temperature in December was 7.8°C, which is 0.7°C above the 30 Year (1976/2005) average. The mean night minimum temperature of 3.1°C was similarly above average, but only by 0.1°C. The lowest grass minimum temperature in December was –9.8°C on 20^th. Totals of 7 air frosts and 12 ground frost occurred in the month.

The lowest day maximum temperature of the month was 2.0°C on both 19^th and 20^th, whilst the highest night minimum temperature was 12.0°C on 5^th. It was the coldest December since 2005 only, the very mild conditions of the first week not being maintained for the rest of the month.

The mean earth temperature at 30 cms depth as observed at 0900 GMT daily was 6.0°C. The highest and lowest earth temperature values in December were 8.9°C and 4.0°C on 7^th, and 21^st and 24^th, respectively.

December was a rather dry month with, for example, only 0.6 millimetres rainfall being recorded in the two week period 10^th/23^rd inclusive. Total rainfall for the month was 41.9 millimetres, which is 79% of the 30 Year (1971/2000) average.

The number of "rain days" (0.2 millimetres or more) in December was 12, with rainfall equalling or exceeding 1.0 millimetre on 10 of these days. The wettest day of the month was 24^th with 7.7 millimetres rainfall. It was the driest December since 2005 only.

Snow, hail and thunder were all absent in December.

It was a very sunny December in Royston and the month’s 30 Year (1971/2000) average had been exceeded by 16^th. The rest of the month was not quite as sunny but by the end of the month the accumulated sunshine total of 74.8 hours was nonetheless 57% above average. The highest daily sunshine value was 6.7 hours, on both 11^th and 23^rd. The number of completely sunless days in December was 6. It was the sunniest December in Royston since 2005 only.

December commenced on a windy note with the highest gust speed of 55 mph occurring on 1^st (also 53 mph on 7^th). The mean wind speed over the whole month was 6.6 mph. The highest daily (00/00 GMT) mean wind speed was 13.9 mph on 2^nd, whilst the lowest daily mean wind speed was 0.2 mph on 14^th.

Winds with S & W components predominated in the month as illustrated by the following analysis of wind directions at 0900 GMT daily: NORTH 0 days, NE 3, E 1, SE 3, S 9, SW 8, W 5, NW 1, and CALM 1.

The mean MSL air pressure in December was 1017.3 mB, which is 3 mB above the 30 Year (1976/2005) average. The highest and lowest air pressure values recorded in December were 1039 mB and 974 mB on 14^th and 9^th respectively. These two extremes were separated by a continuous rise in pressure from 9^th to 14^th.

The mean relative humidity in December was 91.1%. The highest daily (00/00 GMT) mean relative humidity was 99.5% on 23^rd, whilst the lowest daily mean relative humidity was 79.7% on 7^th. The absolute lowest relative humidity reading in December was 49% at 1311 GMT on 15^th.

Fog (visibility < 1000 metres) was observed at the 0900 GMT observation hour on only 1 day (19^th) in December. On this occasion visibility was reduced to below the ‘thick fog’ threshold of 200 metres.

YEAR 2007: It was the 3^rd equal (with 1990, 1999 and 2003) warmest year since records began in 1972, the wettest year since 2002, and the dullest year since 2005 only. The mean daily maximum temperature in 2007 was 15.1°C, which is 1.0°C above average. The mean night minimum temperature was 0.8°C above average at 7.6°C. Total rainfall was 662.1 millimetres, which is 14% above average, whilst sunshine was 13% above average at 1701.0 hours.

Compared to the 30 Year (1971/2000) average annual rainfall of 582.4 millimetres, the rainfall over the last 10 years (1998/2007) has averaged 648.3 millimetres. However, in the previous 10 year period (1988/1997) the average rainfall was considerably less at 552.2 millimetres.

Richard Barker

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

6^th January 2008

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