Royston (Iceni) Weather Station

Daily Weather Observations - December 2006

Temperatures, rainfall and sunshine were all above average in December in Royston.

The first half of December was very mild with the highest temperature (14.3°C) of the month occurring on 4^th. This was the highest absolute maximum temperature in December since 1994. However, a dull and cold anticyclonic spell which set in just after mid-month reversed this trend for 10 days or so before milder weather returned for the last week of the month.

The mean daily maximum temperature in December was 9.0°C, which is 1.9°C above the 30 Year (1976/2005) average. The mean night minimum temperature of 4.9°C was similarly 1.9°C above average. It was the mildest December in Royston since 1988.

The lowest night minimum temperature in December was -4.0°C on 19^th. At the other end of the scale the highest night minimum temperature was 13.0°C on 4^th/5^th. The lowest grass minimum temperature of the month was –7.1°C on 19^th. Totals of 5 air frosts and 8 ground frosts were recorded in December.

The cold spell referred to above produced several low day maximum temperatures, 1.3°C and 1.6°C on 19^th and 21^st respectively, followed by the lowest day maximum temperature of the month of 1.2°C on 22^nd.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 7.2°C, the highest earth temperature recorded being 9.3°C on 2^nd, and the lowest 4.7°C on 21^st.

Half of the December rainfall fell on the last 3 days of the month to bring the month’s total up to 70.7 millimetres, which is 134% of the 1971/2000 average. The number of "rain days" (0.2 millimetres or more) in the month was 18 with rainfall exceeding 1.0 millimetre on 11 of these days. The wettest day of the month was 30^th with 17.9 millimetres rainfall. Rainfall duration in December was 31.0 hours. It was the wettest December since 2002.

No snow was observed to fall in December making it the 9^th snow free December since 1972.

Despite 12 days with no sunshine at all occurring in December it was still the 4^th consecutive December with sunshine significantly above average, although the month’s total this year was the lowest in this sequence. Total sunshine was 67.4 hours which is 141% of the 1971/2000 average. This figure was achieved despite the 7 day dull spell from 21^st to 27^th inclusive which produced only 3 minutes sunshine, and this at 1515 GMT on the last day of this period!

The December wind regime was noteworthy at both ends of the scale. First came the remarkable stillness of the air from 19^th to 26^th with 6 days in this 8 day period producing mean wind speeds of 0.0 mph (to nearest 1 decimal place), the exceptions being 20^th and 24^th with mean wind speeds of 0.1 mph! Just as remarkable were the maximum gust speeds of only 2 mph on both 19^th and 22^nd. Then on 30^th the month’s highest wind gust speed of 65 mph occurred at 1750 GMT. This was the highest gust speed recorded since 20^th March 2004 (71 mph). Also worthy of mention is the gust of 53 mph on 3^rd. The highest daily (00/00 GMT) mean wind speed in December was 12.7 mph on 7^th.

The mean MSL air pressure in December was 1016.3 mB which is 2 mB above the 30 Year (1976/2005) average. The highest and lowest pressure values in December were 1044 mB and 981 mB on 22^nd and 8^th respectively.

The air was highly saturated between 18^th and 23^rd with the relative humidity hovering close to 100% most of the time. The highest daily (00/00 GMT) mean relative humidity was, in fact, 100% on both 22^nd and 23^rd, whilst the lowest daily mean relative humidity was 82.1% on 6^th. The absolute lowest relative humidity reading in December was 49% at 1415 GMT on 3^rd.

Fog (visibility < 1000 metres) was recorded at the 0900 GMT observation hour on 21^st, 22^nd and 23^rd, whilst on 21^st and 22^nd visibility was below the 200 metres threshold at this time.

YEAR 2006: Despite the first 3 months of the year being colder than average (and also August) it was still the warmest year since at least 1972 in Royston. The mean temperature of 11.7°C exceeded the previous highest in 2002 by 0.2°C. The mean daily maximum temperature over the whole year was 15.4°C, which is 1.3°C above the 30 Year (1976/2005) average. The mean night minimum temperature was 8.0°C, which is 1.2°C above average. The last 10 years have all experienced an above average mean temperature.

Total rainfall in 2006 was 638.1 millimetres, which is 110% of average. The number of "rain days" was 167 and the wettest day was 26^th July with 75.5 millimetres rainfall. The Royston rainfall has remained relatively stable over the last 150 years or so and averaged 601.4 millimetres per annum from 1853 to 2006, compared to the current 30 Year standard average of 582.4 millimetres for the period from 1971 to 2000 (this lower figure reflecting the droughts of the 1970’s and 1990’s).

As well as being warmer and wetter than average the year 2006 was also a sunny one, the total sunshine of 1761.5 hours being 117% of the 1971/2000 average.

Richard Barker

Richard Barker, Met. Observer, Royston (Iceni) Weather Station

4^th January 2007

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