Royston (Iceni) Weather Station

Daily Weather Observations - December 2003

The year finished with a very sunny December relative to normal, and with temperatures and rainfall not far from average.

One of the outstanding features of the 2003 weather has been the abundant sunshine and December did not disappoint in this respect. As might be expected December is the dullest month of the year on average in Royston and 13 days without sunshine this month might have suggested a rather low sunshine total.

However, the opposite proved to be the case and by the end of the month a total of 76.6 hours sunshine had been accumulated, this being 163% of the December normal. This was the 5^th sunnier than average December in the last 6 years, but despite the high sunshine total it was only the 3^rd sunniest December in this period.

December commenced on a very dull note with only 3.4 hours sunshine in the first 6 days, but then on 7^th the sunniest day of the month occurred with 6.8 hours sunshine. Thereafter 10 days experienced more than 4 hours sunshine.

As regards temperatures December was yet another warmer than average month, albeit by only a modest margin, and it was less mild than December last year. The mean daily maximum temperature in the month was 7.7°C, which is 0.5°C above the 30 Year (1973/2002) average. The mean night minimum temperature was 3.3°C, which was also above average but only by 0.3°C.

The highest day maximum temperature in December was 14.0°C on 14^th, whilst the lowest day maximum temperature was 2.9°C on both 29^th and 30^th.

The lowest night minimum temperature of the month was –4.1°C on 8^th. In contrast the highest night minimum temperature was 12.1°C on 13^th. The lowest grass minimum temperature in December was –6.9°C on 8^th. There were 5 air frosts and 11 ground frosts in the month, the latter figure being 2 below the ground frost total in October! Air frost duration in December was 50.7 hours.

The mean earth temperature at 30 cms depth as observed at 0900 GMT daily was 5.8°C. The highest and lowest earth temperatures in the month were 7.9°C and 3.9°C on 5^th and 23^rd respectively.

Although cumulative rainfall throughout most of December kept below average, a total of 19.0 millimetres over the last 3 days of the month increased the monthly total to 8% above the 30 Year (1971/2000) Royston average, yielding a monthly total of 57.2 millimetres. The wettest day of the month was 20^th with 8.3 millimetres rainfall.

The number of "rain days" (0.2 millimetres or more) was 18, whilst on 13 of these days rainfall exceeded 1.0 millimetre. Rainfall duration in the month was 80.1 hours.

The mean wind speed over the whole month was 3.7 mph. The highest daily (00/00 GMT) mean wind speed was 11.2 mph on 13^th, whilst on 30^th the mean wind speed (to the nearest 1 decimal place) dropped to 0.0 mph! Although a maximum gust of 5 mph plus a few other minor movements in the air were achieved on that day they were not enough to raise the mean wind speed above the zero mark. The day was essentially one of dead calm and it was the only day in the last 6 years with a mean wind speed of 0.0 mph. The maximum wind gust speed recorded in December was 46 mph on both 20^th and 21^st.

Due north is the only wind direction not represented in the monthly analysis of wind directions as observed daily at 0900 GMT. This analysis is as follows: NORTH 0 days, NE 5, E 1, SE 5, S 3, SW 7, W 2, NW 4, and CALM 4.

The mean MSL air pressure in December was 1017.2 mB, which is approximately 3 mB above average. The highest and lowest pressure values (reduced to mean sea-level) in the month were 1035 mB and 988 mB on 6^th (and 7^th), and 28^th respectively.

The mean relative humidity in December was 90.9%. The highest daily (00/00 GMT) mean relative humidity value was 98.9% on 1^st and the lowest daily mean relative humidity was 79.2% on 22nd. The absolute lowest relative humidity reading in December was 59% at 1320 GMT on 7^th.

Fog (visibility < 1000 metres) was recorded at the 0900 GMT observation hour on 2 days (3^rd and 10^th) in the month. On both occasions the visibility was, in fact, below the 200 metres threshold.

Finally, a brief note about the year 2003 as a whole. It was the equal second warmest year (after 2002) in Royston in the last 30 years, both 1990 and 1999 having an equivalent mean temperature. It was the driest year since 1997. The mean daily maximum temperature in 2003 was 15.6°C, which is 1.6°C above the 30 Year (1973/2002 average). The mean night minimum temperature was 0.4°C above average at 7.1°C. Total rainfall was 501.3 millimetres, which is 86% of the 1971/2000 average. Sunshine duration was exceptionally high at 1997.5 hours, which is 134% of the 1961/1990 average. The excess of sunshine over average was no less than 507 hours!

Richard Barker

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

10^th January 2004

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