Royston (Iceni) Weather Station

Daily Weather Observations - May 2010

The first half of May was rather cool as illustrated by a mean temperature of only 8.5°C for the period 1^st/16^th. In contrast in the period 17^th/31^st the mean temperature increased to 14.6°C.

The mean daily maximum temperature in May was 17.3°C, which is 0.3°C above average. The mean night minimum temperature was 0.7°C below average at 6.7°C.

The highest temperature recorded in May was 28.1°C on 23^rd which was the highest such value in May since 27^th May 2005 when 30.3°C was recorded. The lowest temperature recorded in the month was –0.5°C on 12^th. This gave the first air frost in May since 1996. This was the only day in May with an air frost, although 7 ground frosts were recorded, the most severe of which was on 12^th when the grass minimum temperature fell to –4.1°C. Air frost duration in May was 2.4 hours.

The lowest day maximum temperature in May was 9.2°C on 8^th, whilst the highest night minimum temperature was as high as 15.5°C on 24^th.

Following the trend set by January, March and April this year it was a drier than average May. Total rainfall was 37.1 millimetres, which is 79% of the 30 Year (1971/2000) average for Royston.

This total would have been considerably lower had it not been for the 19.4 millimetres rainfall recorded on 1^st, this making it the wettest day of the month and also the wettest 1^st May since at least 1972. The number of "rain days" (0.2 millimetres or more) in May was 9 with rainfall exceeding 1.0 millimetre on 6 of these days.

The cumulative total of rainfall for 2010 to the end of May was 185.8 millimetres, which is 84% of average.

The sunshine total this May fell short of the May total last year by 64.1 hours and it was the dullest May since 2007. Total sunshine in May was 175.2 hours, which is 92% of the 1971/2000 average. The sunniest day of the month was 23^rd with 15.7 hours sunshine. No day in May had no sunshine at all, but 8^th and 29^th came close with only 1 minute and 3 minutes sunshine respectively.

The mean wind speed in May was 4.6 mph and the highest wind gust speed recorded was 35 mph on 30^th. The highest and lowest daily (00/00 GMT) mean wind speeds were 11.7 and 2.1 mph on 30^th and 13^th respectively.

The distribution of wind directions as observed at 0900 GMT daily was NORTH 5 days, NE 11, E 1, SE 1, S 0, SW 3, W 6, NW 4, and CALM 0.

The mean MSL air pressure in the month was 1016.9 mB, which is 0.6 mB above the 30 Year (1976/2005) average. The highest air pressure reading in May was 1031.9 mB on 20^th, whilst the lowest such value was 1004.3 mB on 29^th.

The mean relative humidity over the whole month of May was 70.4%. The highest and lowest daily (00/00 GMT) mean relative humidity values were 87.9% on 8^th and 56.5% on 28^th respectively. The absolute lowest relative humidity reading in May was 30% at 1545 GMT on 23^rd.

Fog (visibility < 1000 metres at 0900 GMT) was not observed on any day in May.

SPRING (March/May) 2010:

Perhaps the most significant feature of the Spring weather this year was that it was the driest Spring since 1997. It was also the coldest Spring since 2006. Even so the mean temperature was 0.4°C above average.

The mean daily maximum temperature in Spring was 14.6°C, which is 1.1°C above the 30 Year (1976/2005) average. The mean night minimum temperature was 4.8°C, which is 0.3°C below average. Total rainfall in Spring was 79.3 millimetres, which is 59% of the 1971/2000 average. Spring sunshine of 452.4 hours just exceeded the 1971/2000 average (by 2%).

Met. Observer, Royston (Iceni) Weather Station

8^th June 2010

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