Royston (Iceni) Weather Station

Daily Weather Observations - May 2002

May was a duller and wetter month than average in Royston, although the mean temperature was a little above normal.

Despite the mean temperature being 0.9°C above average it was, in fact, the coolest May since 1996. Further, the highest temperature achieved during the month, 23.5°C on 16^th, was the lowest May maximum temperature since 1994. Otherwise May temperatures were somewhat unexceptional.

The lowest temperature recorded in May was 1.5°C on 5^th, and on the same day the only ground frost of the month occurred, the grass minimum temperature registering –1.6°C. In contrast, some relatively high night minima were recorded during the third week of May with a highest minimum value for the month of 13.0°C on 21^st. The lowest daily maximum temperature was 11.5°C on 9^th.

The mean daily maximum temperature in May was 17.0°C, which is 0.5°C above the 25 Year (1973/1997) average for Royston. The mean night minimum temperature for the month of 8.3°C was similarly above average, but by the larger margin of 1.3°C.

The mean earth temperature at 30 cms depth as observed at 0900 GMT daily was 13.8°C. The maximum and minimum earth temperatures recorded during the month were 16.0°C on 18^th and 21^st, and 11.0°C on 1^st, respectively.

Total rainfall in May was 60.6 millimetres, which is 129% of the 30 Year (1971/2000) average. The number of "rain days" (0.2 millimetres or more) was 16, whilst on 11 of these days the rainfall recorded was greater than 1.0 millimetre. The wettest day of the month was 13^th with 11.2 millimetres, closely followed by 21^st with rainfall of 11.0 millimetres.

Hail was observed to fall on just 1 day (3^rd) in May, and thunder was heard on 2 days (3^rd and 26^th). On the night of 17^th/18^th deposits of fine orange/brown dust were washed out in light rainfall. These deposits most likely originated from the deserts of North Africa, having been blown aloft by strong winds and then carried northwards across Europe. The weather conditions over the previous few days were favourable for transportation of such fine desert material to these shores, but a careful analysis of wind trajectories would be required to confirm its source.

After the exceptionally sunny April the May sunshine was somewhat disappointing. The sunshine total for the month was 173.3 hours (90% of normal), which is 35 hours below the April value, and 62 hours below May last year! The sunniest day of the month was 16^th with a total of 14.3 hours. Only 3 other days in May managed a sunshine total in excess of 10 hours, these being 15^th (11.6hours), 27^th (10.8 hours), and 31^st (12.5 hours). Three days (9th, 10^th and 13^th) in May were completely sunless.

The last 10 days of May were rather windy and helped increase the mean wind speed for the month to 4.7 mph. Daily maximum gust speeds fell below 25 mph on only 1 day (31^st) in this period, and on 24^th a wind gust of 46 mph was recorded, this being the highest of the month.

The highest daily (00/00 GMT) mean wind speed in May was 9.6 mph on 14^th, whilst the lowest daily mean wind speed was 1.4 mph on 7^th.

The analysis of daily wind directions as observed at 0900 GMT was NORTH 4 days, NE 3, E 1, SE 2, S 7, SW 9, W 2, NW 3 and CALM 0.

The mean air pressure (reduced to mean sea-level) was 1012.1 mB in May. This value is approximately 4 mB below normal. The highest and lowest pressure values in the month were 1027 mB on 31^st and 991 mB on 22^nd respectively.

The mean relative humidity in May was 79.9%. The highest daily (00/00 GMT) mean relative humidity in the month was 93.7% on 13^th, whilst the lowest daily mean relative humidity was 64.2% on 31^st. The absolute lowest relative humidity reading in May was 36% at 1535 GMT on 27^th.

Fog (visibility < 1000 metres) was noted at the 0900 GMT observation hour on 1 day (8^th) in May.

Summary data for SPRING (March/May) 2002 concludes this report. The mean daily maximum temperature in Spring was 14.8°C, which is 1.7°C above normal. The mean night minimum temperature was 1.0°C above normal at 5.7°C. The highest temperature recorded in Spring was 23.5°C on 16^th May, whilst the lowest temperature recorded was –2.8°C on 2^nd March.

Total rainfall in Spring was 132.0 millimetres, which is 99% of average. The wettest day in Spring was 13^th May with 11.2 millimetres rainfall. The number of rain days" (0.2 millimetres or more) was 35.

The Spring sunshine total was 510.8 hours, which is 115% of normal.

Met. Observer, Royston (Iceni) Weather Station

6^th June 2002

Royston Crow (Corvus Cornix)

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