Royston (Iceni) Weather Station
Daily Weather Observations - May 2001
May 2001 Weather Review
The wet and dull weather of April was reversed in May with a dry and sunny month; also the mean temperature anomaly improved from average in April to above average in May.
The run of 8 wetter than average months which commenced in September 2000 was brought to an end by a dry May. The fact that 80% of the month’s rainfall occurred in the 4 day period from 13th to 16th indicates the dry character of much of the rest of the month.
Total rainfall In May was 28.1 millimetres, which is 60% of the 30 Year (1971/2000) average for Royston. The number of "rain days" (0.2 millimetres or more ) was 7, whilst on 6 of these days 1.0 millimetre or greater was recorded. The wettest day was 15th with 10.1 millimetres. This year the trend to May being drier than average has been extended since in the last 7 years May has experienced above average rainfall on only one occasion (2000).
The thunderstorms which occurred in the early hours of both 10th and 14th did not produce significant amounts of rainfall on either occasion. The only other day on which thunder was heard was 16th, at various times between 1200 and 1500 GMT. The only hail of the month was observed to fall on 16th also.
Sunshine duration in May appeared to heading for an average value until a late flourish from 29th to 31st produced 39 hours sunshine in the last 3 days of the month.. This increased the total sunshine for the month to 235.5 hours, which is 122% of the 30 Year (1961/1990) average.
The sunniest day of May was 22nd with 15.0 hours sunshine. Six other days (11th, 12th, 23rd, 24th, 29th and 30th) experienced a sunshine total in excess of 14 hours. Only one day (14th) in May was completely sunless.
Temperatures in May were above average, particularly by day. The mean daily maximum temperature was 18.9°C, which is 2.4°C above the 25 Year (1973/1997) Royston average. The positive anomaly on the daily minimum temperatures was only 0.6°C, the daily mean for the month being 7.6°C. This temperature pattern is typical of anticyclonic conditions when clearer skies allow a larger degree of heating from the sun by day, but equally allow heat loss by increased radiation at night.
The highest temperature recorded in May was 25.9°C on 11th. This is in stark contrast to 17th when at noon the temperature registered only 6°C, and achieved a 24 hour maximum of only 10.7°C. The lowest minimum and grass minimum temperatures both occurred on 5th, 1.1°C and –1.5°C respectively. The night of 27th/28th was particularly warm, the minimum temperature being 16.3°C.
The mean earth temperature at 30 cms depth at the 0900 GMT observation hour was 14.2°C. The highest earth temperature recorded was 18.0°C on 31st, whilst the lowest earth temperature was 10.7°C on 7th.
Winds from the north and north-east were much in evidence in May, giving rise to the drier conditions and also some of the sunnier and warmer days. The analysis of daily wind directions as observed at 0900 GMT is as follows:
NORTH 6 days, NE 11, E 2, SE 0, S 0, SW 5, W 4, NW 3, and CALM 0.
The mean wind speed for the whole month was 3.2 mph. The highest wind gust recorded was 41 mph on both 16th and 17th. The windiest day overall was 17th with a mean wind speed of 9.4 mph. At the other end of the scale the calmest day was 23rd with a mean wind speed of 0.6 mph.
The mean MSL air pressure in May was 1018.5 mB which is 2 mB above the May average. The highest and lowest pressure values in the month were 1031 mB on 8th and 996 mB on 17th respectively.
The mean relative humidity in May was 74.8%. As regards daily (00/00 GMT) mean relative humidity values the highest was 95.5% on 14th and the lowest 61.7% on 30th. The absolute lowest relative humidity reading was 33% at 1415 GMT on 11th.
With regard to SPRING (March/May) 2001 in Royston temperatures and sunshine were near normal but rainfall was above average. The mean daily maximum temperature over the whole Spring season was 13.5°C, which is 0.4°C above average. The mean daily minimum temperature in Spring was 4.9°C, which is 0.2°C above average. Total sunshine in Spring was exactly average at 443.2 hours, whilst total rainfall of 189.3 millimetres was 141% of normal.
Richard Barker
Richard Barker
Met. Observer, Royston (Iceni) Weather Station
5th June 2001
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