Royston (Iceni) Weather Station
Daily Weather Observations - May 2012
(All differences from average quoted in this review relate to the 30 Year period 1981/2010)
May 2012 was a dull and fairly wet month in Royston, although the mean temperature was near normal.
The very wet second half of April overlapped into early May with rainfall of 28.4 millimetres on 2nd. Thereafter rainfall amounts were relatively small and the second half of the month was mainly dry.
Total rainfall in May was 65.0 millimetres, which is 127% of average. It was the wettest May in Royston since 2008, all of the three intervening years being drier than normal. The wettest day of the month was 2nd with 28.4 millimetres rainfall as mentioned above. The number of “rain days” (0.2 millimetres or more) in May was 13, with rainfall exceeding 1.0 millimetre on 11 of these days.
Thunder was heard on 2 days (28th and 30th) in May. In the late afternoon of 28th a thunderstorm skirted just to the east of Royston, and gave only a rainfall “trace” at this station. On 30th the thunder was accompanied by a brief but very heavy shower which yielded 7.6 millimetres rainfall.
Although April was very wet and May rainfall was above average, the effects of the drought of the last 18 months or so appear to be far from over. Total rainfall for 2012 to the end of May was 263.7 millimetres, which is 118% of average, so rainfall so far this year has begun to address last year’s serious deficit but this trend will need to continue for some months yet for the underground aquifers to return to normal levels.
Day maximum temperatures were well below average until 22nd when a hot spell commenced and by the end of the month this had raised the mean daily maximum temperature to 17.2°C and within 0.1°C of average. The highest temperature recorded in May was 26.9°C on 28th. In contrast on 3 days (3rd, 4th and 6th) near the start of the month the temperature failed to reach 10°C. The coldest of these days was 3rd with a day maximum temperature of only 8.0°C.The mean night minimum temperature in May was 8.3°C, which is 0.6°C above average. The lowest temperature recorded in the month was 2.0°C on 7th. The lowest grass minimum temperature in May, -1.6°C, also occurred on this date, and gave the only ground frost of the month. The highest night minimum temperature in May was 13.6°C on 10th. It was the coolest May since 2010 only.
The mean earth temperature at 30 cms depth as observed at 0900 GMT daily was 13.2°C, which is 0.8°C below average. The highest and lowest earth temperatures in the month were 18.0°C and 9.8°C on 29th and 7th respectively.
The first 3 weeks of May were exceptionally dull and sunshine was running at only 50% of the average for this period by 21st. A very sunny spell then ensued with 99.2 hours sunshine in the 9 days from 22nd to 30th inclusive. This brought the sunshine total for the month as a whole to 156.0 hours, which is 84% of average, making it the dullest May since 2007.
The sunniest day of May was 27th with 15.7 hours sunshine, just 2½ minutes more than the previous day. Only 2 days (9th and 21st) in the month were completely sunless. Despite the sunny end to May the month still registered 32.0 hours less sunshine than in March 2012!
The mean wind speed in May was 5.2 mph and the maximum wind gust speed recorded was 42 mph on 10th. The windiest day (00/00 GMT) of the month was 10th with a mean wind speed of 13.6 mph, whilst the calmest day (00/00 GMT) was 27th with a mean wind speed of 2.1 mph.
Winds with a northerly component featured prominently in May as the following analysis of wind directions observed at 0900 GMT daily shows: NORTH 5 days, NE 10, E 1, SE 1, S 2, SW 4, W 2, NW 6, and CALM 0.
The mean MSL air pressure in May was 1016.1 mB, which is 0.4 mB above average. The highest and lowest air pressure values recorded in May were 1038.1 mB and 1003.1 mB on 12th and 18th respectively.
The mean relative humidity in the month was 82.3%, the daily (00/00 GMT) mean peaking at 96.8% on 3rd and falling to its lowest value of 62.7% on 13th. The absolute lowest relative humidity reading in May was 37% at 1530 GMT on 25th, and again at 1530 GMT on 27th.
Fog (visibility < 1000 metres @ 0900 GMT) was observed on 2 days (23rd and 24th) in May.
Spring (March/May) 2012: It was the coolest Spring since 2010 only, and the wettest Spring since 2008. The mean daily maximum temperature in Spring was 14.6°C, which is 0.7°C above average. The mean night minimum temperature in Spring was 0.1°C above average at 5.5°C. Total rainfall in Spring was 198.6 millimetres (148% of average), whilst sunshine of 487.1 hours was 7% above average.
4th June 2012
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