It was a rather cold May in Royston, but rainfall and sunshine were both near normal.

Temperatures in May were below normal and continued the trend of the previous two Spring months (see end of this report for Spring data). In the event it was the coldest May since 1996.

Ten of the last 12 months have now experienced below average mean temperatures, only August and December 2012 actually being warmer than normal, but then only by small margins. Clearly this a significant statistic and suggests a reversal of the trend to higher temperatures in recent times.

The mean daily maximum temperature in May was 16.0°C, which is 1.3°C below average. The mean night minimum temperature in the month was 1.1°C below average at 6.6°C. The highest temperature recorded in May was 22.4°C on 6^th. This was the lowest absolute maximum temperature in May since 1994. At the other end of the scale the lowest daily maximum temperature was 10.1°C on 24^th, this being a particularly cool day for the end of May.

May commenced on a frosty note with ground frosts recorded on the first three days of the month. The lowest temperature recorded in May was 0.0°C on 1^st, just avoiding an air frost, whilst the lowest grass minimum temperature of –3.6°C occurred also on this date. The highest night minimum temperature in May was 10.3°C on 21^st. In total 4 ground frosts were registered in May.

The mean earth temperature at 30 cms depth as read at 0900 GMT daily was 12.3°C, which is 1.7°C below average. The highest and lowest earth temperature values in May were 13.8°C and 10.3°C on 7^th and 1^st respectively.

The dry end to April followed through into May and only 1.5 millimetres rainfall was recorded between 1^st and 9^th. From 10^th the May rainfall was always struggling to keep up with the average and a dry month looked likely until near to the end of the month, when rainfall in the last few days managed to nudge the rainfall total of 52.6 millimetres to 3% above average.

The number of "rain days" (0.2 millimetres or more) in May was 17, with rainfall exceeding 1.0 millimetre on 11 of these days. The wettest day of the month was 11^th with a modest 8.7 millimetres rainfall. Hail was observed to fall on just 1 day (11^th) in May.

Rainfall in 2013 to the end of May was 196.5 millimetres, which is 88% of normal.

The first 6 days of May were very sunny with 68.1 hours being accumulated in this period. However, the month’s sunshine total of 186.7 hours was only normal, being 101% of average.

The sunniest day in May was 26^th with 15.4 hours sunshine. No day in May was completely sunless, but the 3 day period 28^th/30^th managed an aggregate sunshine total of only 4 minutes!

The mean wind speed in May was 6.2 mph and the maximum wind gust recorded was 39 mph on 9^th. The highest and lowest daily (00/00 GMT) mean wind speeds were 12.4 mph and 2.1 mph on 9^th and 28^th respectively.

The analysis of wind directions as observed daily at 0900 GMT is as follows: NORTH 8 days, NE 2, E 2, SE 2, S 1, SW 6, W 9 NW 1, and CALM 0.

Mean sea-level air pressure averaged out at 1011.0 mB for the month, which is 4.7 mB below average. The highest air pressure reading in May was 1026.7 mB on 1^st, whilst the lowest such value was 985.6 mB on 15^th.

The mean relative humidity in the month was 77.5%. The highest and lowest daily (00/00 GMT) mean relative humidity values were 98.1% and 50.4% on 29^th and 2^nd respectively. The absolute lowest relative humidity reading in May was an unusually low 21% at 1145 GMT on 2^nd.

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

SPRING (March/May): It was the coldest Spring on record (1972), this being a significant event given the steady trend to warmer Springs in recent years. The mean daily maximum temperature was 11.7°C, which is 2.2°C below average. The mean night minimum temperature in Spring was 2.1°C below average at 3.3°C.

Total rainfall in Spring was 124.6 millimetres, which is 93% of average. It was the driest Spring since 2011 only, which had less than a quarter of this Spring’s rainfall. The Spring sunshine total was 453.5 hours, which is precisely average.

Met. Observer, Royston (Iceni) Weather Station

6^th June 2013