Royston (Iceni) Weather Station

Daily Weather Observations - May 2005

Temperatures and sunshine were both a little above average in May, but rainfall was below normal.

The weather highlight of the month was undoubtedly the maximum temperature on 27^th. On this day the maximum temperature was 30.3°C, this being the highest value recorded in May since records began at Royston (Iceni) Weather Station in 1972. The heat was short lived, however, since the maximum temperature the next day was almost 10°C lower!

Although May commenced on a warm note temperatures soon declined and then remained generally below average until the last week of the month. Despite the mean temperature for the month as a whole being 0.5°C above average it was still the coolest May in Royston since 1996. May also experienced the most ground frosts (7) since 1996 (14), but no air frost was recorded.

The mean daily maximum temperature in May was 17.3°C, which is 0.5°C above the 30 Year (1973/2002) average. The highest temperature recorded in the month was 30.3°C on 27^th (as noted above). In contrast, the temperature on 11^th reached a maximum of only 11.1°C, this being the lowest daily maximum temperature of the month.

The mean night minimum temperature in May was 7.6°C, which is 0.4°C above the 30 Year average. The lowest temperature recorded in the month was 1.0°C on 12^th, and on this date the lowest grass minimum temperature (-1.6°C) of the month was also recorded. The warmest night of the month was 26^th/27^th with a minimum temperature of 14.5°C.

Rainfall in May was once again below average, this being the 6^th consecutive month with a rainfall deficit compared to the 30 Year (1971/2000) average. Total rainfall in the period December 2004/May 2005 was 196.9 millimetres, which is only 72% of normal.

It was the driest May since 2001 only. Rainfall in May was 35.3 millimetres, which is 75% of the 30 Year (1971/2000) average. The number of "rain days" (0.2 millimetres or more) in May was 15, whilst on 6 of these days rainfall exceeded 1.0 millimetre. Rainfall duration in May was 31.2 hours. Hail was observed to fall on 2 days (7^th and 8^th) in the month, whilst thunder was heard on 3 days (1^st, 8^th and 20^th).

After a dull February and March, and an April with sunshine marginally below average, the May sunshine total was actually above average but only by 12% compared to the 1971/2000 normal. Total sunshine in the month was 212.0 hours, making it the sunniest May since 2001.

The sunniest day of the month was 15^th with 14.5 hours sunshine. Sunshine exceeded 12 hours on 12^th, 27^th and 28^th also. No day in May was completely sunless although 19^th came close with only 2 minutes sunshine!

The mean wind speed in May was 3.1 mph. The highest daily (00/00 GMT) mean wind speed was 8.4 mph on 28^th, and the lowest daily mean wind speed was 0.1 mph on 11^th. The maximum wind gust speed recorded in the month was 41 mph on 7^th.

The analysis of wind directions as observed daily at 0900 GMT shows a fairly even distribution throughout the month except for the E and SE directions which featured on only 1 day each, as per the following: NORTH 3 days, NE 5, E 1, SE 1, S 6, SW 6, W 5, NW 4 and CALM 0.

The mean MSL air pressure in May was 1015.4 mB, which is 0.5 mB below the 30 Year (1973/2002) average. The highest and lowest air pressure values recorded in May were 1028 mB and 1011 mB on 11^th and 22^nd respectively.

The mean relative humidity in May was 74.8%, with highest and lowest daily (00/00 GMT) mean relative humidity values of 91.2% and 57.6% on 4^th and 18^th respectively. The absolute lowest relative humidity reading in May was 32% at 1535 GMT on 27^th.

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

This report is concluded with a few summary statistics for SPRING (March/May) 2005. The mean daily maximum temperature in Spring was 14.2°C, which is 0.9°C above the 30 Year (1973/2002) average for Royston. The mean night minimum temperature was 5.7°C, which is 0.8°C above average. For the first time since records began at Royston (Iceni) Weather Station in 1972 the temperature reached 30 °C in Spring (30.3°C on 27^th May as noted above).

Rainfall in Spring was 111.0 millimetres, which is 83% of the 30 Year (1971/2000 average). The Spring sunshine total was 440.4 hours, which is precisely equal to the 30 Year (1971/2000) average.

It was the coldest Spring in Royston since 2001, and the driest Spring since 2003.

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

4^th June 2005

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