Royston (Iceni) Weather Station

Daily Weather Observations - May 2008

It was a warm but wet May in Royston, whilst sunshine was near average.

It was, in fact, the warmest May since 1992, and the second warmest May since records commenced at Royston (Iceni) Weather Station in 1972. The main warmth came in the first half of the month with a mean daily maximum temperature of 21.2°C being registered for the period 1^st/14^th, this value being consistent with the July mean daily maximum temperature.

The mean daily maximum temperature for the whole month was 19.2°C, which is 2.2°C above the 30 Year (1976/2005) average. The mean night minimum temperature was 1.5°C above average at 8.9°C.

The highest daily maximum temperature in the month was 26.5°C on 11^th. The lowest night minimum temperature was 1.5°C on 19^th. The lowest grass minimum temperature was –2.0°C on 20^th, this being the lowest grass minimum temperature in May since 1996. The 3 ground frosts in May occurred on the successive nights of 19^th/21^st and caused widespread damage to tender plants in Royston, for instance on the Green Street allotment site which is situated in the lower and flatter northern part of the town.

The mean earth temperature at 30 cms depth as recorded at 0900 GMT daily was 14.5°C. The highest and lowest earth temperature values recorded were 16.4°C and 11.4°C on 14^th and 1^st respectively.

The first two weeks of May were very dry with only 1.8 millimetres rainfall recorded from 1^st/14^th. Despite a further dry spell from 18^th/23^rd (with no rainfall at all) the final rainfall total for the month of 73.7 millimetres was 157% of the 1971/2000 average.

The wettest day in May was 26^th with 18.5 millimetres rainfall (also noteworthy was 18.2 millimetres rainfall on 15^th). The number of "rain days" (0.2 millimetres or more) was 13, with rainfall exceeding 1.0 millimetre on 9 of these days. Rainfall duration in the month was 59.4 hours. It was the 3^rd consecutive wetter than average May in Royston.

Thunder was heard on 2 days (15^th and 28^th) in May.

The first two weeks of May were much sunnier than the rest of the month with sunshine of 138.7 hours from 1^st/14^th compared with 66.3 hours for 15^th/31^st. Total sunshine in May of 205.0 hours was 108% of the 1971/2000 average. The sunniest day of the month was 11^th with 14.2 hours sunshine, whilst 6^th and 8^th both recorded above 14 hours sunshine. The number of completely sunless days in May was 4, these being 16^th and 25^th, 26^th and 27^th. It was the sunniest May since 2005 only.

May was a relatively calm month with a mean wind speed of 3.0 mph, and a maximum gust speed of 39mph on 26^th. The highest and lowest daily (00/00 GMT) mean wind speeds in the month were 8.3 mph and 0.9 mph on 26^th and 10^th respectively.

Winds from between NE and SE dominated the month as per the following analysis of wind directions as observed daily at 0900 GMT: NORTH 1 day, NE 16, E 2, SE 7, S 1, SW 2, W 1, NW 1 and CALM 0.

The mean MSL air pressure in May was 1012.9 mB, which is 3 mB below the 30 Year (1976/2005) average. The highest air pressure value recorded in the month was 1024 mB on 5^th, and the lowest such value was 998 mB on 28^th.

The mean relative humidity in the month was 78.5%, with a highest daily (00/00 GMT) mean value of 97.7% on 16^th, and a lowest daily mean value of 58.8% on 11^th. The absolute lowest relative humidity reading in May was 31% at 1420 GMT on 11^th, and at 1410 GMT on 12^th.

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

SPRING (March/May) 2008:

It was the warmest Spring since 2006 only, but it was the wettest Spring since 1983. Unusually the highest and lowest MSL air pressure values in Spring were recorded only 5 days apart (1034 mB on 5^th March and 958 mB on 10^th March). The mean daily maximum temperature in Spring was 14.2°C, which is 0.7°C above the 30 Year (1976/2005) average. The mean night minimum temperature was 0.4°C above average at 5.5°C. Total rainfall was 204.1 millimetres (153% of 1971/200 average), and total sunshine was 490.9 hours (111% of 1971/2000 average).

Met. Observer, Royston (Iceni) Weather Station

5^th June 2008

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