Royston (Iceni) Weather Station

Daily Weather Observations - May 2009

A warm, dry and sunny May completed a warm, dry and sunny Spring in Royston.

It was a warmer than average May in Royston, but not as warm as in May last year. The mean daily maximum temperature in May was 18.7°C, which is 1.7°C above the 30 Year (1976/2005) average. The mean night minimum temperature in the month was 1.1°C above average at 8.5°C.

Of the temperature extremes in the month, the highest daily maximum temperature recorded was 25.5°C on 31^st, whilst the lowest daily maximum temperature was 13.6°C on 4^th. The lowest night minimum temperature in May was 3.4°C on 11^th whilst in contrast the highest night minimum temperature was 12.8°C on 28^th.

The lowest grass minimum temperature in the month was 0.5°C on 11^th, thus indicating that ground frost was avoided in the month at the Royston (Iceni) Weather Station site, although it is possible that the grass minimum temperature may have dipped below 0°C in lower lying parts of Royston on this date.

The mean earth temperature at 30 cms depth as observed at 0900 GMT daily was 14.2°C. The highest and lowest earth temperature values recorded in May were 16.4°C and 12.9°C on 31^st and 1^st respectively.

It was the driest May in Royston since 2001. Total rainfall in the month was 34.9 millimetres, which is 74% of the 1971/2000 average. Of this total, 19.5 millimetres rainfall was recorded on 25^th (the wettest day of the month), whilst much of the rest of the month was dry, the resultant dry ground conditions being accentuated by the dry preceding months of March and April.

The number of "rain days" (0.2 millimetres or more) in May was 11, whilst on 6 of these days rainfall exceeded 1.0 millimetre. It was the first May since 1996 without thunder being heard.

May was a relatively sunny month and it was the sunniest May since at least 1997. Total sunshine in the month was 239.3 hours, which is 126% of the 1971/2000 average. The sunniest day of the month was 30^th with 16.0 hours sunshine making it the sunniest day in any May since at least 1997. The 29^th and 31^st were also very sunny days and the 3 day sunshine total for 29^th/31^st was a very respectable 45.4 hours.

Only 2 days in May were completely sunless, these being 13^th and 14^th.

Much of May was fairly breezy without any really strong winds being experienced, giving a mean wind speed over the whole month of 5.4 mph. The highest daily (00/00 GMT) mean wind speed in May was 10.4 mph on 27^th, whilst the lowest daily mean wind speed was 0.6 mph on 14^th. The highest wind gust recorded in the month was 45 mph on 8^th.

The analysis of wind directions as observed at 0900 GMT daily is as follows: NORTH 2 days, NE 3, E 5, SE 0, S 3, SW 9, W 8, NW 1 and CALM 0.

The mean MSL air pressure in May was 1016.0 mB, which is equal to the 30 Year (1976/2005) mean. The highest MSL pressure value recorded in the month was 1032 mB on 29^th, and the lowest pressure reading was 998 mB on 15^th.

The mean relative humidity in May was 74.3%. The highest daily (00/00 GMT) mean relative humidity was 94.7% on 14^th, whilst the lowest daily mean relative humidity value was 61.1% on 23^rd. The absolute lowest relative humidity reading in the month was 36% at 1340 GMT on 12^th.

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

SPRING (March/May) 2009:

After 6 of the previous 7 months (August 2008/February 2009) experiencing below average temperatures, this trend was reversed in Spring and each of the three Spring months was warmer, drier and sunnier than average. It was the warmest Spring since 2007, the driest Spring since 2003, and the sunniest Spring since at least 1997.

The mean daily maximum temperature in Spring was 15.6°C, which is 2.4°C above the 30 Year (1976/2005) average. The mean night minimum temperature in Spring was 6.0°C, which is 1.2°C above average. Total Spring rainfall was 85.9 millimetres, which is 64% of the 1971/2000 average. Total Spring sunshine was 35% above the 1971/2000 average at 597.9 hours. This value was 4.4 hours above the very sunny Spring in 2003.

Met. Observer, Royston (Iceni) Weather Station

6^th June 2009

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