Royston (Iceni) Weather Station

Daily Weather Observations - April 2009

April was a warmer, drier and sunnier month than average, although it was the warmest, driest and sunniest April in Royston since 2007 only!

It was the 3^rd warmest April since at least 1972. The highest temperature recorded (21.0°C on 15^th) was not exceptional, however, having been exceeded in April in 14 years since 1972. The lowest temperature recorded in April was 1.7°C (on 21^st) and this was the highest such absolute minimum temperature in April since 1987 (but equal with 1993) and the 3^rd highest since at least 1972.

The mean daily maximum temperature in April was 16.1°C, which is 3.0°C above the 30 Year (1976/2005) average. The mean night minimum temperature was 1.5°C above average at 6.0°C.

The lowest grass minimum temperature in the month was –1.6°C on 5^th, and the grass minimum temperature dipped below 0°C on 6 other days, giving a total of 7 ground frosts for the month.

The mean earth temperature at 30 cms depth as observed at 0900 GMT daily was 10.9°C. The highest and lowest earth temperature values in April were 13.0°C and 8.4°C on 25^th and 1^st respectively.

April was a dry month, particularly so in the second half of the month when only 2.4 millimetres rainfall was recorded from 17^th to 30^th. Total rainfall for the month was 19.4 millimetres, which is 44% of the 30 Year (1971/2000) average. April 2007 was a lot drier, however, with a rainfall total of only 1.6 millimetres, whilst some places in East Anglia recorded nothing at all that month.

The number of "rain days" (0.2 millimetres or more) in April was 9, whilst on 6 of these days rainfall equalled or exceeded 1.0 millimetre. The wettest day of the month was 16^th with 8.1 millimetres rainfall. Rainfall duration in April was 15.3 hours.

Thunder was heard in April on just one day (15^th), firstly at 0235 GMT and then again at 2330 GMT. This first storm temporarily knocked out some of the automatic equipment at Royston (Iceni) Weather Station!

April was a very sunny month but along with temperatures and rainfall could not compete with the exceptional April of 2007. The month’s sunshine total was 196.2 hours, which is 135% of the 1971/2000 average, compared with the sunshine total of 232.1 hours recorded in April two years ago. No less than 8 days accumulated more than 10 hours sunshine and three other days over 9 hours sunshine.

The sunniest day of the month was 22^nd with 13.3 hours sunshine, but 21^st and 24^th recorded sunshine of 13.2 hours and 13.1 hours respectively. The 12 day period between 18^th and 29^th was particularly sunny for any month of the year and sunshine averaged over 10 hours per day in this period.

Only 3 days (11^th, 16^th and 17^th) in April were completely dull and registered NIL sunshine.

The mean wind speed in April was 3.5 mph and the maximum wind gust speed recorded was 48 mph on 8^th. The windiest day of the month was 8^th with a 24 hour (00/00 GMT) mean wind speed of 9.7 mph. At the other end of the scale the calmest day over 24 hours was 12^th with a mean wind speed of 1.0 mph.

The analysis of wind directions as observed at 0900GMT daily is as follows: NORTH 6 days, NE 6, E 3, SE 2, S 5, SW 4, W 4, NW 0 and CALM 0.

The mean MSL air pressure in the month was 1010.9 mB, which is 4 mB below the 30 Year (1976/2005) average, perhaps somewhat surprising for such a dry month. The highest and lowest air pressure values recorded in April were 1027 mB and 991 mB on 20^th and 27^th respectively.

The mean relative humidity in April was 80.4%. The highest daily (00/00 GMT) mean relative humidity was 99.7% on 11^th, whilst the lowest daily mean relative humidity was 59.9% on 26^th. The absolute lowest relative humidity reading in April was 28% at 1330 GMT on 22^nd.

Fog (visibility < 1000 metres @ 0900 GMT) was observed on 4 days (2^nd, 3^rd, 12^th and 17^th) in April.

Met. Observer, Royston (Iceni) Weather Station

4th May 2009

Boadicea – Queen of the Iceni

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