Royston (Iceni) Weather Station

Daily Weather Observations - April 2005

It was a warmer than average April in Royston, with near normal sunshine and rainfall a little below average. No long-term records were broken in the month.

A remarkable sequence of warm Aprils has occurred in recent years and it was, in fact, the 14^th consecutive April with a mean temperature average or above. The last ‘cold’ April was in 1989, the April mean temperature in 1990 being marginally above average and in 1991 marginally below average.

The mean daily maximum temperature in April was 14.5°C, which is 1.6°C above the 30 Year (1973/2002) average. The mean night minimum temperature was 5.4°C, which is 1.1°C above average. Despite these above average values it was the coolest April since 2001, although the mean temperature was the same as in April 2002.

The highest temperature recorded in April was 21.1°C on 30^th, whilst the lowest temperature was –2.1°C on 9^th. On this date the grass minimum temperature also achieved its lowest value (–5.0°C). One air frost and 5 ground frosts were recorded in the month.

The lowest daily maximum temperature was 7.7°C on 8^th. The highest night minimum temperature was 12.5°C on 29^th/30^th.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 10.6°C. Highest and lowest earth temperature values were 13.5°C and 8.6°C on 30^th and 9^th respectively.

For the 5^th successive month rainfall was below normal. In the period December 2004 to April 2005 inclusive rainfall was only 72% of the 1971/2000 average (161.6 millimetres compared with the average for this period of 225.8 millimetres).

Total rainfall in April was 39.8 millimetres, which is 90% of the 1971/2000 average. The number of "rain days" (0.2 millimetres or more) was 13. On 9 days in the month rainfall exceeded 1.0 millimetre. Rainfall duration over the whole month was 32.9 hours.

Thunder was heard on 3 days (7^th, 14^th and 25^th) in April, this being the highest total in April since 1998. Hail was observed to fall on 2 days (7^th and 8^th) in the month. Snow (only a few flakes) fell on just 1 day (8^th).

Sunshine lagged behind average for most of April but very sunny days on 27^th and 29^th raised the month’s total of 143.5 hours to within 2% of the 1971/2000 normal. The sunniest day of the month was 29^th with 12.8 hours sunshine. Sunshine exceeded 10 hours on 4 other days (17^th, 21^st, 22^nd and 27^th). Only 1 day (15^th) in the month was completely sunless.

The mean wind speed in April was 2.5 mph. The highest daily (00/00 GMT) mean wind speed was 10.4 mph on 6^th, whilst the lowest daily mean wind speed was 0.1 mph on both 15^th and 20^th. The highest wind gust speed recorded in the month was 44 mph on 6^th.

The dominant wind direction in April was SW as the following analysis of wind directions as observed at 0900 GMT daily shows: NORTH 3 days, NE 0, E 2, SE 1, S 4, SW 11, W 3, NW 4, and CALM 2.

The mean MSL air pressure in the month was 1012.3 mB , which is 3 mB below the 30 Year (1973/2002) average. The highest and lowest pressure values recorded in April were 1031 mB and 990 mB on 11^th and 18^th respectively.

The mean relative humidity in April was 78.7%. The highest daily (00/00 GMT) mean relative humidity value was 93.9% on 15^th, whilst the lowest daily mean relative humidity value was 67.4% on 2^nd. The absolute lowest relative humidity reading in April was 33% at 1445 GMT on 21^st.

Fog (visibility < 1000 metres) was observed at the 0900 GMT observation hour on just 1 day (15^th) in April.

Richard Barker

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

4^th May 2005

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