Royston (Iceni) Weather Station

Daily Weather Observations - April 2004

It was a warm and wet April in Royston, but sunshine was only average.

In fact, it was the warmest April since 1987, although the mean temperature of 10.3°C was equalled in 1999. However, the highest temperature of the month, 21.7°C on 24^th, was not exceptional and has been exceeded in April in 9 years since 1972 (the highest April temperature in this period being 25.5°C on 16^th April 2003).

At the other end of the scale the lowest temperature recorded in April was 1.2°C on 11^th, and this positive value indicates that no air frost occurred in the month, this being the first such occasion in April since 1994. Also, this was the highest April absolute minimum temperature since 1993. Thelowest grass minimum temperature of the month, -2.6°C, was also recorded on this date. Only 2 ground frosts occurred in April, the lowest total for the month since 1987.

The mean daily maximum temperature in April was 14.6°C, which is 1.7°C above the 30 Year (1973/2002) Royston average. The mean night minimum temperature was also 1.7°C above average at 6.0°C. The latter figure is the highest in April since 1993.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 10.5°C. The lowest earth temperature of the month was 8.5°C on both 8^th and 9^th, whilst the highest earth temperature recorded was 14.0°C on 28^th.

Although April contained two completely dry spells of 7 (10^th/16^th inclusive) and 5 days (22^nd/26^th inclusive) respectively it was , in fact, a wet month. April ended on a rather wet note contributing to a rainfall total for the month of 71.2 millimetres, which is 161% of the 30 Year (1971/2000) average.

The number of "rain days" (0.2 millimetres or more) in April was 15, whilst on all but one of these days the rainfall exceeded 1.0 millimetre. The wettest day of the month was 27^th with 15.1 millimetres rainfall, whilst the next wettest was 18^th with 11.6 millimetres. Rainfall duration over the whole month was 71.7 hours. It was the wettest April in Royston since 2001 only.

Hail was observed to fall on 4 days (4^th, 5^th, 6^th and 8^th) in April. Thunder was heard on 1 day (5^th).

April was heading for a low sunshine total until the 5 day spell from 22^nd to 26^th inclusive produced a  total of 60.5 hours sunshine! This boosted sunshine to above average at that point but the final 4 days of the month could manage only 0.7 hours sunshine on 27^th (with the last 3 days of the month being completely sunless), and this reduced the month’s sunshine total of 144.9 hours to 99% of the 30 Year (1971/2000 average). This total was 48 hours below the April sunshine total of last year, and 63 hours below April 2002.

The sunniest day in April was 24^th with 13.5 hours sunshine, but sunshine on 23^rd was only fractionally below this. No sunshine at all was recorded on 5 days (10^th, 18^th, 28^th, 29^th & 30^th) in the month.

The mean wind speed in April was 3.1 mph. The highest daily (00/00 GMT) mean wind speed was 10.3 mph on 4^th, whilst the lowest daily mean wind speed was 0.1 mph on 11^th. The maximum wind gust recorded in the month was 41 mph on both 3^rd and 4^th.

The analysis of wind directions as observed at 0900 GMT daily gives the following results: NORTH 6 days, NE 0, E 0, SE 3, S 8, SW 4, W 2, NW 6, and CALM 1.

The mean MSL air pressure in April was approximately 2.5 mB below the average of the last 30 years at 1012.7 mB. The highest and lowest pressure values recorded were 1029 mB and 975 mB on 24^th and 18^th respectively.

The mean relative humidity in April was 80.6%. The highest daily (00/00 GMT) mean relative humidity was 99.5% on 29^th and the lowest daily mean relative humidity was 62.4% on 16^th. The absolute lowest relative humidity reading in the month was 35% at 1435 GMT on 23^rd, and 1510 GMT on 26^th.

Fog (visibility < 1000 metres) was not recorded on any day of the month at the 0900 GMT observation hour.

Met. Observer, Royston (Iceni) Weather Station

5^th May 2004

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