Royston (Iceni) Weather Station

Daily Weather Observations - April 2008

April was a little wetter and sunnier than normal, but temperatures were about average.

A surprise snowfall occurred in Royston on 6^th April as cold air plunged south between a low pressure system over southern Scandinavia and an anticyclone close to Iceland. At the 0900 GMT observation hour a level depth of snow of 3 cm was measured with tree branches being covered and conifers particularly being weighed down.

Royston appears to have been the northern limit of snow lying as a trip northwards across the South Cambridgeshire Plain revealed no evidence of any lying snow at all. The snow quickly thawed in Royston and had all disappeared by the end of the day. A small amount of snow fell on the following day but did not settle.

Rainfall lagged behind the 30 Year daily averages until almost the end of the month but heavier rainfall on the last two days increased the monthly total to 52.2 millimetres, which is 118% of the 1971/200 average. The number of "rain days" (0.2 millimetres or more) was 16, with rainfall equalling or exceeding 1.0 millimetre on 12 of these days.

The wettest day of the month was 29^th with 11.2 millimetres rainfall. Hail was observed to fall on 3 days (12^th, 14^th and 24^th) in the month, and thunder was heard on 2 days (11^th and 28^th). On 28^th a thunderstorm could be seen tracking to the SE of Royston and two loud peals of thunder were heard in the town. Rainfall duration in April was 40.1 hours.

Although it was the coolest April since 2001 it was, in fact, the 7^th consecutive April with an above average mean temperature (but only just above average this year).

The mean daily maximum temperature in April was 13.4°C, which is 0.3°C above the 30 Year (1976/2005) average. The mean night minimum temperature of 4.5°C was precisely equal to the 1976/2005 average.

The highest daily maximum temperature recorded in the month was 21.8°C on 26^th. In contrast the lowest daily maximum temperature was 7.1°C on 6^th. The lowest night minimum temperature in April was –0.5°C on 6^th, whilst the highest night minimum temperature was 12.0°C on 27^th. The lowest grass minimum temperature in the month was –4.9°C on 15^th. Totals of 3 air frosts and 11 ground frosts were recorded in April. This was the highest number of ground frosts in April since 1997. Air frost duration in April was 5.3 hours.

The mean earth temperature at 30 cms depth as recorded at 0900 GMT daily was 9.7°C. The highest and lowest earth temperature values in the month were 12.1°C and 8.3°C on 27^th and 1^st respectively.

Only 1 day in April was completely sunless and the total sunshine for the month of 168.4 hours was 116% of the 1971/2000 average. Despite this total being above average it was nonetheless 63.7 hours below the sunshine total of April last year!

The sunniest day of the month was 26^th with 11.2 hours sunshine. Only 1 other day (15^th) in the month exceeded 10 hours sunshine, and then by only 3 minutes.

After the strong winds experienced in each of the first 3 months of the year April was comparatively a much calmer month. The mean wind speed was 5.1 mph and the highest wind gust recorded was 41 mph on 1^st. The highest and lowest daily (00/00 GMT) mean wind speeds were 11.9 mph and 1.5 mph on 1^st and 16^th respectively.

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

The mean MSL air pressure in April was 1007.7 mB, which is 7 mB below the 30 Year (1976/2005) average. The highest air pressure reading recorded in April was 1029 mB on 3^rd, and the lowest 983 mB on 30^th.

The mean relative humidity in the month was 81.2%. The highest and lowest daily (00/00 GMT) mean relative humidity values were 96.7% and 69.4% on 20^th and 10^th respectively. The absolute lowest relative humidity reading in April was 33% at 1520 GMT on 15^th.

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

Met. Observer, Royston (Iceni) Weather Station

5^th May 2008

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