Royston (Iceni) Weather Station
Daily Weather Observations - May 2000
Monthly Weather Review - May 2000
Although temperatures in May were once again above average it was otherwise a dull and wet month in Royston.
The first half of May was mainly dry in Royston with only 10.7 millimetres rainfall being recorded by 16th, of which 8.2 millimetres fell on just one day (14th). This was not a foretaste of things to come, however, as the rest of the month was very wet with measurable rainfall being recorded on every day except one (29th).
Total rainfall for the month was 85.3 millimetres, which is 174% of the 35 Year (1961/1995) average for Royston. This makes it the wettest May in Royston since 1983, following on from the very wet April this year, and ends a sequence of a drier than average May in each of the last 5 years. The number of "rain days" (0.2 millimetres or more) was 21 whilst the wettest day of the month was 27th with 18.8 millimetres rainfall. This brought the two day total for 26th/27th to over 30 millimetres, 12.9 millimetres rainfall having been recorded the previous day. Rainfall duration over the whole month was 82.1 hours.
The first half of the month was both hail and thunder free, but then both of these phenomena were recorded on 19th, 27th and 28th. In light overnight rainfall on 6th/7th a fall of reddish brown dust was observed, the most likely source of origin for this material being the deserts of North Africa.
Despite being the coolest May since 1997, the mean temperature in May this year was 1.3°C above the 25 Year (1973/1997) Royston average, thus continuing the trend to milder conditions in most months over the last 3 years or so. On 15th the temperature reached 26.0°C, giving Royston its warmest day of the month and the warmest May day since 5th May 1995. The temperature exceeded 20°C on a total of 6 days in May, all in the first half of the month.
The mean daily maximum temperature in May was 17.4°C, which is 0.9°C above average. The mean night minimum temperature was 1.6°C above average at 8.6°C. The lowest temperature recorded was 5.5°C on 1st, 20th and 27th, whilst the lowest grass minimum temperature was 2.0°C on 1st. Thus there was not even any ground frost in May, for only the 5th time since 1972. As recently as 1996 the number of ground frosts in May was as high as 14 (plus 4 air frosts that year which was highly unusual since air frost has otherwise been registered in May in Royston only once since 1972 (in 1979).
The mean earth temperature at 30 cms depth as measured daily at 0900 GMT was 14.7°C. The highest and lowest earth temperature values were 17.5°C on 16th and 12.0°C on 5th respectively.
Although only two days (3rd and 23rd) in May were completely sunless the final sunshine duration total of 172.8 hours was somewhat disappointing as it was 10% below average. The sunniest day was 15th with 13.1 hours sunshine, whilst 10 hours sunshine was exceeded also on 5th, 6th, 14th, 16th, 25th and 29th.
Winds with a northerly component dominated the weather for the first 13 days of May, and being of an anticyclonic nature were responsible for the dry start to the month. Thereafter winds with a westerly component prevailed. The daily wind directions as observed at 0900 GMT were N 9, NE 3, E 0, SE 0, S 3, SW 6, W 6, NW 4, CALM 0.
The mean wind speed in May was 3.6 mph. The windiest day (00/00 GMT) was 17th with a mean wind speed of 10.4 mph, and the calmest day was 9th with a mean wind speed of 0.5 mph. The highest wind gust speed recorded was 39 mph on each of 17th, 19th and 23rd.
As regards mean sea-level air pressure this was about 2 mb below average at 1014.4 mb. The highest pressure reading observed was 1027 mb on 2nd, whilst the lowest pressure reading was 993 mb on 28th.
The mean relative humidity over the whole month was 82.6%. The highest daily (00/00 GMT) relative humidity was 95.5% on 3rd and the lowest 67.4% on 29th. The lowest absolute relative humidity value recorded was 39% on both 15th and 29th.
The three day period from 8th to 10th produced fog each morning with the visibility below 1000m at the 0900 GMT observation hour.
To conclude this report some data for the season Spring 2000 (March/May) is given. The mean day maximum and night minimum temperatures were 13.9°C (0.8°C above average) and 6.0°C (1.3°C above average) respectively. Spring rainfall was 191.0 millimetres (139% of average), whilst Spring sunshine was 2% below average at 434.5 hours. It was the coolest Spring since 1996 and the wettest Spring since 1983.
Richard Barker
Richard Barker
Met. Observer, Royston (Iceni) Weather Station
7th June 2000
Royston Crow (Corvus Cornix)
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