Royston (Iceni) Weather Station
Daily Weather Observations - September 2001
September 2001 Weather Review
The September weather got the Autumn off to a disappointing start in Royston, since it was cooler, wetter and duller than average.
In fact, it was the coolest September in Royston since 1994. The cool weather was principally due to the presence of an anticyclone over the Atlantic Ocean to the west of the British Isles for much of the month which resulted in a rather cool north-westerly flow over the country. Several warmer days occurred in the last week of the month, however, including 28th which was not only the warmest day of the month with a maximum temperature of 22.0°C, but also the wettest and sunniest day of the month also! This unusual combination occurred when a warm, sunny day was followed by a wet night.
The mean daily maximum temperature in September was 17.4°C, which is 1.2°C below the 25 Year (1973/1997) average for Royston. The lowest day maximum temperature was 13.6°C on 17th. This value is 4.6°C below the 25 year average for this date. The temperature exceeded 20°C on only 3 days in the month, 2nd, 3rd, and 28th.
The mean daily minimum temperature was also below average, by 0.3°C, at 10.1°C. The lowest night minimum temperature of the month was 6.3°C, also on 17th, whilst the highest night minimum temperature was 15.6°C on 2nd. The lowest grass minimum temperature was 4.0°C on 23rd.
The mean earth temperature at 30 cms depth as observed daily at 0900 GMT over the whole month was 15.4°C. The highest earth temperature in September was 17.8°C on 3rd, and the lowest 14.0°C on 18th, 19th, and each of the days 23rd to 26th.
September was wetter than average again this year, making it the 4th successive wet September and the 8th wetter than average September in the last 10 years ( the exceptions were September 1996 and 1997). Total rainfall in September was 69.0 millimetres, which is 121% of the 30 Year (1971/2000) average.
The number of "rain days" (0.2 millimetres or more) in September was 16, with 1.0 millimetre or more rain falling on 14 of these days. The wettest day of the month, as indicated above, was 28th with 13.0 millimetres.
Only May and June have been drier than average months so far this year and total rainfall for 2001 to the end of September was 550.4 millimetres, which is 95% of the annual average.
Thunder was heard on only 1 day in September, at 0350 GMT on 4th.
September was a rather dull month, and could manage only 16 hours more sunshine than in January this year! Total sunshine for the month was 112.2 hours, which is 78% of normal. Only 2 completely sunless days, 18th and 23rd, occurred during the month, however.
The sunniest day of the month was 28th with 10.4 hours sunshine. This was the only day in the month to exceed 10 hours sunshine.
Moving on to wind speed and direction, the mean wind speed during the month was 4.2 mph. The maximum wind gust speed was 35mph which occurred on 30th. The highest daily (00/00 GMT) mean wind speed was 8.5 mph on 7th, whilst the lowest daily mean wind speed was 0.2 mph on 22nd.
The analysis of wind direction as observed daily at 0900 GMT was as follows: NORTH 2 days, NE 1, E 0, SE 2, S 2, SW 4, W 4, NW 12, and CALM 3. As can be seen from this analysis, on half of the days in the month the wind had a northerly component, an occurrence which has been of a rare nature in recent years in Royston.
The mean MSL air pressure in September was some 3 mB below average at 1013.4 mB. The highest MSL air pressure recorded in the month was 1022 mB on 17th, and the lowest air pressure was 993 mB on 30th.
The mean relative humidity in September was 84.6%. The highest and lowest daily (00/00 GMT) mean relative humidity values were 99.1% on 18th and 69.3% on 10th respectively. The absolute lowest relative humidity reading in the month was 44% which was recorded at various times during the afternoon of 8th.
Richard Barker
Richard Barker
Met. Observer, Royston (Iceni) Weather Station
9th October 2001
Royston Crow (Corvus Cornix)
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