Royston (Iceni) Weather Station

Daily Weather Observations - September 2006

It was a very warm and sunny September in Royston, with rainfall below average.

The outstanding feature of the September weather was the exceptional warmth. Not only was it the warmest September since at least 1972 but the mean temperature was even 0.5°C above the 30 Year (1976/2005) normal for August, the hottest month of the year on average over this period.

The mean temperature also exceeded the previous warmest September (1999) by 1.1°C and was 1.2°C above the mean for August this year. A further indication of the notable warmth of September was that the daily maximum temperatures exceeded their respective 30 Year (1976/2005) daily means on all but 3 days in the month. Both daily maximum and night minimum temperatures recorded their highest means since at least 1972.

The mean daily maximum temperature in September was 22.3°C, which is 3.3°C above the 30 Year (1976/2005) average. The mean night minimum temperature was 3.0°C above average at 13.8°C. The highest daily maximum temperature recorded in September was 28.6°C on 11^th, this value only having been previously exceeded (going back to 1972) in 1999, 1991 and 1973. In contrast the lowest daily maximum temperature was 17.2°C on 22^nd.

The lowest night minimum temperature in the month was 7.2°C on 8^th, whilst the highest night minimum temperature was 18.3°C on 5^th. The lowest grass minimum temperature in September was 4.6°C on 8^th.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 17.9°C, reaching a peak of 19.4°C on 7^th and falling to a low of 17.0°C on both 27^th and 28^th.

The first 10 days of September were very dry with only 0.8 millimetres rainfall recorded in this period. Thereafter it was wetter but not notably so and the month’s rainfall total of 44.6 millimetres was only 78% of the 1971/2000 average. The number of "rain days" (0.2 millimetres or more) was 14, whilst on 10 of these days rainfall exceeded 1.0 millimetre. The wettest day of the month was 13^th with 10.9 millimetres rainfall. Rainfall duration in the month was 31.7 hours. Rainfall for 2006 to the end of September stands at 438.6 millimetres, which is 104% of average.

Thunder was heard on just 1 day (13^th) in September. It was the driest September since 2004 only.

September was a very sunny month with a total of 191.7 hours sunshine, which is 135% of the 1971/2000 average. However, it was the sunniest September since 2003 only, as in that year the September sunshine was 25.8 hours higher at 216.8 hours.

The sunniest day of the month was 8^th with 12.2 hours sunshine. The following 3 days were also very sunny with amounts of 11.7, 12.0 and 10.3 hours respectively. Only 1 day (25^th) in the month was completely sunless.

September was for the most part a relatively calm month with an overall mean wind speed of 2.5 mph and a maximum wind gust speed of 46 mph on 3^rd. The highest daily (00/00 GMT) mean wind speed was 9.2 mph on 3^rd and the lowest daily mean wind speed was 0.1 mph on 8^th.

In the analysis of wind directions as observed at 0900 GMT daily, winds with southerly and westerly components featured on no fewer than 24 days, as follows: NORTH 3 days, NE 1, E 0, SE 6, S 6, SW 8, W 4, NW 0 and CALM 2.

The mean MSL air pressure in September was 1010.2 mB, which is 6 mB below the 30 Year (1976/2005) normal. The highest and lowest air pressure values recorded in the month were 1031 mB and 998 mB on 8^th and 21^st respectively.

The mean relative humidity in the month was 83.6%, with highest and lowest daily (00/00 GMT) mean values of 96.2% and 68.3% on 25^th and 11^th respectively. The absolute lowest relative humidity reading in September was 29% at 1400 GMT on 11^th.

Fog (visibility < 1000 metres at the 0900 GMT observation hour) was observed on 1 day (16^th) only in September.

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

4th October 2006

Boadicea – Queen of the Iceni

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