Royston (Iceni) Weather Station

Daily Weather Observations - September 2007

Temperatures, rainfall and sunshine each showed a different characteristic this month in that temperatures were about average, rainfall was below average, and sunshine was above average.

It was the 5^th consecutive warmer than average September in Royston, although the mean temperature was only 0.2°C above average this year and it was, in fact, the coolest September since 2002.

The mean daily maximum temperature in September was 19.4°C, which is 0.4°C above the 30 Year (1976/2005) average. The mean night minimum temperature was 0.1°C below average at 10.7°C.

The highest temperature recorded in the month was 24.0°C on 5^th. This was the lowest absolute maximum temperature in September since 2001. The lowest temperature recorded was 5.0°C on 30^th. The lowest grass minimum temperature of 1.4°C was also recorded on this date, and was the lowest grass minimum temperature in September since 1996. The lowest day maximum temperature in the month was 11.8°C on 26^th, whilst the highest night minimum temperature was 14.4°C on 6^th/7^th.

An active cold front which crossed the area on 24^th resulted in the temperature falling sharply from 16.8°C at 0640 GMT to 13.1°C at 0700 GMT and to 11.8°C at 0745 GMT. This cold front spawned several well publicised tornadoes in other parts of the country.

The mean earth temperature at 30 cms depth as recorded at 0900 GMT daily was 16.5°C, the earth temperature reaching a maximum of 18.0°C on 7^th and falling to a minimum of 14.1°C on 28^th, 29^th and 30^th.

After a run of 4 wetter than average months the rainfall of 27.8 millimetres in September was only 49% of the 30 Year (1971/2000) average. In a 23 day spell from 23^rd August to 15^th September inclusive only 1.6 millimetres rainfall was recorded, and extending this spell by a further week to 22^nd September added only another 5.0 millimetres. However, it was the driest September since 2004 only.

The number of "rain days" (0.2 millimetres or more) in September was 11, and on 8 of these days rainfall equalled or exceeded 1.0 millimetre. The wettest day of the month was 23^rd with 9.2 millimetres rainfall. Thunder was heard on just 1 day (25^th) in the month. Hail was also observed to fall on this day, this being the first hail recorded in September since 1994. Rainfall duration in September was 24.5 hours. Rainfall for the first 9 months of the year totalled 529.0 millimetres, which is 126% of average.

September was a sunny month with the sunshine total of 169.6 hours being 20% above the 1971/2000 average. It was still the dullest September since 2002, however. The sunniest day in the month was 11^th with 11.8 hours sunshine, whilst 15^th and 16^th both experienced 11.7 hours sunshine. Only 1 day (28^th) in September was completely sunless.

The mean wind speed in September (calculated from 5 minute means over the whole month) was 2.4 mph. The highest and lowest daily (00/00 GMT) mean wind speeds in the month were 8.2 mph and 0.1 mph on 24^th and 12^th respectively. The highest wind gust speed recorded in September was 46 mph on 24^th.

The analysis of wind directions as observed at 0900 GMT daily is as follows: NORTH 7 days, NE 1, E 0, SE 0, S 1, SW 8, W 5, NW 7 and CALM 1.

The mean MSL air pressure in September was 1017.8 mB, which is 1 mB above the 30 Year (1976/2005) average. The highest and lowest air pressure values in September were 1030 mB and 998 mB on 7^th and 24^th respectively.

The mean relative humidity in the month was 81.7%. The highest daily (00/00 GMT) mean relative humidity was 99.1% on 28^th, whilst the lowest daily mean value was 68.0% on 4^th. The absolute lowest relative humidity reading in September was 31% at 1420 GMT on 18^th.

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

Met. Observer, Royston (Iceni) Weather Station

4^th October 2007

Royston Crow (Corvus Cornix)

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