Royston (Iceni) Weather Station

Daily Weather Observations - September 2003

September followed the trend set in the Summer and was a warm, dry and sunny month.

The most notable influence of the temperature regime in September concerned the high day maximum readings. The mean daily maximum temperature of 21.3°C was 2.6°C above average and was the highest such value in September in at least the last 30 years in Royston (although it was equalled in 1999). Night temperatures did not exhibit this positive anomaly, however, and the mean night minimum temperature of 10.3°C was actually 0.2°C below the 30 Year (1973/2002) average. Overall it was the warmest September since 2000 only.

The highest temperatures occurred around mid-month and on 16^th the highest temperature of the month, 27.2°C, was recorded. As in August temperatures reduced appreciably in the last week of the month, the coolest such day being 23^rd when a maximum temperature of only 14.5°C was achieved.

The lowest temperature recorded in September was 4.9°C on 24^th, whilst the lowest grass minimum temperature of the month, 1.5°C, also occurred on this date. In contrast, only 2 days earlier on 22^nd the night minimum temperature was as high as 15.2°C, making it the warmest night of the month.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 16.2°C. The earth temperature reached a peak of 18.1°C on 6^th, and fell to a lowest value of 13.6°C on both 29^th and 30^th.

September was yet another dry month in a dry year so far and the total rainfall of 17.1 millimetres was only 30% of the 30 Year (1971/2000) average. This brings the total rainfall for the year to date to 306.2 millimetres, which is only 73% of average. This represents a serious deficit for the 9 month period which for water supply purposes is ameliorated only by the wet nature of the previous 3 years.

An obvious contributory factor to this deficit was the very low August rainfall; the drought which began on 2^nd August lasted until 5^th September, giving a consecutive 34 day period when no measurable rainfall was recorded. Even then the rainfall on 5^th September was the lowest value (0.2 millimetres) to count as an official "rainfall day"!

The number of "rain days" (0.2 millimetres or more) in September was 7, whilst on 5 of these days the rainfall equalled or exceeded 1.0 millimetre. The wettest day of the month was 6^th with 5.9 millimetres. This relatively small amount did little to reverse the effects of the preceding drought, however. Rainfall duration in the month was 15.7 hours. It was the driest September in Royston since 1997. Thunder was heard on just 1 day (6^th) in the month.

It was an exceptionally sunny September and the reduced cloud cover was undoubtedly partly responsible at least for the warm days and relatively cooler nights mentioned above. The month’s sunshine total of 216.8 hours was 151% of the 30 Year (1971/2000 average). It was also the 5^th consecutive month this year with over 200 hours sunshine (in the 30 Year averages no month in the year reaches this value!).

The sunniest day of the month was 4^th with 12.3 hours sunshine. Sunshine exceeded 10 hours on no less than 9 other days (13^th, 14^th, 16^th, 17^th, 19^th, 20^th, 24^th, 25^th and 29th) in the month. No day in the month was completely sunless, although 10^th came very close with only 8 minutes sunshine.

The mean wind speed in September was 2.2 mph. The highest wind gust speed recorded was 39 mph on 22^nd. The highest daily (00/00 GMT) mean wind speed was 6.1 mph on 22^nd, whilst the lowest daily mean wind speed was 0.2 mph on 27^th.

As regards daily wind directions as observed at 0900 GMT the analysis of these gives the following results: NORTH 1 day, NE 1, E 0, SE 5, S 4, SW 8, W 5, NW 4, and CALM 2.

The mean MSL air pressure in September was some 4 mB above average at 1020.3 mB. The highest and lowest pressure values recorded in the month were 1034 mB and 1000 mB on 24^th and 22^nd respectively.

The mean relative humidity over the whole month was 71.0%. The highest daily (00/00 GMT) mean relative humidity was 91.1% on 10^th. The lowest daily mean relative humidity was 52.7% on 16^th. The absolute lowest relative humidity reading in the month was 24% at 1340 GMT on 24^th.

Fog (visibility < 1000 metres) was not noted on any day in September at the 0900 GMT observation hour.

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

6^th October 2003

Royston Crow (Corvus Cornix)

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