Royston (Iceni) Weather Station

Daily Weather Observations - September 2004

Overall September was a warm, dry and sunny month in Royston.

The Autumn season commenced with a very warm start to September. The mean daily maximum temperature of 25.1°C over the first 10 days of the month was almost 5°C above the 30 Year (1973/2002) average for this period! This included the highest daily maximum temperature of the month, 28.5°C on 5^th. This value has been exceeded in September in only 3 years since 1972, namely 1973, 1991 and 1999.

Thereafter day maximum temperatures gradually reduced until 20^th but still remained mostly above average. Cooler conditions then followed and the lowest day maximum temperature of the month, 14.2°C, occurred on 24^th.

The lowest temperature recorded in September was 5.9°C on 24^th. In contrast, the night minimum temperature on 10^th/11^th was 15.8°C, making this the warmest night of the month. The lowest grass minimum temperature of the month was 2.1°C on 16^th.

The mean daily maximum temperature over the whole month was 20.6°C, which is 1.9°C above the 30 Year (1973/2002) average. The mean night minimum temperature was 1.2°C above average at 11.7°C. It was the warmest September since 1999.

The mean earth temperature at 30 cms depth in September was 16.7°C. The earth temperature reached a maximum of 18.8°C on 7^th and fell to a minimum of 14.2°C on 26^th.

The first rainfall in September did not occur until 10^th, the first 9 days having been completely dry in a glorious spell of weather with high temperatures, as noted above, and abundant sunshine. The wettest day of the month, 30^th, could only manage rainfall of 3.9 millimetres.

Total rainfall in September was 16.0 millimetres, which is 28% of the 1971/2000 average. The number of "rainfall days" (0.2 millimetres or more) was 12, whilst on 6 of these days rainfall exceeded 1.0 millimetre. Rainfall duration over the month was 22.9 hours. It was the driest September since 1997. No thunder was heard in September. Rainfall for the first 9 months of 2004 stands at 476.3 millimetres, which is 113% of average.

The first half of September was exceptionally sunny with total sunshine of 144.0 hours from 1^st to 16^th, but this did not last and only 34.4 hours sunshine was recorded from 17^th to 30^th. This gives a total for the month of 178.4 hours, which is 126% of the 1971/2000 average.

The sunniest day of the month was 4^th with 12.4 hours sunshine. Sunshine exceeded 10 hours on 7 other days in the month. Only 1 day, 25^th, was completely sunless. This day brought to an end a consecutive run of 115 days (since 1^st June) with at least some sunshine recorded each day.

September was a rather breezy month at times and the mean wind speed for the month as a whole was 4.5 mph. The highest daily (00/00 GMT) mean wind speed was 11.9 mph on 20^th, whilst the lowest daily mean wind speed was 0.2 mph on 5^th. The maximum wind gust speed recorded in the month was 44 mph on both 11^th and 14^th.

As in August, winds with a westerly component predominated during the month, as illustrated by the analysis of daily wind directions @ 0900 GMT which is as follows: NORTH 0 days, NE 5, E 0, SE 0, S 3, SW 10, W 9, NW 2 and CALM 1.

The mean MSL air pressure in September was 1017.9 mB, which is 2 mB above the 1973/2002 average. The highest and lowest air pressure values recorded in September were 1036 mB and 997 mB on 8^th and 14^th respectively.

The mean relative humidity in September was 77.1%, with the daily (00/00 GMT) mean peaking at 92.8% on 30^th and recording a lowest value of 63.4% on 3^rd. The absolute lowest relative humidity reading in the month was 31% at 1535 GMT on 5^th and 1320 GMT on 8^th.

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

Richard Barker

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

7^th October 2004

Royston Crow (Corvus Cornix)

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