Royston (Iceni) Weather Station

Daily Weather Observations - September 2002

Autumn in Royston commenced with a mostly dry and sunny September with temperatures marginally above average.

The long dry spell lasting from early August through September and into early October was punctuated by just one very wet day. This was on 9^th September when 32.9 millimetres rainfall (which represented 87% of the month’s total) was recorded. This was followed by a day of brilliant sunshine totalling 11.4 hours on 10^th!

The heavy rainfall on 9^th was preceded by a 30 day spell in which only 5.7 millimetres rainfall was recorded. A further dry spell set in immediately after 9^th and up to the end of the month rainfall increased by only a further 2.4 millimetres.

Total rainfall in September was 38.0 millimetres, which is 66% of the 1971/2000 average for Royston. The number of "rain days" (0.2 millimetres or more) in the month was 6, whilst on 3 of these days rainfall was 1.0 millimetre or more.

It was the driest September in Royston since 1997. Rainfall duration over the whole month was 16.6 hours, by far the lowest such monthly value so far this year. No thunder was heard in September. Rainfall for the year to the end of September was 435.8 millimetres, which is 104% of average.

September was a mainly anticyclonic month with respectable daily sunshine totals to match except for the 7 day period from 14^th to 20^th inclusive when only 2.1 hours sunshine was recorded! A high pressure system to the north-west of the British Isles fed in northerly and north-easterly winds during this week and whilst it was dry and the winds were light a cloud cover persisted with only glimpses of sunshine. This was a classic case of anticyclonic gloom.

Despite this very dull period the total sunshine for the month of 158.1 hours was nonetheless 10% above the September average. No day in the month was completely sunless although on 3 days (14^th, 16^th and 27^th) the daily sunshine total was less than 1 minute! On 18^th the sunshine total was only 1.5 minutes.

The sunniest day of the month was 1^st with 11.7 hours sunshine; the only other days to achieve over 10 hours sunshine were 10^th (11.4 hours) and 13^th (10.1 hours). It was the sunniest September in Royston since 1999 and the sunshine total for the month exceeded September last year by no less than 46 hours.

The first half of September was pleasantly warm but thereafter temperatures were lower until increasing again near the end of the month. The warmest day of the month was 13^th with a maximum temperature of 24.5°C, but this was followed by one of the coolest days of the month on 14^th when a maximum temperature of only 16.5°C was achieved. The coolest day of September was 18^th with a maximum temperature of 15.8°C.

The lowest temperature of the month was 4.7°C on 24^th. This was the lowest temperature recorded in Royston in September since 1990. The lowest grass minimum temperature recorded in September was 2.1°C, also on 24^th.

The mean daily maximum temperature in the month was 19.6°C, which is 1.0°C above average, whilst the mean night minimum temperature was 0.3°C below average at 10.1°C.

As to be expected at this time of the year the earth temperature at 30 centimetres depth as observed at 0900 GMT daily showed a steady decline throughout the month from a maximum of 18.0°C on 1^st to a minimum of 14.4°C on 30^th. The mean earth temperature over the whole month was 16.3°C.

Winds were mainly light in September and it was, in fact, the second calmest month (after August) so far this year. The mean wind speed as measured continuously over the month was 1.7 mph. The windiest day of the month was 6^th with a mean wind speed (00/00 GMT) of 7.9 mph, whilst the calmest day of the month was 20^th when the mean wind speed was only 0.1 mph.

As regards the wind direction as observed daily at 0900 GMT it was dead calm at this time on no less than 5 days, and of the remainder winds with a northerly component were dominant. The daily wind direction analysis was NORTH 7 days, NE 7, E 0, SE 2, S 2, SW 4, W 0, NW 3, and CALM 5.

The mean air pressure reduced to mean sea-level in September was 1020.9 mB, which is approximately 4 mB above average. The highest and lowest pressure values recorded in the month were 1033 mB and 1001 mB on 1^st and 9^th respectively.

The mean relative humidity in September was 79.0%. The highest daily (00/00 GMT) mean relative humidity was 97.5% on 9^th, whilst the lowest daily mean relative humidity was 67.3% on 5^th. The absolute lowest relative humidity reading in the month was 35% at 1235 GMT on 3^rd

Fog was recorded on 2 days at the 0900 GMT observation hour in September. On 4^th the visibility was below 200 metres, but the fog was not as thick on 29^th with visibility classified as being below 1000 metres only.

Richard Barker

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

7^th October 2002

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