Royston (Iceni) Weather Station
Daily Weather Observations - September 2000
SEPTEMBER 2000 WEATHER REVIEW
The Autumn this year in Royston was ushered in by a September which was warm and wet, and with about average sunshine.
September is on average the wettest month of the year in Royston, but it was only the 3rd wettest month (after April and May) so far this year. Even so it was a wet month, in fact the 3rd successive wet September.
Total rainfall in September was 78.2 millimetres, which is 141% of the 35 Year (1961/1995) Royston average. The number of "rain days" (0.2 millimetres or more) was 13, and the wettest day of the month was 25th with 17.9 millimetres rainfall. Rainfall duration over the whole month was 73.2 hours.
Thunder was heard on just 1 day (24th at 1330 GMT) in September.
The mean temperature in September was once again above average, thus continuing the trend to warmer conditions in recent years. In the 45 months since the end of 1996 no less than 35 months have experienced an above average mean temperature.
Also of significance is the fact that this was the 4th consecutive warmer than average September, after a consecutive run of 5 cooler than average Septembers in the years 1992/1996 inclusive.
The highest temperature of the month, 27.5°C, occurred on 11th. Whilst this is high for September it is not exceptional, having been exceeded in 3 Septembers in the previous 28 years (in 1973, 1991, and 1999). The mean daily maximum temperature was 1.1°C above the 25 Year (1973/1997) average at 19.7°C.
The lowest night minimum and grass minimum temperatures of the month both occurred on 4th. These were 6.5°C and 3.8°C respectively. The mean night minimum temperature in September was 12.2°C, which is 1.8°C above average.
The highest night minimum temperature of the month was 17.4°C, recorded overnight on 7th/8th. Somewhat surprisingly this was followed by a maximum temperature of only 18.5°C on 8th! No really cool days occurred in September, the lowest day maximum temperature being 15.5°C on 19th.
A rather dull spell around mid-month put a dent in the September sunshine value, and in the event the month’s total sunshine of 139.7 hours was 3% short of normal. Statistically this gives an about average month for sunshine, but the total was almost 30 hours below the sunshine value achieved in September last year.
The sunniest day of the month was 4th with 11.7 hours, closely followed by 10th with 11.5 hours, and 11th with 10.7 hours. No sunshine at all was recorded on 4 days in the month, these being 8th, 15th, 18th and 19th.
Turning now to wind speed and direction in September, it was a fairly breezy month (but without any notable strong winds) with wind directions containing a southerly component (as observed at 0900 GMT) on as many as 21 days.
The mean wind speed for the month was 4.0 mph and the maximum wind gust recorded was 37 mph on 27th. The highest daily (00/00 GMT) mean wind speed was 9.2 mph, also on 27th. The lowest daily mean wind speed value was 1.0 mph on 4th, 9th, and 30th.
Daily wind directions as observed at 0900 GMT were North 2, NE 0, E 0, SE 3, S 7, SW 11, W 5, NW 2, and CALM 0.
The mean MSL air pressure was some 6 mb below normal at 1010.4 mb. The highest air pressure value recorded was 1024 mb on 4th, and the lowest 992 mb on 19th.
The mean relative humidity value in September was 83.8%, the second highest value this year (after January). The highest daily (00/00 GMT) mean relative humidity was 97.3% on 19th, whilst the lowest daily mean relative humidity was 69.5% on 4th. The absolute lowest relative humidity value recorded in the month was 35% at 1445 and 1505 GMT on 6th.
As can be seen from the above report, 4th September experienced several of the month’s extreme values – the lowest minimum and grass minimum temperatures, the highest air pressure, the highest daily sunshine amount, the lowest daily mean relative humidity value, and the joint lowest daily mean wind speed. This day was also notable for a particularly fine pair of parallel aircraft contrails visible over Royston in the morning. These could also be seen on high resolution satellite pictures.
Richard Barker
Richard Barker
Met Observer, Royston (Iceni) Weather Station
4th October 2000
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