Royston (Iceni) Weather Station

Daily Weather Observations - September 2005

It was a warm and sunny September in Royston, with rainfall close to normal.

The first half of September was very warm with mean daily maximum and minimum temperatures for the period 1^st/15^th reaching 23.4°C and 14.1°C respectively. The mean of these values is no less than 3.5°C above the 30 Year (1973/2002) mean for this period!

In contrast the second half of the month was somewhat cooler with mean daily maximum and minimum temperature values of 18.8°C and 9.8°C respectively. Despite these reductions in temperature overall it was still the warmest September since 1999.

The mean daily maximum temperature for the whole month was 20.9°C, which is 2.2°C above the 30 Year (1973/2002) average. The mean night minimum temperature of 12.0°C showed a smaller positive anomaly of 1.5°C above the average.

The highest temperature recorded in September was 27.4°C on 4^th. This value has been exceeded in September in only 5 years since 1972. At the other end of the scale of daily maximum temperatures in the month only 15.3°C was reached on 16^th. The lowest temperature recorded in the month was 4.0°C on 17^th, whilst the highest night minimum temperature recorded was 16.6°C on 5^th.

The lowest grass minimum temperature in September was 1.9°C on 17^th. Thus any ground frost in the month was avoided although this is a rare occurrence for September, the last such occasion being in 1990. Since 1972 the only other years in which ground frost was recorded in September were 1986, 1987 and 1988.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 17.3°C. Highest and lowest earth temperature values in the month were 19.3°C and 14.5°C on 5^th and 30^th respectively.

September commenced on a wet note with 23.3 millimetres rainfall falling in the thunderstorms in the early morning of 4^th, but with drier conditions thereafter it appeared that it would end as a drier than average month. However, 11.0 millimetres rainfall over the last 2 days of the month raised the monthly total to just above normal.

Total rainfall in September was 59.5 millimetres, which is 104% of the 30 Year (1971/2000) average. The number of "rain days" (0.2 millimetres or more) was 14, whilst on 10 of these days rainfall exceeded 1.0 millimetre. The wettest day of the month was 4^th as indicated above. It was the wettest September since 2001. Rainfall duration in the month was 50.6 hours.

Thunder was heard on 4 days (4^th, 5^th, 9^th and 10^th) in September. This is the highest September total since 1999.

Despite the September sunshine total of 174.1 hours being 23% above the 30 Year (1971/2000) average it was nonetheless the dullest September since 2002! Only 2 days (10^th and 11^th) in the month were completely sunless, whilst the sunniest day in September was 2^nd with 11.7 hours sunshine.

September was a relatively calm month and the highest wind gust speed recorded was a modest 37 mph on 28th. The mean wind speed for the month was only 1.8 mph. As regards daily (00/00 GMT) mean wind speeds, the highest was 6.1 mph on 28^th and the lowest 0.1 mph on 2^nd, 6^th and 17^th.

Wind directions between S and W were in the ascendancy during September as indicated by the following analysis of wind directions as observed at 0900 GMT daily: NORTH 3 days, NE 4, E 1, SE 0, S 5, SW 10, W 4, NW 1 and CALM 2.

The mean MSL air pressure in September was 1016.3 mB, which is equivalent to the 30 Year (1973/2002) average for Royston. The highest and lowest air pressure values in September were 1029 mB and 1006 mB on 3^rd and 25^th respectively.

The mean relative humidity in the month was 82.3%, the mean daily (00/00 GMT) value peaking at 99.8% on 11^th, and falling to its lowest value of 69.9% on 2^nd. The absolute lowest relative humidity reading in September was 34% at 1335 GMT on 2^nd.

Fog (visibility < 1000 metres) was noted on just 1 day (11^th) in September.

Richard Barker

Richard Barker,

Met. Observer, Royston (Iceni) Weather Station

5th October 2005

Return to Daily Weather Observations - September 2005 header page