Royston (Iceni) Weather Station

Daily Weather Observations - September 2009

It was a very dry September in Royston, whilst temperatures and sunshine were a little above normal.

The outstanding feature of the September weather was undoubtedly the very low rainfall total. After 7.3 millimetres rainfall on 1^st/2^nd only 1.0 millimetre rainfall was recorded over the rest of the month. The monthly total of 8.3 millimetres was only 15% of the 30 Year (1971/2000) average.

It was the driest September in Royston since 1959, and the 5^th driest September since rainfall records commenced in Royston in 1853. Also, it was the 4^th consecutive drier than average September and the 7^th drier than average September in the last 8 years.

The wettest day in the month was 2^nd with 5.8 millimetres rainfall. The number of "rainfall days" (0.2 millimetres or more) in September was 3. Rainfall duration over the whole month was only 6.9 hours. Not surprisingly no thunder was heard in the month and no hail was observed to fall.

Rainfall for 2009 to the end of September stands at 411.5 millimetres, which is 98% of average.

Temperatures in September were unexceptional and it was the warmest September since 2006 only. The highest temperature recorded in the month was 27.1°C on 8^th, whilst the lowest temperature recorded was 6.5°C just two days later on 10^th. The lowest grass minimum temperature, 2.6°C, also occurred on 10^th.

The lowest daily maximum temperature in September was 16.2°C on both 13^th and 17^th; in contrast the highest night minimum temperature was 17.0°C on 8^th.

The mean daily maximum temperature in September was 20.1°C, which is 1.3°C above the 30 Year (1976/2005) average. The mean night minimum temperature was 0.1°C above average at 11.1°C.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 16.1°C. The highest earth temperature recorded was 18.0°C on 1^st and the lowest earth temperature recorded in the month was 14.6°C on 27^th.

The 17^th was a significant day in the September sunshine regime as this was the first sunless day since 14^th May 2009, thus ending a consecutive run of 125 days with at least some sunshine recorded. However, this was the only such day in September and the next day, 18^th, proved to be the sunniest day of the month! On this day a total of 11.5 hours bright sunshine was logged.

Total sunshine in September was 154.0 hours, which is 109% of the 1971/2000 average. It was the sunniest September since 2007 only.

The mean wind speed in September was 5.7 mph. The highest and lowest daily (00/00 GMT) mean wind speeds in the month were 15.0 mph and 1.0 mph on 3^rd and 19^th respectively. The highest wind gust speed recorded in September was 39 mph on 3^rd.

The analysis of wind directions as observed at 0900 GMT daily was as follows: NORTH 6 days, NE 6, E 0, SE 0, S 1, SW 5, W 11, NW 1, and CALM 0.

The mean MSL air pressure in September was 1020.4 mB, which is 4 mB above the 30 Year (1976/2005) average. The highest air pressure value recorded in the month was 1037.1 mB on 11^th whilst the lowest air pressure reading was 989.6 mB on 3^rd.

The mean relative humidity in September was 74.9%. The highest daily (00/00 GMT) mean relative humidity in the month was 86.6% on 15^th whereas the lowest such mean value was 67.1% on 1^st.

The absolute lowest relative humidity reading in September was 39% at 1340 GMT on 1^st.

Fog (visibility < 1000 metres @ 0900 GMT) was not observed on any day in September.

Met. Observer, Royston (Iceni) Weather Station

8th October 2009

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