September 2011 was the warmest for four years and the fourth warmest September I have recorded. The mean temperature of 15.0°C was 1.2°C above the long-term average. The month ended with a mini-heat wave when the thermometer peaked at 26.6°C on the 29th and just lower than this on the following day with 26.4°C. This was brought about by a large high pressure area to the east of us allowing very warm and dry air to flow north from the Mediterranean. This system became a blocking high stopping weather fronts from making progress from the Atlantic. A record was set for this station on the 10th when the minimum temperature was a very warm 16.4°C., which was warmer than three of the day time temperatures in the month. It was also a relatively dry month; the total of 46.9mm was 74% of the long-term average. After a warm start for the first two days we had nine successive days of rainfall, but no great daily totals. The wettest day was the 18th with 8.1mm.
September was an unusual month. I cannot remember when the mean temperatures and total rainfall were almost identical to the long-term averages. The mean maximum was identical at 18.7°C and the mean minimum just 0.2°C below. The total rainfall was 63.0mm compared with the long-term average of 63.6mm. The first 12 days often brought days with glorious sunshine, light breezes and above average maxima with a maximum of 22.6°C on the 10th. With winds veering into a more northerly direction in the middle of the month there were some very cool nights with a minimum of 2.6°C in the early hours of the 18th. The wettest day was the 22nd with 16.9mm. The latter part of September brought depressed maxima combined with windy and wet days.
September has been such a contrast to previous months. A mild first eight days, with the wettest day of the month on the 3rd (8.3mm) changed when the jet stream at last moved north allowing pressure to build. The downside was the arrival of strong northeasterly winds, a maximum gust of 31mph occurred on the 16th, which were unpleasant and lasted until the 19th. A light shower was noted on the 22nd and 23rd with the remainder of the month being warm and dry with very pleasant autumn sunshine.
The average temperature was 0.2° above the long-term average with no air frosts but a minimum of 2.7°C on the morning of the 27th. This was the driest September I have recorded and is identical to September 2003, which also had a total of 11.1mm and is just 17% of the long-term average. The figure for evapotranspiration, the amount of moisture that evaporates form the ground and plant life, was 59.2mm and explains why the ground is now so dry after a wet summer.