September 2012

Temperatures
(all temperatures in degrees Celsius; all averages 1961 – 1990)

Mean daily maximum: 16.8
Difference from average: +0.1

Mean daily minimum: 8.0
Difference from average: -0.7

Mean air temperature: 12.4
Difference from average: -0.3

Absolute maximum: 25.8 (7th)

Absolute minimum: 1.6 (22nd)

Mean grass minimum: 5.5

Absolute grass minimum: 0.1 (22nd)

Number of ground frosts: 0

Number of air frosts: 0

Mean soil temperature at 30cm depth: 14.9

Mean soil temperature at 100cm depth: 14.9

Rainfall
(all totals in millimetres; all averages 1961 – 1990)

Total for the month: 122.4
Difference from average: +66.8

Percentage of the average: 220%

Wettest day: 47.4 (24th)

Number of rain days (>0.25mm): 11

Number of wet days (=> 1mm): 9

3-month total rainfall to 30th September: 323.8
Difference from average: +149.6

6-month rainfall total to 30th September: 661.6
Difference from average: +336.3

12-month rainfall total to 30th September: 852.6
Difference from average: +203.6

Humidity
(Averages for 09:00 hours GMT)

Relative Humidity: 82%

Sunshine
(all averages 1961 – 1990)

Total for the month: 132.8 hours
Difference from average: +8.8

Percentage of the average: 107%

Mean daily sunshine: 4.4 hours
Difference from average: +0.3

Sunniest day: 9.7 hours (8th)

Number of days with no recorded sunshine: 1

Cumulative total sunshine since 1st January: 989.6 hours
Difference from average: -134.8

ISSN1350-483

Comments on September 2012
This was the coolest September in Durham since 1994. The mean maximum was only the lowest since 2008 but the mean minimum was the lowest since 1994 (equal to 1997). And yet, the warmest day of the year (maximum 25.8°C) was on the 7th! Nor were there any air or ground frosts.

Of course, all the conversation has been about the heavy rainfall and flooding towards the end of the month. The rainfall was spread across two days, with 47.4mm recorded on the 24th and 40.8mm on the 25th. The former is the 27th wettest day on record at Durham since 1850 and the latter was the 41st wettest. Looking at 2-day totals, the combined total of 88.2mm is the 5th highest 2-day total since 1850, although well less than the record holder, 10th / 11th September 1976 when a total of 120.1mm was recorded. Of all possible 6-month periods (n=1948), this has been the wettest ever. Of all possible 12-month periods, the last 12-month period ranks 21st wettest out of 1952 such periods. For the 3-month, this was the 67th wettest 3-month period in a total of 1943. It was the wettest September since 1976 and the 7th wettest on record. The heaviest rain fell late on the evening of the 24th when 16mm fell in 3 hours.

Notwithstanding the large amount of rainfall, sunshine was just above average; the number of rain days (11) was just below average (14).

Professor Tim Burt
Department of Geography
Durham University