July 2009

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

Mean daily maximum: 19.7
Difference from average: +0.4

Mean daily minimum: 11.1
Difference from average: +0.7

Mean air temperature: 15.4
Difference from average: +0.5

Absolute maximum: 26.8 (1st)

Absolute minimum: 8.0 (11th)

Mean grass minimum: 8.9

Absolute grass minimum: 4.1 (31st)

Number of ground frosts: 0

Number of air frosts: 0

Mean soil temperature at 30cm depth: 17.3

Mean soil temperature at 100cm depth: 15.8

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

Total for the month: 167.4
Difference from average: +115.9

Percentage of the average: 325%

Wettest day: 64.2 (17th)

Number of rain days (>0.25mm): 22

Number of wet days (=> 1mm): 16

3-month total rainfall to 31st July: 286.2
Difference from average: +131.1

6-month rainfall total to 31st July: 379.7
Difference from average: +84.4

12-month rainfall total to 31st July: 774.6
Difference from average: +125.6

Humidity
(Averages for 09:00 hours GMT)

Relative Humidity: 82%

Sunshine
(all averages 1961 – 1990)

Total for the month: 131 hours
Difference from average: -23.9

Percentage of the average: 85%

Mean daily sunshine: 4.2 hours
Difference from average: -0.8

Sunniest day: 11.7 hours (2nd)

Number of days with no recorded sunshine: 1

Cumulative total sunshine since 1st January: 928.4 hours
Difference from average: +83.5

Wind

The mean wind speed: 12.4km/h [6.7kt]
Difference from average: +0.9km/hr

The highest gust: 53.9km/h [29.1kt] from 292° at 14:50 GMT on 17th

ISSN1350-483

Comments on July 2009
The most extraordinary weather episode occurred on the 17th when very high rainfall caused severe flooding in Durham; the University’s Maiden Castle sports centre was inundated for only the 3rd time since being built in the early 1960s. The rain began on the 16th with a fall of 11.6mm in one hour (1400-1500 GMT): altogether 15mm fell that day. A total of 64.2mm was recorded on the 17th; there were only 3 hours that were completely dry. Remarkably, 6.6mm fell between 1400 and 1500 GMT and 9.2mm in the next hour. The highest gust of the month was recorded during this period. 17th July 2009 is the 6th wettest day on record at Durham since 1850. The last time this total was exceeded was 25th August 1986 when 69.2mm fell. The wettest day on record at Durham is 11th September 1976 with 87.8mm.

All this rain meant the 4th wettest July on record, easily beating last year (134mm), but still exceeded by 1930 (183.9mm), 1867 (184.9mm) and 1888 (206.6mm). It was the 17th wettest month on record since 1850. Interestingly, 8 of these 17 have been in summer, 4 in autumn and five in winter, none in spring; this reflects the fact that Durham tends to have wet summers (marginally wetter than autumn) with a tendency to heavy falls of rain. As a result, all long-term rainfall totals are now well above average again.

In terms of temperature it was an unexceptional month, a little above average in all respects. The absolute maximum on the 1st was the highest since 2006, but thereafter temperatures were not exceptional. The absolute minimum was the highest since 2004.

Professor Tim Burt
Department of Geography
Durham University