December 2009

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

Mean daily maximum: 5.2
Difference from average: -1.4

Mean daily minimum: -0.5
Difference from average: -1.6

Mean air temperature: 2.4
Difference from average: -1.5

Absolute maximum: 10.6 (6th)

Absolute minimum: -7.8 (22nd)

Mean grass minimum: -1.1

Absolute grass minimum: -5.7 (1st)

Number of ground frosts: 22

Number of air frosts: 16

Mean soil temperature at 30cm depth: 4.9

Mean soil temperature at 100cm depth: 7.7

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

Total for the month: 83.6
Difference from average: +26.2

Percentage of the average: 146%

Wettest day: 12.2 (16th)

Number of rain days (>0.25mm): 25

Number of wet days (=> 1mm): 20

3-month total rainfall to 31st December: 276.4
Difference from average: +104.4

6-month rainfall total to 31st December: 497.4
Difference from average: +151.2

12-month rainfall total to 31st December: 749.1
Difference from average: +100.1

Humidity
(Averages for 09:00 hours GMT)

Relative Humidity: 91%

Sunshine
(all averages 1961 – 1990)

Total for the month: 70.2 hours
Difference from average: +24.6

Percentage of the average: 154%

Mean daily sunshine: 2.3 hours
Difference from average: +0.8

Sunniest day: 6.5 hours (23rd)

Number of days with no recorded sunshine: 1

Cumulative total sunshine since 1st January: 1383.1 hours
Difference from average: +53.9

Wind

No data due to faulty equipment

ISSN1350-483

Comments on December 2009
After a mild November, December has been the complete reverse – very cold indeed. December was not nearly as wet as November (still well above average) but most of the precipitation in the second half of the month fell as snow. We no longer keep records of “snow lying” but this must have been the month with the highest number of days with snow lying for well over a decade.

December 2009 was the only month below average temperature in the whole year. It was the coldest December since 1995 (1.8°C) – indeed the coldest of any month since then, a run of 168 months in all. Mean maximum temperature was the lowest for December since 1996 and mean minimum temperature the lowest since 1995. It was the 30th coldest December in 160 years; all being equal, this would mean a December like this every five years on average but of course this is now much rarer, more like once every 15 years, it seems. Nevertheless, there were several days above 10°C at the start of the month, the last as late as the 10th.  The absolute minimum air temperature (-7.8°C) was the lowest only since 2001.

Notwithstanding a cold December, the year as a whole saw an average mean air temperature of 9.5°C, the 14th equal warmest in 160 years. In this decade only 2000, 2001 and 2008 have been colder. All years this decade are in the top 21 warmest out of 160 years. One cold month cannot disguise the fact that this has been the warmest decade on record at Durham since 1850.

The “rainfall” total, much of it falling as snow, was well above average, as noted above. Given the exceptionally wet November, this meant the year as a whole was also well above average, even though eight months were below average. There were 176 rain days – the most since 2000.

Despite all the precipitation, December was sunnier than normal. 2009 was the sunniest year since 2003, just above average.

The anemometer seems to have been faulty all month so no data is reported. The grass minimum figures must also be doubtful, given the amount of snow lying. In the old days of manual readings, the thermometer would have been kept clear of snow!

Professor Tim Burt
Department of Geography
Durham University