By Philip Eden
Although the days are at their shortest in December, it is not usually the coldest month of the winter. In the last 100 years December has had the lowest mean monthly temperature 17 times compared with 46 times for January and 37 times for February.
Snowy Decembers are rare: there were just two during the 20th century, in 1950 and 1981. Over lowland Britain there is an average of just two days with snow falling, and three days with snow covering the ground. Much more often December seemed to be a late-autumn month characterised by frequent rain and gales. Once upon a time December was an even gloomier month than now, but the clean air legislation of the 1950s resulted in a marked improvement in the sunshine records of all our towns and cities.
The mean December temperature, according the Professor Gordon Manley's Central England Temperature (CET) record, was 3.9°C during the eighteenth century, 4.0°C in the nineteenth, and 4.7°C during the twentieth century. The average for the standard reference period 1981-2010 was 4.7°C, apparently showing a stalled warming. But that warming process in December has been very erratic. The warmest decades of all were the 1970s and 1980s but these followed a run of cold Decembers in the 1960s; meanwhile the 1990s and 2000s brought several colder months including 2010, the coldest December since 1890.
December rainfall has increased significantly during the last two centuries or so. Averaged over England and Wales the monthly total was 79mm between 1766 and 1870, compared with 95mm from 1871 to date. In the middle of the 19th century it was actually one of the driest months of the year, while during the last decade of the 20th century and the first of the 21st it was the wettest month of all.
Sunshine recording began in the UK in the 1870s, and the rise in the December sunshine figures during the last 125 years is largely the result of the reduction in smoke-based pollution. Taking the country as a whole the average up to the 1960s was 56 hours compared with 65 hours since then, but the change has been much more dramatic in urban areas. In central London, for instance, the mean December sunshine was 16 hours from 1921-50, but 53 hours from 1981-2010, and 61 hours in the last ten years.
By Philip Eden