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 19 times compared with 43 times for January and 38 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 per month with snow falling, and three days per month with snow covering the ground. Much more often December seems to be a late-autumn month characterised by frequent rain and gales, occasional brief colder interludes with fog and frost, and distinguishable from November only by the shortness of daylight. Once upon a time December was an even darker and gloomier month than it is now but the clean air legislation of the 1950s and 1960s 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 18th century, 4.0°C in the 19th, and 4.7°C during the 20th. The average for the standard reference period 1971-2000 was 5.1°C, apparently emphasising the rising trend in recent decades which shows up so clearly in the records for most months of the year. But that warming process in December has been very erratic. The warmest decades of all were the 1970s and 1980s but these followed an exceptional run of cold Decembers during the 1960s; meanwhile the 1990s also brought several colder months.

Any month with a CET of 1°C or below can be regarded as severe: there were seven examples among the Decembers of the 1800s but only one since - in 1981. At the other end of the spectrum the only two Decembers with a CET above 8°C both occurred in the 1900s - in 1934 and 1974.

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, whereas in the early and late decades of the twentieth century 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 50 hours from 1971-2000. Similar improvements can be seen in the records for the central parts of all our main conurbations, and particularly for Manchester and Glasgow.

(c) Philip Eden