Extra - January 20, 2015
January Down The Ages
January warmer, wetter and sunnier

By Philip Eden

The character of January in the UK has changed markedly over the last three centuries: it is now warmer, wetter, sunnier, and less snowy than at any other time in the period of instrumental measurements.

The Central England temperature record, compiled by Professor Manley, shows clearly how our climate has warmed over a period of 356 years. January was consistently cold during the first two hundred years of the record and the coldest decade of all were the 1770s with a mean temperature of 1.3°C. From that low point early in the 19th century a steady warming trend is evident until the 1920s, by which time the mean January temperature had risen to 4.7°C. Between 1940 and 1970 there was something of a hiccup, the monthly temperature slipping back by a degree or so, but the upward trend resumed during the last three decades of the 20th century such that the mean figure for the 1990s had recovered to 4.7°C, and for 2005-14 it had reached 4.8°C.

Rainfall records representative of the whole of England and Wales extend back to 1766, with rather more limited data between 1697 and 1765. An analysis of these reveals a strong correlation between rainfall and temperature in January. In other words, warmer Januarys tend to be wetter while colder ones are also relatively dry. This relationship is clearly demonstrated by the ten-year averages: the 1770s and 1810s were the driest decades with a mean rainfall of 61mm per year, while the wettest decades were the 1920s, 1930s and 1990s, each with a mean of 100mm per year but it had fallen back to 90mm for 2005-14.

The systematic recording of sunshine began in 1876. The marked improvement in January sunshine records in the last 120 years is partly a consequence of the cleaner air in our urban areas, so the change has been most apparent in the large conurbations â' from a monthly average of 19 hours in the first quarter of the twentieth century to 52 hours in the last quarter. But there has been a small increase in rural areas too, from 48 hours to 56 hours.

As one would expect, snow falls more frequently and lies much longer during colder months, and vice versa. But huge year-to-year variations mask any underlying trend. The snowiest January was that of 1963 when much of the UK was snow-covered throughout.

By Philip Eden

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