Weather wise

WHEN we talk about "unusual" weather, it is not always clear what we mean. Last summer the temperature reached or exceeded 25C (77F), a rough definition of a "hot day" in Britain, for several days in a row in August in south-east England, and we talked about a "heatwave". But surely a heatwave requires weather truly out of the ordinary. In fact, on average, 25C is exceeded on an average of 11 days every summer in London. So a week or two of continuous fine weather in the South-east is not really a "heatwave" at all.

What about higher temperatures? Most people talk about the magical 90F mark (32C) as the definition of exceptional summer weather. In fact, between 1960 and 1993, 32C was exceeded somewhere in Britain in 14 of the 23 years. So, it is fair to say that in an average summer, somewhere in the country will hit 90F.

A true heatwave, such as the summer of 1911, 1976 or 1990, is marked by long periods of summer heat. In 1911, central London reached or exceeded 27C on 40 days. In 1976, the average daily maximum in central London was 27C for the month of July. In 1990, several days exceeded 30C in southern England and 3 August that year saw the highest temperature ever recorded in the UK, 37.1C (98.8F), at Cheltenham. So for once, the heatwave headlines were justified.

Cold weather too tends to be over-reported. Snow is not exactly unheard of in southern England, yet its arrival always catches us unawares. Between 1960 and 1993, only 10 years saw not a single day's snow cover at Kew, west London (one of Britain's least snowy spots). Nine of those years saw 10 days or more of snow cover, so snow, even in London, is not newsworthy.