Wintry snap frost and snow in late-October 2003


Snow in October may seem a rare enough event to require some special explanation, but last weeks wintry episode has plenty of precedents, including a notably cold spell just after mid-month last year. It was, perhaps, a rather more extreme example of a northerly outbreak which occurs once in every two or three Octobers, on average, and snow over the Scottish hills during this month is hardly uncommon.


Cold air originating within the Arctic Circle reached northern Scotland last Sunday when the temperature failed to reach 8C at several sites, some 68degC lower than had been achieved on the previous Friday. The cold snap reached its greatest intensity over Scotland on Tuesday when the afternoon maximum temperature was just 3C at Aviemore, Glenlivet, and Biggar.


This was an outbreak of what meteorologists call deep cold air; the temperature normally decreases with altitude, but in this sort of air mass that decrease is more pronounced than usual. Thus on the summit of Cairn Gorm the temperature on Tuesday afternoon was 5C, contrasting with 11C four days before. The lowest layer of an Arctic air mass is warmed considerably during its passage across the waters of the Norwegian Sea on its journey towards the UK the oceans are relatively warm at this time year and this accentuates the temperature difference between sea-level and 100 metres aloft. The bigger this temperature difference the more unstable the lower atmosphere becomes, with strong convective currents (updraughts and downdraughts) producing active cumulonimbus clouds which in turn deliver sudden heavy squalls of rain, hail, sleet and snow, sometimes accompanied by thunder and lightning.


As often happens in such a northerly airflow, the area around Huntly and Dufftown, in the rolling hill country between Aberdeen and Elgin, caught the first heavy snow with 10cm on the ground on Monday morning, seriously disrupting road traffic. There was, of course, much more over the higher slopes of the Cairngorms. Later on Monday and during Tuesday a prolonged snowfall affected the Lammermuir and Moorfoot hills which lie between Edinburgh and the Scottish Borders.


In southern Britain Wednesday was the coldest day, with snow reported from the higher parts of the Devonshire moors; the maximum temperature that afternoon at Liscombe on Exmoor was 4C, and on low ground Exeter failed to exceed 6C the lowest October maximum here since at least 1974.


Philip Eden



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