snap – frost and snow in late-October 2003
October may seem a rare enough event to require some special explanation, but last week’s 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.
originating within the Arctic Circle reached northern Scotland last Sunday when
the temperature failed to reach 8ºC at several sites, some 6–8degC 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 3ºC 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 –5ºC, contrasting with 11ºC 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.
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.
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 4ºC, and on low ground Exeter failed
to exceed 6ºC – the lowest October maximum here since at least 1974.