The Snow Eater

  • The Snow Eater
    09.12.2010 12:46

    Winds from the Atlantic are going to bring an-all-too brief respite to wintry Britain through Friday and the weekend. It will feel noticeably milder, with temperatures rising close to the early-December average, and on Friday night they should stay above freezing just about everywhere for once.

    There will be a thaw of lying snow and ice but Saturday night and Sunday night will turn colder. Although temperatures will not be anywhere near as low as recently, some areas are going to have a hazardous re-freeze of melt-water as the mercury just dips below zero degrees.

    The thaw will bring other dangers, such as sudden falls of snow from trees and roofs, and a risk of melted snow dripping onto freezing surfaces for a while to create ice.

    Scotland has already had some rapid rises in temperature. Glenochar in South Lanarkshire saw the mercury rise 11 degrees, from -1 to 10 degrees Celsius, in only one hour in the early hours of Thursday.

    When considering large and dramatic temperature rises, however, that becomes small beer when compared with Altnaharra, Sutherland. On 30 December 1995. The temperature rose from a record-equalling low of -27.2 at midday to -1.0 degrees Celsius at 3pm, a leap of 20.2 degrees in just three hours. The 24-hour range from 9am to 9am finished up at 29.3 degrees, the highest daily range ever recorded in Britain.

    Cold, stagnant air in the valley resisted any attempts to shift it for several days, until the wind switched direction and freshened from the southeast, clearing it out of the way and allowing milder air to pour over the uplands.

    Glenochar’s temperature change came with the onset of fresh westerly winds and the passage of a warm front from the Atlantic.

    For the most startling warming, however, we have to look to the city of Spearfish, South Dakota, USA, which holds the world record for the fastest temperature change.

    On 22 January 1943 the temperature at Spearfish was -20 degrees Celsius just after sunrise. Then a strong Chinook wind set in, the westerly breeze that rapidly warms as it pours down the eastern flanks of the Rockies.

    After only two minutes it had caused the temperature to shoot up to 7 degrees Celsius, a rise of 27 degrees, or nearly one quarter of a degree per second.

    By 9am it was up to 12 degrees C but the wind then suddenly dropped, and the temperature tumbled back to -4 degrees Celsius, falling 32 degrees in only 27 minutes.

    The largest 24-hour temperature change ever known came under similar circumstances on 15 January 1972, when the sudden onset of the Chinook made the temperature at Loma, Montana, rise from -48 to 9 degrees Celsius, a change of 57 degrees.

    The Chinook is also known as the “Snow Eater”, sometimes devouring thick layers of snow in a day or two. While the change in Britain will not be as striking, a lot of snow and ice is going to disappear; at least until the wintriness returns next week.

    By: Stephen Davenport