Cold Fronts and dramatic weather changes

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  • This graphic shows the maximum temperatures across the central Plains of the USA for Monday 10th and Tuesday 11th November 2014. The position of the cold front and its effect on the temperature is stark.

    Weather chart for 18:00 on 28th January 2004, showing a strong cold front surging southwards across southern England, with a bitterly cold arctic airmass following in its wake. The orange arrow ahead of the cold front suggests that temperatures were relatively mild. Image from www.wetterzentrale.de (NCAR/CFS Archive).

    Strong wintertime cold fronts can lead to rapidly deteriorating conditions on the roads as they pass through. Often it is raining one minute and then freezing with snow or widespread black ice the next. Gritting lorries cannot put down salt in the rain as it gets washed off, and then they only have a limited time to get all roads salted once the rain stops and skies clear. Salting the roads is not a magic wand to prevent accidents, so drive with care in these conditions! Photo: John Giles, PA Archive.

  • Cold Fronts and dramatic weather changes
    14.11.2014 15:37

    Most people will have heard of a cold front and will be familiar with its appearance on a weather map; the solid blue line with blue triangles, most often scything its way southwards or eastwards across the chart. By definition, a cold front is the leading edge of a mass of cooler air, replacing (at ground level) a warmer mass of air. Warm fronts do the opposite of course, although they often move more slowly and tend to produce less sudden changes in temperature.

    When cold fronts pass over large landmasses in winter, they are often accompanied by a large change in temperature in just a few hours. This is because these locations are positioned well away from moderating influence of ocean breezes. Large changes in temperature are a feature of the climate of North America in late autumn. Residual summer heat from the Gulf of Mexico still travels northwards across Plains, but intensifying cold air over Canada and the Arctic can also show its hand, the two contrasting airmasses meeting like oil and water over the central US states. Rapid arrival of arctic air across the Plains is colloquially known as a 'Blue Norther', so called, because of the intensity of the deep blue skies that accompanies the chill.

    A 'Blue Norther' occurred at the start of this week. On Monday daytime, temperatures soared to between 25 and 28C across western Kansas and Texas, in balmy southerly breezes. Then, on Monday evening, in just a few hours, the temperature plunged by 20-25C as the cold front raced south-east. Strong northerly winds and clear skies then followed, with temperatures widely below 0C before dawn on Tuesday. Kansas residents were probably switching off their air conditioning and firing up the heating instead, all in one evening! However, the 'Great Blue Norther' of 11th November 1911 still sets the benchmark, with date records for maximum and minimum temperature being set in Oklahoma on the same day. Hot conditions were followed by heavy rain (and a few tornadoes) in the afternoon, with snow and frost in the evening as the temperature nose-dived by more than 30C in a few hours.

    Away from local frost hollows, such huge temperature flips are unlikely over the UK thanks to the modifying effect of the adjacent Atlantic Ocean. However, lively cold fronts can sometimes cause the mercury to plunge at a rapid rate. 10 years ago, on 28th January 2004, one of the most potent winter cold fronts over the UK in recent decades brought a temperature drop of about 8C within 1-2 hours to parts of the Midlands and southern England. After a rather mild and bright early afternoon, heavy rain set in from the north. This was shortly followed by hail, squalls and lightning, and then a short period of heavy snow commenced. Clear starry skies and a widespread frost completed the 60 minute transition.

    So beware the cold front. Or embrace it! It often fulfils an old saying that typifies mid-latitude climates - "if you don't like the weather, wait five minutes".

    By: Matthew Dobson