From warmth to cold

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  • Jet stream digging south over the Atlantic and bulging north of the British Isles on 22 March.

    The northward loop of the jet stream has now shifted westwards, leaving the British Isles under a trough and susceptible to cold and unsettled incursions.

    A warm day at Warkworth Hermitage in Northumberland on Thursday March 29, 2012. Photo: Owen Humphreys/PA Wire

    Daffodils wilting in the snow in the Brecon Beacons on Wednesday April 4, 2012. Photo: Tim Ireland/PA Wire

  • From warmth to cold
    10.04.2012 08:01

     

    “Topsy-turvy weather” … “Can’t make its mind up” … “Summer, spring and winter all in one season”.

    These are some of the exclamations since the weather changed from dry balminess to cold and snow in the space of a week from the end of March to early April.

    How unusual was this change, though? The answer is: not very.

    During the last couple of years we have become used to equable and warm, springs, especially during April and May. Perhaps this has made it easy to forget that the UK can have very variable weather at this time of year – or indeed at any time of year, as those who in June 1975 witnessed snow as far south as the Midlands can attest.

    Spring is particularly susceptible, however. The seas surrounding our islands are still rather cold, just emerging from their late winter and early spring nadir, so it doesn’t take much to turn chilly if conditions are right; and the jet stream that steers weather systems across the middle latitudes is often over the British Isles.

    Its northward loops and southward plunges can mean cold and warm air masses laying virtually side by side, producing large differences in temperature over fairly short distances. A shift in the dividing line can therefore bring a sudden swing. Think of a nearly deflated balloon – squeeze it one end and the remaining air bulges into the other. Just as easily it can be squeezed back again.

    In March the jet stream soared far north of Europe with high pressure building strongly at the surface and delivering daytime warmth and sunshine with fine but chilly nights.

    Now, however, the atmosphere has “retrogressed” – which is to say the long-wave atmospheric pattern has drifted westward against the normal eastward progression.

    This has placed that big northwards loop of the jet steam over the Atlantic, and high pressure has become re-centred west of Ireland with the jet plunging to the south of Spain

    The door has opened to winds from the northerly quarter, dropping temperatures and bringing snow to many areas on a southwards moving cold front.

    So on March 27th Aboyne, Aberdeenshire, broke the Scottish high temperature record for March when it reached 23.6 degrees Celsius, while on the morning of April 5th Braemar, also in Aberdeenshire, recorded a minimum temperature of -8.2 degrees – not far from a  record for the date.

    Those who like warmth have little cause to be optimistic just yet: there is nothing remotely summery on the horizon in the next week or two. However, eventually there should be another retrogression to see us back under a warmer anticyclone, even if that’s not until later this month or next month.

    By: Stephen Davenport