A warm dry and calm April

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  • A warm dry and calm April
    06.05.2011 13:07

     

    Not only was last month the warmest April on record it was also very dry, and the combined two-month period of March and April was the driest since 1938.

    This followed a drier-than-average winter, notwithstanding December’s snows. The aridity and warmth are manifesting themselves in wildfires and low river levels in places but thus far the rapidly burgeoning vegetation across the country is not really showing signs of stress.

    The warm spring thus far has caused many plants to bloom early and has done butterflies a favour, with Butterfly Conservation reporting that 22 out of 28 British species have been observed one month earlier than in 2009.

    That was a very different spring. Much but not all of the country, for example, has just experienced a fine if breezy Bank Holiday weekend with winds from the east, compared with a frigid northerly blow and squally showers at the same time last year.

    Cherry trees were blossoming at the time but pavements and gardens quickly became strewn with pink and white petals in the blusteriness and the showers of heavy rain and hail. “Rough winds do shake the darling buds of May”, as the Bard wrote.

    This year there were hardly any cherry blossoms left, particularly in the south, for any strong winds to disturb. And this raises another aspect of spring so far, and indeed last winter – a lack of strong winds. So calm has it been not only through the spring and winter but back into 2010 as well that wind farms were idle for long periods, and turbines in Scotland’s onshore wind farms last year operated for less than 22 per cent of the time.

    Normally during spring time there might be a sequence of depressions from the Atlantic hurtling across the British Isles and bringing gales with them. This year, however, high pressure has been rather dominant, with the atmosphere in our part of the globe in a ‘blocked’ pattern. 

    It was blocked in winter as well, but the resultant east and north-easterly winds were then coming from a very cold direction. So a similar atmospheric situation has given both a very cold December and a very warm April, with the time of year and slight difference in the position of high pressure making the difference to our weather.

    Quite why the atmosphere has been blocked for such long periods in the last 12 months or more is a matter of conjecture and research. If it remains so for the summer then we could be in for further sustained weather patterns. Whether this bodes well or ill depends on where the putative high pressure develops. Often the pattern changes between spring and summer, and dry Aprils are frequently followed by wet summers.

    That might be a reasonable rule of thumb but there are exceptions, and it is possible that summer may turn out to be both hotter and wetter than average. Although that might sound contradictory, warm sunshine could be interrupted by sudden, short-lived, heavy, thundery bursts of rain rather than frequent persistent spells of rain from leaden skies. 

    At the  moment this is not possible to say with anything approaching certainty, and the summer might simply behave according to type. However, two summers that followed a dry April may be lodged in many people’s memories – 1984 and the long, hot summer of 1976.

    By: Stephen Davenport