April showers

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  • A wet day at Clevedon, Somerset, on 9 April 2012. Photo: Tim Ireland/PA Wire

    Developing shower clouds seen by the MODIS instrument aboard NASA's Aqua satellite on 17 April 2012. Image courtesy of NASA/GSFC, Rapid Response.

    A distant cumulonimbus anvil over the Medway Towns, Kent, on the early evening of 17 April. Cumulus humilis and fractus in the foreground, remnants of a recent heavy shower. Photo: Stephen Davenport/MeteoGroup.

    Some of the hailstones that hit Horsham, West Sussex, on 5 September 1958.

  • April showers
    20.04.2012 07:29

     

    After a couple of warm and dry Aprils in 2010 and 2011 we have returned to a more traditional sort of mid-spring weather regime.

    Admittedly, it is cooler than average and a bit wetter but we would usually expect to see a changeable period during April. In the Prologue to the Canterbury Tales, of course, Chaucer kicked off by describing the weather thus:

    Whan that Aprille with his shoures soote

    The droghte of Marche hath perced to the roote’...

    And we have certainly seen a lot of showers, making the dry and warm March rather a distant memory.

    But some of the showers have not been so 'sweet', delivering snow to high ground in the north of the country and hail to many places, as well as thunderstorms and some heavy downpours and squalls. None of which are sufficient yet to put much of a dent in the drought blighting some eastern and southern regions.

    Spring is an ideal time of year for heavy showers to form during the day under the atmospheric conditions we have just now. There is a broad trough of low pressure and cold air in the upper atmosphere, under which low pressure systems have slowed to rotate around the British Isles.

    With cold air aloft conditions are unstable. In other words, when the sun warms the ground and thereby the layer of air with which it is in contact, air ‘bubbles’ can rise uninhibitedly, ballooning up into large shower or thunder clouds if there is enough moisture – cauliflowers of cumulus and towering cumulonimbus clouds.

    In mid April, just over two months from midsummer, the sun’s radiation is getting pretty strong, so there is plenty of energy to cause these clouds to build and then dump their moisture in heavy showers.

    Most of these will be rain but cumulonimbus clouds can produce hail. They are so tall and have such strong updraughts that raindrops freeze as they carried to the top of the cloud, partially melt as they fall, then re-freeze as they are again carried upwards.

    The cycle can repeat many times with layers of ice forming during the melting and re-freezing sequences, and as more water droplets are gathered. A hailstone, if cut in half, can often show concentric rings of transparent and opaque ice to illustrate this process. Eventually a hailstone will become too heavy to be borne by the buoyancy of the air or it will become caught in a downdraught, and it will fall out of the cloud

    In the most enormous  clouds, such as the ‘super-cells’ in the Plains of the USA that also produce tornadoes, the updraughts can be so strong that hailstones can grow to enormous sized before they eventually fall.

    Parts of Oklahoma recently saw hailstones the size of baseballs or cricket balls. They can grow, albeit rarely, to the size of grapefruits.

    They don’t have to grow as large as that to be dangerous, though. Hailstones the size of golf balls caused at least three fatalities in China this week.

    Hail can be mistaken for snow but although snow is very rare in temperate climates during the warmer half of the year, hail can happen at any time; and as we have seen, extra warmth can help it form.

    It can drift like snow, and a heavy fall of hail and rain in Texas last week led to hailstones accumulating to a depth of four feet as floodwaters piled them up.

    Because they can melt so quickly it is hard to know what the world’s largest hailstone might have been; but one that fell on Vivian, South Dakota, on 23 July 2010 has a claim. It was eight inches (20 cm) across when measured, and tipped the scales at 1.93 pounds (0.9 kg). 

    The largest known in the UK fell on Horsham, West Sussex, on 5 September 1958. Hailstones were described as between egg and cricket ball sized, and one weighed in at about 0.19 kg.

    By: Stephen Davenport