Violent skies in Europe

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  • Powerful lightning strikes set fire to properties

    Large hailstones can be a real danger

    Roads, homes and businesses were flooded

  • Violent skies in Europe
    21.08.2011 08:43

     

    It has been a particularly tempestuous few days in the skies over Northern Europe, especially so across parts of the Netherlands, Belgium, Germany and Poland, where violent thunderstorms have raked their way eastwards, producing significant damage.

    In the UK, we’re fairly accustomed to summer thunderstorms, although they have been fewer in number over the last few years. However, we rarely experience the levels of severity that thunderstorms exhibit in other parts of the world. For a thunderstorm to become severe, potentially obtaining legendary ‘supercell’ status, a number of key ingredients are needed. The most obvious, perhaps, is abundant heat and moisture, in essence the fuel for thunderstorm growth. However, for thunderstorms to grow in size and maintain themselves for a significant length of time, wind-shear is also vital.

    Wind shear basically describes the way in which winds turn and vary their speed with height, perhaps blowing from a south-westerly direction at ground level but from more of a south-easterly direction a few thousand metres up. This differential in wind direction and speed separates out the hot, rising air within a thunderstorm, from the cold downdrafts of rain the storm produces. In every-day thunderstorms, cold rain falling though the storm starves the cloud of heat, eventually killing it off. However, where the warm updrafts and cold downdrafts are separated, something akin to a battery cell is created which can allow the storm to propagate further, dramatically extending the storm’s life. The turning with height also produces atmospheric ‘spin’, and as this is ingested into the thunderstorms, the storm itself can actually rotate.

    Rotating storms are called supercells, and are often accompanied by violent winds, huge hailstones, and even tornadoes. In the UK we often have conditions which are either warm, humid and calm, or cool with large amounts of strong wind shear. It is very rare for hot, humid conditions to occur at the same time as strong wind shear in the UK.  On the continent though, as with other parts of the world such as the central USA, such conditions are more frequent.

    Back to events on the near continent, over the last seven days or so, very warm and humid air has sat across much of France and central Europe. However, a sharp boundary to much cooler air has been in place over the UK and the North Sea. This stationary frontal boundary has been the focus for widespread thunderstorm development, which has made headlines worldwide. Perhaps the most tragic of these thunderstorms erupted over Belgium last Thursday 18th August. By cruel chance, the storm ravaged its way across the Pukkelpop Music Festival, about 40 miles from Brussels. Very strong winds associated with the storm ripped apart fences and marquee tents, culminating in the toppling of a major soundstage at the festival. A number of people were killed as the stage crumpled to the ground, an eerily similar tragedy to deaths caused by a storm-buffeted stage at a festival in Indiana in the USA a few days earlier.

    More violent storms erupted over parts of Germany and Poland on Saturday 20th August, creating more havoc and damage. At least two people were killed northwest of Warsaw in Poland, where violently strong winds coincided with torrential rain and hail. Streets and homes were flooded, and some houses were even set alight by powerful lightning strikes. There were also reports of damage in eastern Germany, with unconfirmed reports of a tornado in Heinsberg. It is quite likely that some of these thunderstorms were indeed supercells, with radar and satellite showing potential rotation and cell-longevity.

    The very warm air boundary which has spawned this heavy weather looks set to move northwards into southern parts of the UK later on Monday and through Tuesday. With it might well come heavy rain and even a few thunderstorms, although it is unlikely that any embedded thunderstorms will contain the sort of severity seen over the past few days on the near continent.

    By: Brendan Jones