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    Waterspouts.
    From: George Huxtable
    Date: 2003 Sep 17, 18:22 +0100

    Waterspouts.
    
    At a recent meeting of our local cruising club (information about that club
    can be found on  ) there was
    interest in reports of recent sightings of  waterspouts, on two successive
    days off the North Brittany coast, by Nav-L member Clive Sutherland.
    
    It turned out that several other members had observed waterspouts, in
    European coastal waters, at different times, so these phenomena are not
    particularly rare. None were close-quarters encounters, perhaps fortunately
    so. Me, I've never been aware of one, glad to say.
    
    There was general agreement that (from a distance, anyway) these appeared
    to be undramatic, without any menacing roar, and rather stable, lasting for
    20 to 30 minutes, travelling slowly without significant change. Weather
    conditions in the vicinity were usually light winds and heavy overcast low
    cloud. Nobody was brave (or foolhardy) enough to head towards the spout for
    a closer look. It was hard to be sure how wide these spouts were and how
    far away, but some estimates were of "a couple of boat-lengths" across
    (perhaps 20 metres) with disturbed sea surface over that diameter and a
    grey column of about that diameter, with a sharply defined edge, rising
    from the surface, and then slanting away, at sometimes a shallow angle, to
    disappear into the cloud base. Other waterspouts were reckoned to be much
    bigger, perhaps hundreds of meters across.
    
    These raise a few question. Have mariners, (perhaps Nav-L members, even) in
    large vessels or small, passed through the eye of a waterspout? What's it
    like in there? Can sails or rigging survive if not furled in time? Do
    published accounts exist?
    
    Presumably the pressure within the column is significantly less than that
    of the surroundings. Is it ever low enough to give rise to a noticeable
    mounding of the water surface below it? The turbulence will make any
    precise observation rather difficult, to say the least.
    
    How does a waterspout function?  I have read that both tornados and
    waterspouts can grow downwards, swirling from the cloud layer above. I
    think I can understand a tornado, over Sun-heated land, as it takes in hot
    air from the layer just above the surface, and the rising of that hot air
    within the column that provides the energy to drive it. It's the dust and
    debris drawn from the ground that darkens the air-column to make it so
    visible. Is that acceptable as a plausible model?
    
    How does that work for a waterspout, however? Presumably, the dark column
    is visible because it carries a load of water or spray, drawn up from the
    surface. But once the column has grown downwards to reach the water
    surface, surely the effect of any water and spray that it picks up will be
    to quench and cool the column of air: just the opposite to a land tornado.
    In the summer the sea temperature will surely always be much cooler than
    the air above it, and any evaporation will cool the column further still.
    Is there any published study of how all this works? Do list members have
    any explanations of their own?
    
    George.
    
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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