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    Re: Refraction
    From: George Huxtable
    Date: 2005 Aug 7, 10:09 +0100

    Replying yesterday to Robert Eno's recent question about weather conditions
    that can result in dip differing from its predicted value, I omitted to
    mention the effect of sea-ice. A pity, because it may be of special
    interest to Robert, in his Northern latitudes.
    
    I've seen reports of large deviations, by many arc-minutes, of the dip of
    the horizon when measured over a sheet of sea-ice. Also, in such
    circumstances, mirage effects near the horizon seem to be common.
    
    I can speculate about a possible reason. Where there's a water surface, its
    temperature doesn't vary much, because the surface water mixes to some
    extent with that of the body of the seawater just below, and that immense
    heat-reserve keeps things rather constant. As soon as the surface has
    frozen, however, the ice surface can no longer exchange heat with the water
    body, but is insulated from it depending on the ice thickness.
    
    At the end of a cold, clear, Arctic night, then, heat radiation into space
    from the ice surface could well end up with it being much colder than the
    air layer above it, and the resulting temperature gradient in the air
    layers close above that surface could greatly affect the dip. It's the
    converse effect to that of the Sun heating up the surface layer of desert
    sand, referred to in an earlier posting.
    
    I've seen reports from Arctic (perhaps Antarctic?) land-explorers, sledging
    over an ice-shelf that simply has to be flat, that the surrounding horizon
    often appears to the eye as though they are travelling at the bottom of an
    immense saucer depression, seeming to be uphill in every direction. A
    dispiriting prospect indeed! No matter how far they travel on, they still
    seem to stay at the bottom of that saucer. Presumably this is the result of
    a large negative value of dip.To be apparent to the eye, without using
    instruments, such a distortion of the horizon level must be immense; a
    matter of some degrees, at a guess. Presumably this is the result of warmer
    air overlying a very cold ice surface, and with rather still air so that
    little mixing occurs in the lower air layers.
    
    George.
    
    ===============================================================
    Contact George at george---.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.
    
    
    

       
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