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    Re: Refraction
    From: Robert Eno
    Date: 2005 Aug 8, 16:52 -0400

    I have read similar accounts of such phenomenon in the Arctic, however, I do
    not think that I would want to trust an ice horizon but would instead,
    resort to an artificial horizon of some sort. Ice tends to dampen the waves
    and therefore there is always the possibility of being able to use a bubble
    horizon in ice-infested waters. I have never had to navigate from a vessel
    travelling through ice, so I cannot provide first-hand comments about using
    an ice horizon. Small vessels such as mine, take to the beach when the ice
    starts to move in during the fall.
    Most, if not all, of the classic polar explorers -- at least those who
    travelled over the ice in quest of the north pole -- used artificial
    horizons because of the notoriously unpredictable dip.
    ----- Original Message -----
    From: "george huxtable" 
    Sent: Sunday, August 07, 2005 5:09 AM
    Subject: Re: Refraction
    > 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@huxtable.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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