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    Re: Refraction at the horizon.
    From: Frank Reed
    Date: 2008 Mar 15, 23:23 -0400

    Marcel, you wrote:
    "My suggestion would be that one measures the temperature of the water
    and of the air. If this temperature difference is "considerable",
    extreme refractions may be likely. Unfortunately I'm not in a position
    to quantify the "considerable". From my own experience at my present
    location these effects are more likely during spring and autumn. In
    spring the water is still cold from winter and the air may already be
    warm; this leads at the surface to inversions with ducting. In autumn
    the sea is still warm from summer and the air already cold; this leads
    to "Omega" sunsets or inferior mirages."
    
    This was tried out in the late 19th/early 20th centuries, primarily for
    reducing dip anomalies. It seems that in practice it wasn't sufficiently
    predictive. I would imagine there are formal papers on the results of these
    experiments from a century ago, but I haven't seen any. Here's a brief
    extract from Chauvenet who discussed this possibility:
    "For an altitude of a few feet, the difference of pressure will not sensibly
    affect the value of D', and may be disregarded, especially since a very
    precise determination of the dip is not possible unless we know the density
    of the air at the visible horizon, which cannot usually be observed. We may,
    however, assume the temperature of the water to be that of the lowest
    stratum of the air"
    
     -FER
    
    
    
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