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    Re: Refraction
    From: Marcel Tschudin
    Date: 2007 Dec 11, 11:24 +0200

    Depending on what one intends to do, the different approximate
    functions which have been suggested for calculating refraction don't
    provide useful results near the horizon. The scatter of the observed
    refraction values is just too high. Victor Reijs has a nice plot on
    his Web-page http://homepages.iol.ie/~geniet/eng/tropospherelayers.htm
    showing how observed refractions do scatter around a calculated
    approximate value. The plot shows also nicely the (systematic)
    difference between sunrise and sunset. One set of data points shows
    also the result from Sinclair's approximate formula. This graph may
    help you to understand why one shouldn't use observations below about
    3 to 5 degrees above the horizon for navigational purposes. The reason
    for the large scatter of observed refractions are the (diurnal)
    changes in the planetary boundary layer, i.e. the lowest part of the
    atmosphere where the heat exchange between the earth (or sea) and the
    atmosphere takes place. Those effects are high over land, less near
    the coast and minimal over the sea.
    
    Marcel
    
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