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    Re: Refraction at the horizon.
    From: Marcel Tschudin
    Date: 2008 Mar 18, 12:20 +0200

    George observed:
    
    >  However, two of the observations were from Mauna Kea, Hawaii, at the great
    >  height of 4205 metres, at which height much of the atmosphere has been left
    >  behind. Based on the arguments above, one might expect to see the total
    >  refraction significantly increased, with the extra refraction on the way up
    >  being quite a large fraction of what it was on the way down. However, the
    >  results show total refractions of 16' and 30'. Not good statistics, but
    >  giving no support at all to the notion of a highly augmented total
    >  refraction.
    >
    >  In the case of Mauna Kea, the sea horizon is over 120 miles away, and less
    >  than 30 miles of that path is over land, and that at significantly reduced
    >  pressure. Would sunset times at that location therefore be expected to show
    >  little fluctuation?
    
    The refractions shown for these two observations can really be
    questioned. With a standard atmosphere I would expect at this location
    rather refraction values somewhere around 50 min arc. Various
    calculations done so far indicate that super adiabatic lapse rates
    near the surface tend to reduce the total refraction. Looking at the
    soundings made near the time of those two observations:
    
    http://weather.uwyo.edu/cgi-bin/sounding?region=naconf&TYPE=TEXT%3ALIST&YEAR=1988&MONTH=11&FROM=3000&TO=3012&STNM=91285
    and
    
    http://weather.uwyo.edu/cgi-bin/sounding?region=naconf&TYPE=TEXT%3ALIST&YEAR=1989&MONTH=01&FROM=2800&TO=2812&STNM=91285
    indicate that the first of the two observations may have had (at least
    partially) such an influence.
    It would be interesting to hear how the observers explain those low
    refraction values.
    
    Marcel
    
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