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    Re: Refraction
    From: Marcel Tschudin
    Date: 2005 Aug 8, 20:28 +0300
    Clive wrote
    The following  is one I use,
    Ref = Tan  (90-Altitude - 0.999139 * Altitude - (7.31 / (Altitude + 4.4)))
    between -20 < Air Temp < +40, and  970 <air pressure mbs < 1050
    or else use the correction below for abnormal temperature and Pressure
    Abn = (( Baro Pressure - 80 ) / 930) * (1 / ( 1 + 0.00008 * (Refraction + 39) * (Temperature - 10))), 
    using mbs,deg celsius and deg Altitude.
    Refraction = Ref  * Abn and  True Altitude  = Observed Altitude - Refraction
    Does anyone of you know where this correction for abnormal condition comes from?
    What is actually ment with "abnormal"? As an example, the standard atmosphere also has temperatures below -20°C and air pressures below 970 mBar above a certain height, but this I would consider rather "normal". Could it be that the abnormal conditions refer to sea level conditions?
    Thanks Marcel
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