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    Re: Corrections for refraction in deserts
    From: Frank Reed CT
    Date: 2006 Mar 6, 18:02 EST

    Geoffrey K, you wrote:
    "Of course, due to the  high temperatures (40 to 50 Centigrade) corrections
    will have to me made to  the standard Altitude Correction tables. But it
    occurred to me that the  Refraction Correction Tables for Non Standard
    Conditions in the Nautical  Almanac may not be appropriate in the middle of
    the Sahara, where the hot  sand will bake the air near the ground.
    
    Does anyone have any data on  altitude corrections for desert conditions?"
    
    Sure. The main difference in  the atmosphere over the Sahara as far as
    refraction is concerned, apart from the  temperature, is the lapse rate. In "normal"
    parts of the world, the lapse rate  may show slowly decreasing, stable, or
    rising temperatures for the first few  hundred meters of altitude and then
    temperatures usually fall off along "moist  adiabatic" curves which average about
    -6.5 deg Celsius per kilometer. Over the  oceans, it's not unreasonable to use
    a moist adiabatic curve from sea level all  the way up to the tropopause
    (details above the tropopause and the exact height  of the tropopause don't
    matter). In deep desert areas, the air is extremely dry.  So the air cools with
    altitude at the "dry adiabatic rate" which is close to  -9.75deg C/km. When you do
    refraction integrations for these various lapse rate  structures, one feature
    that emerges over and over again is that the structure  doesn't matter when
    you do sights more than 3 or 4 degrees from the horizon.
    
    Here's a refraction table prepared for ocean conditions for every half
    degree of altitude from 0 to 5 degrees above the  horizon:
    33.8'
    28.3
    24.1
    20.8
    18.2
    16.1
    14.3
    12.9
    11.7
    10.7
    9.8
    And  here's a similar refraction table for desert  conditions:
    32.4'
    27.5
    23.6
    20.5
    18.0
    15.9
    14.3
    12.9
    11.7
    10.7
    9.8
    As  you can see, they match for altitudes of 3 degrees and higher. Both
    tables are  for standard temperature and pressure at sea level so you still would
    need to  apply corrections for non-standard T and P and for altitude above sea
    level.
    
    Of course, tables like these are only right when the actual conditions  match
    the input conditions. It's easy to imagine conditions in the desert, for
    example, right after sunrise, when the temperature distribution would not match
    the assumptions above at all.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

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