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    Refraction for beginners
    From: Bill B
    Date: 2004 Jul 9, 15:06 -0500

    Jeez guys, all I asked about was what corrections to make to a
    bubble-horizon observations 
    
    If I put it all together the refraction issue relating to HO229 comes out
    something like this:
    
    Dr. Carney, meteorologist and Head of Purdue's Aviation states aviation
    "altimeter" pressure has adjusted local station pressure to sea level plus
    10 feet (hmm) and most broadcast sources have adjusted the local "station
    pressure" to sea level.  As a sanity check, if the pressure in inches
    mercury is above 29.8" in normal weather, it has been adjusted.
    
    His rule of thumb was plus/minus 1" mercury per 1000 ft change.
    
    Quick formula to adjust station pressure to sea level:
    Station pressure + (.0011 x height above sea level in feet)
    
    If I understand the earlier posts, if I were sitting in Denver or on a
    mountaintop, the air would be thinner and therefore less refraction.  As a
    skier, I can attest to the former.  So it is not just what happens at the
    "boundary" of outer space and Earth's atmosphere, but also how much air
    (distance) there is between the boundary and observer, and how dense it is.
    Watching the color temperature of light change at sunset/sunrise and the
    high refraction corrections for a low Hs seems to validate this.
    
    So sea-level pressure works out fine if you are at sea level, but a
    barometric pressure report in Denver that has been adjusted for sea level is
    not a big help.
    
    Getting back to Great Lakes sailing, it would therefore behoove me to use
    the ship's barometer or adjust the broadcast pressure (that has been
    adjusted to sea level) back to local "station pressure."
    
    Since we are only talking about .6" of mercury, that would keep me within
    the limits the temp/pressure table.
    
    If I were in Denver and used local pressure (maybe 25" mercury), I would be
    outside the table's parameters anyway.
    
    Would also guess that the parallax change etc. by moving closer or farther
    from the Earth's center because of tides is minimal, but might encounter
    that and other problems with a mountaintop bubble shoot and HO229.
    
    After all, the cover of my HO229 does say, "Sight Reduction Tables for
    Marine Navigation." Sounds like HO249 (which I have not explored yet) is the
    way to go for mountaintop bubble shoots.
    
    Do I got it?
    
    Bill
    
    
    

       
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