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    Re: U.S. Standard Atmosphere 1976
    From: Frank Reed CT
    Date: 2005 Sep 6, 12:18 EDT

    Paul Hirose wrote:
    "The only temperature vs.  latitude graphs I see are for heights of 40, 60,
    and 80 km. I don't think  these are very useful for computing refraction,
    since above 40 km the air is  so rarefied that surely it can't have much
    effect."
    
    It seems that the  "US Standard Atmosphere" is primarily intended for
    aeronautical engineering of  high-altitude aircraft and hypersonic re-entry vehicles
    (there are still some  thousands of these in silos and submarines around the
    world). The troposphere,  the portion of the atmosphere which is mixed by
    active turbulence and extends  from ground level up to 10 or 15km, is too messy for
    such models. But for  refraction, the troposphere is 99.5% of the the game.
    At and close to sea level,  the variability with season and latitude makes
    almost no difference for  refraction above 3 degrees altitude, but when you get up
    to 5 or 10 km above sea  level that variability makes a measurable difference
    but probably nothing of  practical significance.
    
    With all these small details flying around, it's  probably worth mentioning
    that you get an excellent approximation for refraction  at various moderate
    heights above sea level, e.g. for shooting practice sights  in mile-high Denver,
    by taking the sea level refraction table and multiplying by
    exp(-height/34,000 feet). At an altitude of 5000 feet, this is a factor of about  86.3%. When
    you adjust for tempreature and pressure, the air pressure has to be  the "sea
    level" adjusted pressure which is what's ordinarily reported in weather  data
    so that's no problem. But in addition, before making the  temperature/pressure
    adjustment, the temperature should be adjusted to sea level  at a rate of
    about 6.5 degrees C per kilometer of altitude or 3.5 degrees F per  thousand feet
    of altitude. That is, if you're 5000 feet high and the air  temperature is 32
    F while the barometric pressure is 29.90 inches, this  corresponds to a sea
    level temperature of just about 50 F (and the pressure is  already adjusted to
    sea level) so you're right at the standard conditions for  the usual refraction
    tables --no adjustment necessary. I don't think this second  part has been
    discussed before on the list, and I haven't thought about  thoroughly enough to
    "sign off" on it as correct but I think I've got it  right.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

       
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