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    Re: Refraction and humidity
    From: Frank Reed CT
    Date: 2007 Mar 27, 15:44 -0700

    Bill you wrote:
    "That would indicate my recollection that humidity is a component of
    refraction is correct, and my belief that the color of a star/body may
    affect refraction.  I do not know if dust in the air will affect
    refraction like humidity."
    Humidity has no significant effect on refraction at any altitude. You
    don't have to worry about it. The direct impact of humidity on the
    refraction of air is very small.
    There is, however, an indirect connection with humidity involving the
    lapse rate of temperature in the lower atmosphere. In desert areas,
    the air usually cools quite a bit more rapidly with altitude in the
    lowest few thousand feet than in balmy, tropical areas. This happens
    because humid air releases a lot of energy as tiny droplets of water
    condense out of it when it rises and the pressure falls. These
    differences in the lapse rate change the refraction by 0.1 minutes of
    arc or more for altitudes below THREE degrees. Above those very low
    altitudes, the only thing you have to worry about is temperature and
    pressure (taking into account the observer's height above sea level).
    But stay away from stars below three degrees altitude.
    The color of a star does not much impact the tabulated refraction
    except at the lowest altitudes. This isn't connected with humidity, of
    course, it's just the usual dispersion of colors as in a rainbow. It
    turns bright stars into little "French flags" when they're very low in
    the sky. Since the image of the star is stretched out, it becomes
    difficult to decide where the true center is located. So stay away
    from stars below three degrees altitude.
    Dust does not directly affect refraction. Dust absorbs and scatters
    light, rather than refracting it. It can enhance the "French flag"
    problem I mentioned above by removing the blue end of the spectrum.
    Indirectly, the presence of dust in the air may be a clue that you are
    under a temperature inversion (temperature increasing with altitude
    for a few hundred meters of more), and that means significant changes
    in refraction at very low altitudes. In such a situation, you should
    stay away from stars below three degrees altitude. :-)
    All of these variations in refraction even below three degrees
    altitude are smaller than a minute of arc so they really only matter
    for high-accuracy sights.  For normal celestial navigation, if the Sun
    is a degree high and it's your best opportunity to take a sight,
    there's no reason to avoid it.
    42.0N 87.7W, or 41.4N 72.1W.
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