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    Re: Refraction at the horizon
    From: Marcel Tschudin
    Date: 2008 Mar 16, 10:58 +0200

    Frank, you wrote:
    >  Primarily, this is the temperature structure of the lower atmosphere
    >  (temperature inversions, etc.). There are also small effects from humidity.
    
    Indeed, humidity has a very small influence on the refractivity of
    air. This effect can likely be ignored in most cases. However, the
    water content in the atmosphere may become important to a certain
    extend in a different way. I mentioned in an other mailing that the
    differences of refraction at the horizon between sunrise and sunset do
    change as a result of the diurnal temperature cycle. To explain this I
    compared the diurnal temperature variations within the planetary
    boundary layer (PBL) with the simplified model of a pendulum where the
    length of the pendulum corresponds to the height of the PBL and its
    deflection to the diurnal temperature change at the earth's surface.
    The diurnal temperature has a minimum shortly before or at sunrise and
    a maximum early afternoon. It's in this model where the water content
    of the atmosphere plays a more important role. The difference between
    maximal and minimal temperature is smaller with an increased water
    content in the atmosphere. Tropical locations (Koeppen climate class
    A) have much smaller diurnal temperature variations than e.g. desert
    locations (Koeppen climate class B). I expect therefore that this
    difference may also have a noticeable influence on the diurnal change
    of refraction. Btw it's that what I'm investigating at the moment.
    
    Marcel
    
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