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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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