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I finally had again a closer look at the exponential behaviour of
refraction at large ZDs. I can confirm now that the effects are "real"
and not the result of a strange bug in the program. The reasons are
the following ones:

(1) The effect of a temperature change on the pressure is as described
above. A change of temperature at the observer changes the temperature
ratios between the upper and lower level in all the layers of the
atmospheric model. The pressure values at each layer depend in a
non-linear way (exponentially) from the temperature ratio at the
corresponding layer.

(2) A change of pressure at the observer results in a non-linear
contribution from the changed ratio between the dry part of the air
pressure and the partial water vapour pressure. Note that the graphs
which have been shown earlier were calculated with an atmosphere
having 50% relative humidity. There is also a non-linear effect in the
case of a totally dry atmosphere; this one is much smaller. It looks
like it comes here from a non-linear contribution in the dn/dR
differential caused by the changes in the product refractivity times
pressure.

Btw: the program uses the hydrostatic equations for humid air as
derived and shown in the Hohenkerk/Sinclair paper. For those who are
interested: the above explanations can be compared with the content of
that paper.

Marcel

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