[hdersch]

Atmospheric refraction is due to the vertical gradients of atmospheric pressure, temperature, and composition of the atmosphere, all of which are usually not precisely known, and which vary with time, so one gets larger lines of sights at certain times. For my application I used the standard formulas for astronomical refraction (-> many weblinks) with constant medium gradients. If I recall correctly this results in ~100m height correction for features in 100km distance and ~400m in 200km distance (features appear higher than without atmospheric refraction). For your application it would make sense to use two extreme values for the gradients to get maximum and minimum, as you suggested.

[tombh]

Right, yes variance over time is what I was referring to. I did a bit of research and indeed there seems to be some evidence backed minimum and maximum values I can plug in. But it's quite a variance! Now I'm thinking I should do min, max and average.