  Atmospheric density profiles as well as several light absorption and
scattering processes depend on geographic position and are generally
time-variable. Their impact on the atmospheric Cherenkov technique in general
(imaging or non-imaging) is investigated. Different density profiles lead to
differences in Cherenkov light density of up to 60%. Seasonal variations at
mid-latitude sites are of the order of 15-20%. The quest for improved energy
calibration of Cherenkov experiments also shows the need for improved
transmission calculations, taking all relevant processes into account and using
realistic profiles of absorbers. Simulations including the scattering
mechanisms also reveal the relevance of Rayleigh and Mie scattering for
atmospheric Cherenkov experiments. Refraction and the differences between
treating the atmosphere in plane-parallel or spherical geometry are also
investigated.
