  In Monte-Carlo simulations of gamma-ray or cosmic-ray detector arrays on the
ground (here mainly arrays of imaging atmospheric Cherenkov telescopes), the
atmosphere enters in several ways: in the development of the particle showers,
in the emission of light by shower particles, and in the propagation of
Cherenkov light (or fluorescence light or of particles) down to ground level.
Relevant parameters and their typical impact on energy scale and so on are
discussed here.
