  The attenuation of atmospheric Cherenkov photons is dominated by two
processes: Rayleigh scattering from the molecular component and Mie scattering
from the aerosol component. Aerosols are expected to contribute up to 30
Wm$^{-2}$ to the emission profile of the atmosphere, equivalent to a difference
of $\sim20^\circ$C to the clear sky brightness temperature under normal
conditions. Here we investigate the aerosol contribution of the measured sky
brightness temperature at the H.E.S.S. site; compare it to effective changes in
the telescope trigger rates; and discuss how it can be used to provide an
assessment of sky clarity that is unambiguously free of telescope systematics.
