  Cherenkov telescope experiments, such as H.E.S.S., have been very successful
in astronomical observations in the very-high-energy (VHE; E $>$ 100 GeV)
regime. As an integral part of the detector, such experiments use Earth's
atmosphere as a calorimeter. For the calibration and energy determination, a
standard model atmosphere is assumed. Deviations of the real atmosphere from
the model may therefore lead to an energy misreconstruction of primary gamma
rays. To guarantee satisfactory data quality with respect to difficult
atmospheric conditions, several atmospheric data quality criteria are
implemented in the H.E.S.S. software. These quantities are sensitive to clouds
and aerosols. Here, the Cherenkov transparency coefficient will be presented.
It is a new monitoring quantity that is able to measure long-term changes in
the atmospheric transparency. The Cherenkov transparency coefficient derives
exclusively from Cherenkov data and is quite hardware-independent. Furthermore,
its positive correlation with independent satellite measurements, performed by
the Multi-angle Imaging SpectroRadiometer (MISR), will be presented.
