  The current generation of Imaging Atmospheric Cherenkov telescopes are
allowing the sky to be probed with greater sensitivity than ever before in the
energy range around and above 100 GeV. To minimise the systematic errors on
derived fluxes a full calibration of the atmospheric properties is important
given the calorimetric nature of the technique. In this paper we discuss an
approach to address this problem by using a ceilometer co-pointed with the
H.E.S.S. telescopes and present the results of the application of this method
to a set of observational data taken on the active galactic nucleus (AGN) PKS
2155-304 in 2004.
