  Atmospheric monitoring is an integral part of the design of the Cherenkov
Telescope Array (CTA), as atmospheric conditions affect the observations by
Imaging Atmospheric Cherenkov Telescopes (IACT) in multiple ways. The variable
optical properties of the atmosphere are a major contribution to the systematic
uncertainty in the determination of the energy and flux of the gamma photons.
Both the development of the air-shower and the production of Cherenkov light
depend on the molecular profile of the atmosphere. Additionally, the rapidly
changing aerosol profile, affecting the transmission of the Cherenkov light,
needs to be monitored on short time scales. Establishing a procedure to select
targets based on current atmospheric conditions can increase the efficiency of
the use of the observation time. The knowledge of atmospheric properties of the
future CTA locations and their annual and short-term variations in advance is
essential so that the atmospheric calibration can be readily applied to first
scientific data. To this end, some devices are already installed at one or both
of the selected sites...
