  The Cherenkov Telescope Array (CTA) will be the next high-energy gamma-ray
observatory. Selection of the sites, one in each hemisphere, is not obvious
since several factors have to be taken into account. Among them, and probably
the most crucial, are the atmospheric conditions. Since July 2012, the site
working group has deployed automatic ground based instrumentation (ATMOSCOPE)
on all the candidate sites. Due to the limited time span available from ground
based data, long term weather forecast models become necessary tools for site
characterization. It is then of prime importance to validate the models by
comparing it to the ATMOSCOPE measurements. We will describe the sources of
data (ATMOSCOPE, weather forecasting model and satellite data) for the site
evaluation and how they will be used and combined.
