  Advanced knowledge of the detailed atmospheric properties of both the future
sites of the Cherenkov Telescope Array is essential in preparation of the
arrival of the first scientific data. Meteorological variables are studied
using a dedicated characterization station installed at the southern site in
Chile and a wealth of data from existing observatories around the northern site
on the La Palma island. Campaigns using radiosondes launched on balloons are
foreseen to complement these data in the near future. Cloudiness during the
night has been continuously monitored at both sites for several years using
All-sky Cameras which assess the presence of clouds based on detection of
stars. The integrated aerosol optical depth over the southern site has been
measured using a Sun/Moon Photometer since 2016 and the small robotic FRAM
telescope since 2017; identical instruments have been deployed at the northern
site in autumn 2018. Also in October 2018, the ARCADE Raman lidar (RL) has
started to take measurements on routine basis at the northern site, providing
data on the vertical profile of the aerosol optical properties (i.e.,
extinction and scattering) and of the water vapour mixing ratio. We present the
data currently available from these instruments from both sites with emphasis
on characteristics important for the (future) operation of Imaging Atmospheric
Cherenkov Telescopes.
