  The atmospheric monitoring devices for the planned calibration system of the
Cherenkov Telescope Array (CTA) are undergoing intensive development,
prototyping and testing. The All-Sky Cameras, the Sun/Moon Photometers and the
FRAM telescopes have been gradually deployed at the future CTA sites with the
primary goal of site characterization, simultaneously allowing the assessment
of their operational reliability in realistic environmental conditions. All
three devices have shown the ability to work smoothly in both the extreme
dryness and the large temperature variations of the southern site as well as in
the occasional adverse weather during winter months at the northern site. The
target availability of 95\% of time has not yet been reached mostly due to
minor hardware failures that have proven difficult to fix due to the remoteness
of the installation in the absence of the future CTA infrastructure. The
experience gathered during the prototype operations will contribute to the
improved reliability of the final instruments. The Raman LIDARs, described in
separate proceedings of this conference, and the infrared Ceilometer, ready for
testing in Prague, will complement the set of atmospheric calibration devices
in near future. The final operational procedures for the atmospheric
calibration of the CTA during its operation are being finalized foreseeing the
use of the All-sky Cameras and the Ceilometer for the monitoring of clouds over
the whole sky and the LIDARs and FRAMs for precision measurements of the
atmospheric transmission as a function of altitude and position within the
field-of-view of the CTA array.
