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.