Control of humidity is studied in one of the environmental chambers for the NBS cesium beam clock ensemble. The relative humidity was changed from a few percent to 48%. All of the clocks underwent a change in frequency. The resulting frequency changes were of different sign and of varying magnitudes among the clocks. In some, the changes were an order of magnitude larger than the flicker noise FM levels associated with the corresponding clocks. The implications of such changes are quite significant in the generation of International Atomic Time (TAI) and of Universal Coordinated Time (UCT). The possibility of annual variations in the time scales of the principal timing centers as well as in TAI and UTC is discussed.
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