A case study is presented of the onset of the seasonal transition from the dry conditions characteristic of early summer over southern New Mexico to the more humid conditions of late summer. Observations of C2n from the VHF radar wind profiler at White Sands Missile Range, New Mexico, are used as proxy indicators of humidity in the troposphere. The observations are made each 3 min at heights from about 5 to 20 km and are presented as hourly means. During June 1991, a sudden shift in the mean value of C2n of about 10 dB took place throughout the troposphere, in a time span of only a few hours. Surface and upper air charts, local weather observations, weather surveillance radar, and satellite imagery are used to establish the synoptic events associated with this shift. No distinct feature changed in the flow patterns aloft or in the analyzed air mass boundaries. The shift in moisture observed by the VHF radar is found to persist on time scales of a month or more from inspection of time series of precipitable water from radiosonde data at nearby El Paso, Texas. ¿ American Geophysical Union 1993