Further laboratory results with the aluminum oxide humidity sensor are described. It is concluded that the sensor has sufficient sensitivity, time response, accuracy and stability to be successfully employed in all portions of a balloon flight and parachute descent in the atmosphere. The results of a number of flights are shown. Frost point values between −75C and −96C at the tropopause were observed, indicating that stratospheric water content may be dependent on both location and time. The continuous pattern of increasing water content above the tropopause is consistent with the results of many investigations as compiled by Gutnick and Salmela.