About twenty radiosonde flights to altitudes above 100,000 ft have been made from Belmar, New Jersey (latitude 40.2°N) during the period July 1948 to April 1949, using a balloon whose nominal weight was 10,000 grams. The radiosonde and associated equipment were specially designed to accurately measure pressure, temperature and winds to an altitude of 150,000 ft. It was found that the mean daytime temperature of the stratosphere is about −60C from 50,000 ft to 60,000 ft, above which the mean temperature rises at a rate of about 0.5C per 1000 ft to a temperature of about −30C at 120,000 ft. No abnormally high temperatures at high altitudes resulting, for example, from a solar flare, were detected in any of these flights. It was found that the winds below 60,000 ft were predominantly westerly, with maximum speed at an altitude of about 40,000 ft. Between 60,000 ft and 120,000 ft, the winds were easterly during the summer and westerly during the winter. The pronounced easterly flow commenced about three weeks after the vernal equinox and ceased about three weeks before the autumnal equinox. Usually the wind speed was still increasing at the bursting altitude of the balloon. These results are consistent with the existence of a stratospheric circumpolar vortex which is cyclonic in winter, anticyclonic in summer. The fragmentary wind data obtained from these flights at high altitudes suggest the possible existence of a stratospheric jet stream in middle latitudes. The significance of such a jet stream, if confirmed, is discussed.