Air temperatures to altitudes as high as 100,000 feet have been measured with radiosondes carrying a temperature element which was exposed directly to solar radiation. The temperature element was coated with a special white paint which reflected more than 90 per cent of the solar radiation. Air temperatures measured with the white element should not be in error by more than 0.5C except at the highest altitudes. Air temperatures measured with radiosondes using an air duct for radiation shield were higher than temperatures obtained with the white element by 3–6C at 50,000 feet, by 5–10C at 75,000 feet and by as much as 20C at 100,000 feet. The results of a limited number of flights indicate that to altitudes of 100,000 feet, the heat wake of a balloon does not extend as far as 25 feet below the balloon.