Abstract
Measurements have been made of atmospheric humidity to altitudes as high as 35 kilometers by means of a hygristor — a humidity-sensitive radiosonde accessory which is essentially a miniature version of the Foxboro Dewcel. The hygristor indicates ambient dewpoint so long as the relative humidity is above a threshold which is roughly 15 per cent. When the relative humidity is below the threshold value, the hygristor indicates the upper limit of possible dewpoint values. Since the relative humidity in the stratosphere was usually very low, most of the humidity data obtained with the hygristor above the tropopause are in the latter category. However, on several occasions when precise values of dewpoint were obtained in the stratosphere, it was found that the dewpoint increased above the tropopause and was even higher than the temperature of the tropopause at altitudes near 20 kilometers.
