An infrared radiometer with a pass band of 280 to 520 cm-1 (35.7 to 19.2 &mgr;m) is employed on the NASA Ames Research Center U-2 and C141A aircraft in the measurement of water vapor burden in the upper troposphere and stratosphere. Coincidentally with altitude changes the water vapor mass mixing ratio is also inferred by observing the change in optical depth over a known vertical distance. Data from the December 1980 U-2 water Vapor Exchange Experiment over the Panama Canal Zone adds to the concept that overshooting cumulonimbus towers ''moisten'' the lower stratosphere. The average mass mixing ratio in close proximity to or above such towers ranges from 3.5 to 5.0 parts per million above 18 km while the average background mass mixing ratio is only 2.9 parts per million. Generally the lowest background mixing ratios, averaging 2.6 parts per million occurred in the 18 to 21 km layer. For the same levels background Panama mass mixing ratios averaged from 1.0 to 3.0 parts per million higher than in middle latitudes.