The U.S. National Weather Service has recently corrected an error in radiosonde humidity data reduction algorithms, eliminated a sonde thats processing contained another error, and recently made a further change in the humidity data reduction algorithm. They also introduced new procedures for reporting humidity from radiosondes. These changes will affect the climate records of U.S. upper-air humidity. Because the changes affect observations differently at different temperatures and relative humidities, their impact on monthly mean humidity quantities depends on the Frequency with which certain conditions occur. The authors calculated the effects on monthly mean humidity quantities of five specific changes at a representative set of U.S. radiosonde stations. Some of these effects are quite substantial and should be considered in evaluating climate variability.