Using radiosonde data from 50 stations for 1973--1990, we quantify relationships between surface air temperature (Ts) and precipitable water vapor (W) for different time scales. Monthly mean observations are fairly well described by an equation of the form ln W=A+BTs, but the coefficients A and B depend on the Ts range considered. At high Ts, the relationship is poor. This relationship and relationships between sea surface temperature (SST) and W based on satellite microwave observations over oceans are in remarkably good agreement over restricted SST ranges. Monthly and annual anomalies of W and Ts are well correlated only outside the tropics, but on longer time scales, there is some evidence of positive trends in both W and Ts at most of the stations studied. Thus the relationship between W and Ts depends on the time scales and geographic region considered. ¿ American Geophysical Union 1992