In their 1967 paper “Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity” (JAS, 24, 241-259), Manabe and Wetherald explored the behavior of atmospheric temperature under conditions of fixed absolute and relative humidity. The assumption of a fixed global mean relative humidity was made. In this paper, we present a technique to estimate global lower tropospheric relative humidity from satellite observations. A twelve-year time series of relative humidity (1988 – 1999) is created. The NASA Water Vapor Project (NVAP) global layered water vapor dataset and the Microwave Sounding Unit (MSU) lower tropospheric temperature data are combined to estimate relative humidity. We find a time series with a global mean of 30 % and a standard deviation of 1 %. A seasonal cycle is evident, and there is a slight drop in relative humidity during July of each year associated with heating of the land in the Northern Hemisphere. It appears that Earth has maintained a nearly constant relative humidity during this time period.