How does relative humidity vary with altitude?

Well, there's different types of humidity. You are probably most familiar with Relative Humidity, which reflects the percentage of saturation of the atmosphere. read more

You can see this from observing clouds. When there are clouds visible the air there is at saturation point - 100 % relative humidity. Everywhere else the humidity is lower. As even the most basic of observations show. the locations of clouds varies day by day with the weather - as does the atmospheric humidity. read more

Relative humidity depends only on the temperature and actual water vapor present. It has nothing to do with the air pressure itself. If I am at sea level with a pressure of 1013 mb, and the air temp is 59° F, then 8.5 mb of water vapor content means the RH = 50% (8.5/17). read more