Water vapor is one of the gases in air. Unlike nitrogen and oxygen which are constant in the bottom 100 km of the atmosphere, water-vapor concentration can vary widely in time and space. Most people are familiar with relative humidity as a measure of water-vapor concentration because it affects our body’s moisture and heat regulation. But other humidity variables are much more useful in other contexts.
Storms get much of their energy from water vapor — when water vapor condenses or freezes it releases latent heat. For this reason we carefully track water vapor as it rises in buoyant thermals or is carried by horizontal winds. The amount of moisture available to a storm also regulates the amount of rain or snow precipitating out.
What allows air to hold water as vapor in one case, but forces the vapor to condense in another? This depends on a concept called “saturation”.