A word for the amount of water held in the air as invisible vapour. In this state water is a gas.

The amount of moisture in the air varies with the temperature of the air. Hot air can hold more moisture than cool air. Cool air can hold more moisture than cold air.

So, when air flows toward a mountain, it rises up the mountain side and cools. Because it can hold less moisture, the air starts to release surplus moisture as droplets or ice crystals and we see this as cloud. When the droplets group together they become rain or snow.