Water vapor is constantly present in the atmosphere. This is referred to as humidity. The amount of water vapor can be up to a certain limit. The air is then oxygenated with water vapor. At a certain height, the air temperature decreases, the water vapor there turns into water droplets and "settles inside" the cloud. As the amount of water drops increases, the drops become larger and heavier. It becomes "difficult" for them to remain "suspended" in the air, and water vapor falls to the Earth in the form of atmospheric precipitation.