It is expressed as a percentage. If the relative humidity is 50 percent, the amount of water vapor in the air is half of what it would be if the air were saturated. Lowering the air temperature increases the relative humidity . If the relative humidity is 100 percent, the air is saturated. When air becomes saturated, water vapor begins to condense into droplets of water.
Condensation is the opposite of evaporation.
It is a change from a gas to a liquid. If the air is cold enough, at saturation the water vapor develops into tiny ice crystals. If condensation occurs on a cold surface such as the surface of an automobile window at night, dew or frost forms. Dew and frost do not fall from the sky like rain or snow. Rather, they form when air in contact with a relatively cold surface is chilled to saturation.