See wet bulb temperature below. The phase change of liquid water into ice. The phase change of liquid water into water vapor . The phase change of ice into liquid water . The amount of water vapor actually in the air divided by the amount of water vapor the air can hold.
Relative humidity is expressed as a percentage and can be computed in a variety of ways.
One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent. Click here for text explaining this in more detail. The condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air. The maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature.