When water is evaporated, energy is lost from the air, reducing the temperature. Two temperatures are important when dealing with evaporative cooling systems.
Dry Bulb Temperature

• This is the temperature that we usually think of as air temperature, measured by a regular thermometer exposed to the air stream.

Wet Bulb Temperature

• This is the lowest temperature that can be reached by the evaporation of water only.

When considering water evaporating into air, the wet-bulb temperature, as compared to the air’s dry-bulb temperature, is a measure of the potential for evaporative cooling. The dry and wet bulb temperature can be used to calculate the relative humidity.
Evaporation will take place when the humidity is below 100% and the air begins to absorb water. Any given volume of air can hold a certain amount of water vapour and the degree of absorption will depend on the amount it is already holding.
The term humidity describes how much water is already in the air; relative to the amount it is capable of holding. Air is saturated when it cannot hold any more water. Imagine it as a sponge, if the sponge held half as much water as it was capable of holding, it would be 50% saturated. In the case of air, we would describe the Relative Humidity as being 50%.
Energy is required to change water from liquid to vapour. This energy is obtained in an adiabatic process from the air itself. Air entering an evaporative air cooler gives up heat energy to evaporate water. During this process, the dry bulb temperature of the air passing through the cooler is lowered.