Evaporative cooling occurs naturally all around us. For example, you feel it when you step out of a pool on a hot day and immediately feel a chill. These cooling effects occur because as dry air passes over water, the dry air will absorb some of the water.
This absorption occurs when the temperature and vapor pressure of the water attempt to equalize with the air. As a result, water molecules turn into gas molecules and heat switches from the higher air temperature to the lower water temperature. Since the air circulates naturally, the area around it is cooled.
To consistently cool your home, residential evaporative air coolers build on this natural phenomenon, using a fan to draw warm, stale air inside the unit, where it passes over water-moistened pads to be cooled.