An evaporative air cooler cools air by humidifying it, while an air conditioner pumps heat out of a building using a cooling liquid in a closed system. Evaporative coolers are simpler, more energy efficient devices, but they are much less effective at cooling, especially in humid regions of the country.
An evaporative cooler, sometimes called a swamp cooler, relies on the evaporation of water to function. When hot air passes over water, it causes molecules of water to evaporate from the surface of the liquid. This process requires energy, which removes heat from the hot air. A typical evaporative cooler uses a fan to blow air over a dampened pad or wick, drawing hot air in one end of the unit and expelling cooler, more humid air from the other.
The chief disadvantage of an evaporative cooler is it requires low humidity to work well. If there is already a large amount of water vapor present in the air, it reduces the amount of water that can evaporate from the reservoir, thus limiting the amount of energy removed from the air. The increased humidity can cause condensation and rust, and the unit requires a steady supply of fresh water to function.
An air conditioner cycles a refrigerant that transforms from a gas to liquid and back again, absorbing heat from inside the home and transferring it outside. These devices can work even in very humid climates, and actually dehumidify a home in the process of reducing the air temperature.