Evaporative air coolers cool the air up to 20 degrees Fahrenheit but add humidity to the air, while air conditioners cool by reducing humidity, according to Air and Water. Evaporative coolers are less expensive to operate than air conditioners
Evaporative air coolers work much like perspiration cooling the skin. As water evaporates, it absorbs heat. Evaporative coolers work well in dry climates, because the low humidity of the air allows large amounts of water to evaporate, increasing the unit's cooling ability. In humid areas, the unit becomes less effective. The humid air cannot accept as much water, so there is less cooling. Under these conditions, the additional humidity reduces the swamp cooler's effectiveness at cooling the air.
Air conditioners use evaporation and condensation to pump heat out of an enclosed space. When William Carrier built the first air conditioner to reduce humidity in a print shop, he found it also provided cooling. The system blows air through chilled coils to cause moisture to condense and drop to a tray. As it moves through the coils, it cools an enclosed space.
The best choice for a particular space depends on many conditions and personal preference. Evaporative coolers operate with 75 percent greater efficiency than air conditioners, according to Air and Water. However, if the humidity is already high, they may cause problems, including mold and mildew.