Evaporative coolers (also called swamp, desert, or air coolers) are devices that cool air through the simple evaporation of water. They differ from refrigeration or absorption air conditioning, which use vapor-compression or absorption refrigeration cycles. In the United States, evaporative coolers are often called swamp coolers (possibly due to the odor of algae produced by early units[1]). Air washers and wet cooling towers utilize the same principles as evaporative coolers, but are optimized for purposes other than air cooling.

Evaporative cooling is especially well suited for climates where the air is hot and humidity is low. For example, in the United States, the western/mountain states are good locations, with swamp coolers very prevalent in cities like Denver, Salt Lake City, Albuquerque, El Paso, Tucson and Phoenix, where sufficient water is available. Evaporative air conditioning is also popular and well suited to the southern (temperate) part of Australia. In dry climates, the installation and operating cost of an evaporative cooler can be much lower than refrigerative air conditioning, often by 80% or so. But evaporative cooling and vapor-compression air conditioning are sometimes used in combination to yield optimal performance. Some evaporative coolers may also serve as humidifiers in the heating season.

In moderate humidity locations there are many cost-effective uses for evaporative cooling, in addition to their widespread use in dry climates. For example, industrial plants, commercial kitchens, laundries, dry cleaners, greenhouses, spot cooling (loading docks, warehouses, factories, construction sites, athletic events, workshops, garages, and kennels) and confinement farming (poultry ranches, hog, and dairy) all often employ evaporative cooling. In highly humid climates, evaporative cooling may have little thermal comfort benefit beyond the increased ventilation and air movement it provides.