Evaporative coolers, also called swamp coolers, rely on this principle. By passing outdoor air over water-saturated pads, the water in the pads evaporate, reducing the air temperature by 15°- to 40°F-before it is directed into the home. Evaporative coolers cost about one-half as much to install as central air conditioners and use about one-quarter as much energy. However, they require more frequent maintenance than refrigerated air conditioners and they're only suitable for areas with low humidity.