An evaporative cooler produces effective cooling by combining a natural process - water evaporation - with a simple, reliable air-moving system. Fresh outside air is pulled through moist pads where it is cooled by evaporation and circulated through a house or building by a large blower. As this happens, the temperature of the outside air can be lowered as much as 30 degrees. Evaporative coolers can work wonderfully well, provided the outside air they are drawing in is dry and desert-like. However as the humidity increases, the ability for them to cool the air effectively decreases. Whether it is for a single room or a whole house, there is a simple formula for determining the proper size of evaporative cooler you need. Figure the cubic feet of space you want to cool, and then divide that number by two. The quotient will give you the CFM rating for the proper-sized evaporative cooler.