It uses the natural principle of cooling by evaporation. Wet your finger and hold it in the air and it becomes cool; that is because evaporation is occurring; it is a natural phenomenon. In the Evaporative Air Conditioner, a pump circulates water through a set of cooling pads through which air is forced by a motor-driven fan. The hot outside air passing through the pads evaporates the water in the wetted pads at a very high rate. This cools the air which is then pumped into your building.