The best performing systems draw incoming air through a wetted pad at the inlet of the ventilation system. As air passes across the great amount of surface area within the pad, BTUs of heat are removed from the air, lowering the air temperature. The more water is evaporated, the greater the temperature reduction. Also, the drier the air at low relative humidity, the greater the ability of evaporation to take place. During the hottest time of the day, when cooling is needed, generally the humidity will be low, and the air can be cooled several degrees.