Evaporative cooling is a cooling technique which uses the evaporation of water to cool the temperature. Humans actually use evaporative cooling naturally when they sweat as they heat up, and the sweat evaporates, lowering body temperature. The same principle can be used to bring the air temperature down in warm climates. This type of cooling is generally only suitable for hot, dry climates. In a humid climate, evaporative cooling cannot work as well, and it can cause the humidity to reach an uncomfortable level.
The basic design of an evaporative cooler consists of a fan which pulls hot air through a series of water soaked pads. As the air moves through the pads, it cools down, and the cool air can be vented directly into a room or into a duct system. The humidity will also increase slightly, thanks to the water in the air, which can actually be beneficial in very dry climates where the dry air leads to issues like cracked lips and dry skin.
One advantage to evaporative cooling is that the air feels fresher. Air conditioning works on a closed system, while evaporative cooling actually requires windows to be open for ventilation so that warm air from the house can escape and the evaporative cooling device can push in cooled air. Since recycled air can sometimes be stifling, some people enjoy the sensation of a steady fresh breeze created with evaporative cooling. People can direct the breeze by opening and closing various windows.