Is the principle of the natural method that uses water to reduce the temperature of hot air. The principle is to divide the water particles, there is a very small surface area. When the hot air over the water surface. Hot water is part of the lower temperature endothermic (Latent Heat) and some water gets hot vaporization. The result is a decrease in the temperature of hot air, but the water vapor in the air and water, more and more, there are some very small particles do not change when exposed to heat, as it quickly. The water area to evaporation by heat water evaporation will pull the nearby,where water evaporates to pull the heat to nearby. So the area has a low temperature over a similar atmosphere where there is no water source.