As water (or any substance) evaporates, the liquid left behind becomes cooler. What we think of as temperature is really the average kinetic energy of the molecules of a substance. Molecules with higher kinetic energy move faster and are more likely to leave the liquid as a gas. When they do, they take this energy with them and leave behind molecules with lower kinetic energy, and therefore lower temperature. This process is known as evaporative cooling.
In many living organisms, including humans, evaporative cooling plays a vital role in maintaining body temperature. By releasing sweat, which is 90 percent water, humans can utilize the effects of evaporative cooling to give off heat to the surrounding air, even if that air is warmer than their body temperature.
When humans cool themselves, they give off heat and moisture to the surrounding air, causing it to become hotter and more humid, and reducing its capacity to absorb additional heat and moisture. This layer of hot humid air forms a blanket around the body, reducing its ability to give off excess heat.
The breeze from a ceiling fan disrupts this layer of insulating air and supplies cooler, drier air. This not only allows for faster rejection of heat directly from the skin, but it also increases the rate at which perspiration evaporates, increasing the effects of evaporative cooling.
By working with the body’s natural ability to utilize evaporative cooling, Big Ass Fans® cool occupants by as much as 10°F (6°C), and do so more efficiently than air conditioning, which cools the space rather than the people in it.