The principle of evaporative cooling was discovered by Ben Franklin in the 18th century. He found that the faster a liquid evaporated, the colder the surrounding area became. When he dipped a thermometer in ether and blew on it with a pair of bellows, it froze in the middle of summer. If you live in the right place, you could see similar results when you switch on your evaporative cooler.
How Evaporative Coolers Work
Evaporative coolers, or swamp coolers, suck in hot air in with a fan and through a moistened pad. The hot air evaporates the water in the pad and then gets blown out again, colder than when it came in. How much colder depends one thing: the relative humidity.
How Relative Humidity Works
Relative humidity is the ratio between the amount of water vapor in the air (actual vapor pressure) and the maximum amount of water vapor the air can hold (saturation vapor pressure).
If actual vapor pressure was 10 oz./in3, and saturation water density was 20 oz./in3, then relative humidity would be 50 percent. When it’s 100 percent, water vapor condenses back into water. The key variable in relative humidity is temperature. As air heats up, air molecules are forced farther apart, more vapor is required to fill it and saturation vapor pressure goes up. The opposite happens temperatures drop. Air molecules are forced closer together so less water is needed to fill the space.
What Happens to Evaporation in Low Humidity
Evaporative coolers work when relative humidity is high. It’s a common misconception that they don’t. Evaporative coolers still cool you down even when humidity climbs as high as 70 percent. The amount of cooling is just less dramatic, what you’d expect from an ordinary fan or air conditioner.
When relative humidity is low, evaporative coolers kick into overdrive. Evaporation requires heat and space. When relative humidity is very low, temperatures are generally high, the air can hold more water, and the rate of evaporation skyrockets. If you turned on an evaporative cooler in a house with five percent humidity, it would lower the temperature 20-25 degrees! If it was 110 outside, it would be 77 inside. If it was 75 outside, it would be 55 inside and you’d be breaking out your winter gear.
Areas with Low Humidity
Some areas of the country have low relative humidity year round, primarily the west and southwest. Some only have it in summer, when temperatures peak. To find out if you can expect dramatic results from your swamp cooler, visit the Relative Humidity Map at Weather Central. They track relative humidity across the country. Wherever you see the humidity dip, that’s where your evaporative cooler will work the best.
Want to know more about evaporative cooling? Ask us in the comments!