Swamp coolers use the principles of evaporative cooling to cool the air.

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If you've ever tested the wind by holding a wet finger in the air, you've used evaporative cooling. The same principle cools you off after a swim, and it also powers one of the oldest and simplest forms of air conditioning. Known in the U.S. as swamp coolers, modern evaporative coolers can trace their lineage to ancient Egypt. They're cheap, efficient and good for the environment, but they come with some limitations, so don't push your standard air conditioner out the window just yet.

The ancient Egyptians had a great need for air conditioning. They accomplished it by hanging wet blankets across the doors of their homes or, if they happened to be royalty, having servants fan them across jugs of water. When hot, dry air passes over water (or better yet, through it) the air cools off. Nowadays, we use electric fans instead of servants, but the principle of cooling the air by evaporation remains the same.

Unfortunately, evaporative air coolers don't work everywhere. Swamps, for instance, are lousy places for swamp coolers. It's not entirely clear where they got the nickname, but it probably refers to the humidity they add to the air or the swampy smell that can develop when they aren't cleaned often enough. In order to work, they need a hot, dry climate. In the U.S., swamp coolers work well in the arid southwest. There were more than 20 million evaporative coolers worldwide in 1998, with four million of those in the US. According to the Energy Information Administration's 2001 appliance report, only three percent of U.S. households had swamp coolers, but in the states along the Rocky Mountain Range, they could be found in 26 percent of the homes.

Swamp coolers are based on a simple, efficient technology that has been around a long time. The principles of evaporative cooling worked for the pharaohs, and they can still work for you. Next we'll take a look at how they work.