What the heck is an "evaporative air cooler" or “swamp cooler”?

If applied in the right circumstances (not super high humidity), an evaporative air cooler is a fantastic choice for cooling air.  These units are less expensive than a traditional air conditioner, they use less energy, and they are simpler machines. An evaporative air cool works on a very simple process based on a common endothermic reaction we have all experienced, but seldom think about. Air is passed through a wet media (the wick). Evaporation of the water in the media cools the air. Very similar to how sweating cools us off on a hot day.

Evaporative Air Coolers work better in a dry climate than a humid one because of the fundamental physics behind how they work. To continue the example from above, if it is more humid our sweat system does not work as well. That's because when the air is humid (moisture laden) not as much water can be evaporated into the air (kind of like a how a dry sponge will absorb more liquid than a wet one).

An evaporative air cooler actually works better when given plenty of ventilation to the outside air. This can be a little counter-intuitive to some because many people are used to sealing up their house or car and cranking up the air conditioner. With an evaporative air cooler, you want ventilation to carry away the more humid air it produces.

Okay, we throw around the term “endothermic reaction”. That’s a fancy-sounding physics term to describe a process that absorbs energy in the form of heat. In an evaporative air cooler it is this process that absorbs heat from the air. Think about standing in front of a fan wearing a wet shirt. Even on a hot day, the endothermic reaction will cool you off. If it is a humid day, you will not be cooled as much because the water in your shirt can not evaporate as fast into the already moisture laden air. In this example, like the sweating example above, it is not the water or air absorbing the heat, it is the endothermic reaction.