Evaporative coolers cool the air through the evaporation of water. Similar to getting out of the swimming pool or the shower, when water evaporates it dissipates or allows the heat to escape from the air. Evaporative coolers work on the same principle. Water is dripped into the cooler pad. When the water evaporates it cools the hot air that is drawn through the pads from the outside.
How Cold Is The Air?
Under most common conditions an evaporative cooler will produce air cooler than 80 degrees. Humidity plays a key roll in how cold the air will be. Air evaporates less when the humidity is up because the outside air is wet or damp. A common sponge helps explain this. A wet sponge can't soak up much water when it's wet, i.e. it's full of liquid already. But when it's dry it soaks up lots of liquid. Humid air is wet so it doesn't evaporate as well as non humid air.
Using Tucson, Arizona as an example. If the outside air is 106 degrees and the humidity is 20% evaporative cooled air would be about 76 - 78 degrees.
Other Benefits From Evaporative Cooling
An evaporative cooler uses about 25% of the electricity that a typical air conditioning unit would use. . This means you can save up to 75% of your cooling costs on your electric bill!
When using an evaporative cooler you want the hot air in your house out so you leave a window or door cracked open. This allows new cooled air in and warmer air to escape. This also means that stale air escapes the house. Air conditioners are closed systems, i.e. the use the air from inside your home over and over again while an evaporative cooler constantly draws in fresh air from outside.