However, it probably comes from the fact that if the air outside is hot and already humid, the cooler will blow that hot and muggy air into the room instead of cool air.
Why does that happen? An evaporative cooler is able to produce cool air by the natural process of water evaporation. It then moves that air into the room by means of an air flow system involving a motor and a fan. Hot, fresh air is pulled into the cooler where it comes into contact with moist pads. The hot, dry air evaporates some of the water from the pads and the air is cooled in the process. The pads also happen to act as an air purifier or ionizer, capturing small particles of dust or other pollutants before the air circulates into the room.
But if the air that comes into the evaporative cooler is already saturated with water, as is the case in areas with high humidity, then it can’t pick up any morewater. If the air can’t pick up any more water, then it can’t be cooled by the evaporative cooler. Instead, the hot, humid air will pass through the cooler unchanged, and the result will be evaporative coolers acting more like what earned them the name swamp coolers – they’ll simply blow the hot humid air from outside into your home.
Starting prices for evaporative coolers 
Swamp coolers or evaporative coolers work the best in areas like the southwest of the United States, where the climate is regularly hot and dry. Because the coolers use just a fraction of the electricity that comparable air conditioners use, they can result in a great savings in energy bills as well. In fact, many energy companies in the areas where swamp coolers work the best actually offer incentives to homeowners to install an evaporative cooler. Some offer rebates of up to $300 to install a whole-house evaporative cooler.
But if you don’t need a whole-house system or don’t want to go to the expense of installing one, evaporative coolers are available in window units or portable units as well. That way you can use swamp coolers only in the rooms that gets the hottest, or you can move one from room to room as you need it.
