When It's Too Humid for Swamp Coolers

Q: When it's humid out, I know a swamp cooler is inefficient, but is there a graph that shows at what humidity level they work? For instance, if the humidity is 10 degrees, will it work? At what level does it stop? I just rented an apartment, and the swamp cooler doesn't work.
Rita, Las Vegas, Nevada, USA

Q: What is a good range of humidity percentage for swamp coolers to work?
Martha, Tyler, Texas, USA

Q: What is the optimal relative humidity for a swamp cooler to work?
Sandi, Albuquerque, New Mexico, USA

A: Rains do come to deserts and, when they do (in July and August for the Southwest), swamp coolers don't work well. How well? The chart above tells the story.

Keeping a house cool with a swamp cooler depends on the temperature and humidity of the air coming into the cooler. The chart shows the inlet temperature (the numbers along the red bar) and inlet humidity (the gray bar numbers).

The swamp cooler cools the outside air and blows the cooler air into the house. The green range of cool air temperatures on the graph shows the conditions when the swamp cooler works best — lowering house temperature to the 70's (22 - 26 C).

Humidity makes a difference. At 50% humidity, an evaporative cooler can lower the air coming into the cooler by about 10 degrees (6 degrees C). But at 10% humidity, the cooler can plummet temperatures by about 20 to 30 degrees (13 to 14 degrees C)

So, for example, if the outside temperature is 90 degrees F (32 C) and the relative humidity is 50%, then the swamp cooler can cool the house to 79 degrees (26 C). Moreover, at 90 degrees (32 C) outside and only 10% humidity, the house temperature drops to 67 degrees (19 C).

On the other hand, when the outside temperature soars to 100 degrees (38 C), we're in trouble if the humidity is much above 25%. When the swamp cooler doesn't work well depends both on outside temperature and humidity. The higher the outside temperature, the lower the humidity must be to drop the house temperature into the cool 70's.

By the way, the swamp cooler sends a breeze through the house so the air temperature feels about five degrees cooler that it is.

Further Reading:

What the heck is a swamp cooler and how does it work? WonderQuest

Evaporative cooling (including a good animation showing a swamp cooler in action), California Energy Commission

Ed's Weather by Ed Phillip, Arizona Almanac

(Answered May 28, 2007)