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When it's too humid for swamp coolers; Why the ground darkens with rain

Cut away photo of a swamp cooler.  The fan draws warm outside air through the wet pads and blows the cooled-by-evaporation air throughout the house. Photo courtesy of ASAP Heating & Cooling.

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 a swamp cooler 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 below tells the story.

How well a swamp cooler cools a house.    Information from the Arizona Almanac by Ed Phillips.

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

Q: Why is the ground darker after a rain?  Gyula, Kaposvar, Hungary

A: I looked out my window at sunlight slanting through piñons, and thought about rain.  An object looks black, because it absorbs most light shining on it, leaving little to reflect to our eyes.  So, the ground must look darker after a rain, because the ground absorbs more light when it's wet. 

The question is, why should ground absorb more light, when wet rather than dry? 

Light (depicted as a yellow line) enters dry dirt (top image), and ducks back out, with little chance of being absorbed.  Light enters wet dirt (bottom image), and stays longer before reflecting out.  Its longer path (yellow line) increases the likelihood of the light being absorbed.  The black circles depict dirt particles; the blue circles represent water.  Image courtesy of Henning Nielsen of the Aalborg University, Denmark, modified by author.

Dry dirt is composed of many fairly loose particles, surrounded by air.  When light hits the dirt, and bops around through it, dirt particles scatter the light.  Some light is absorbed, and some reflects back out.  Light might, on the average, follow a path shown (ideally) by the top image in the simplified drawing. 

Rain wets the ground by filling in the cracks and hollows among the particles with water.  Water has optical properties close to those of dirt, writes physicist Craig Bohren in Clouds in a Glass of Beer.  So, wet dirt looks somewhat like only dirt and no air.  Because of this optical homogeneity, light follows a longer path through wet dirt than through dry dirt; moreover, the path goes deeper.  The longer path (shown in the bottom image) increases the chances light will get absorbed.  That's why wet ground absorbs more light.

Like any good scientist, I checked these ideas with an experiment.  I couldn't wait for rain; that can take weeks in New Mexico.  So, I filled a clean coffee pot with water, and poured the water on the ground.  The effect was striking.  As soon as the water touched the ground, the light dirt turned to a rich, deep almost chocolate brown.  That made sense because the dry dirt reflected much light shining on it.  The wet dirt darkened into brown as it reflected less light. 

Craig Bohren describes another experiment in Chapter 15 of Clouds in a Glass of Beer, which you might like to do.  This experiment shows how sand wetted with benzene looks even darker than sand wetted with water, because benzene's optical properties match dirt's better than water.

(Answered May 21, 2007)

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