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Swamp Coolers

Swamp Coolers

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How late in the year do we have days where it is so hot that swamp coolers don't work? A couple years ago we had a record number of days above 100 and alot where swamps didn't work-were those all in July-Or did they spill over into August- and how late in August??


Would a swamp cooler inside a large green house work even less in hot weather because the humdity inside is higher -or would it still follow the dew point outside?

Bonnie **********************************************************

Question 1: Your swamp cooler will work efficiently as long as there is not too much "moisture" in the air. We measure the moisture by what the "dew point" is. As the temperature gets higher, the dew point rises as well; however, generally here in Utah, the dew point doesn't get so high that your swamp cooler won't work reasonably efficient.

The rule of thumb is that your swamp cooler will "cool" the air to about 10 degrees higher than the dew point. If the dew point is 60, then the most efficient swamp cooler would cool the air to 70 and a medium efficient swam cooler would cool it to 75. If the temperature 100, but the dew point is 60 (as in the example above) then you will see a good cooling effect from your swamp cooler. If the temperature is 80 and the dew point is still 60, there is no difference in how your swamp cooler will work. The temperature essentially makes no difference in efficiency. Now, if the dew point goes up to 65, then the swamp cooler will only cool to 75 or 80 and it is not very efficient. The temperature of 100 with a dew point of 65 will keep you very warm in the house. Now, if the dew point is 55, then the temperature can be over 100 and your swamp cooler is efficient!

In the end, it is more important what the dew point is than the temperature. So, when is the dew point at its highest? It happens when you hear us talk about the Monsoon in July and August. When the winds blow the moisture up from Arizona and Mexico into Utah then the dew point rises and your swamp cooler can't cool the house efficiently. If you go to the web site…where the (weather section) is found at:; then look down where the current conditions are will see the "dew point".

Here is what it is saying right now:

Current Conditions

Salt Lake City 54° F

High so far:

57° F

Low so far:

41° F


10 miles


N 8 mph




31° F

Do you see the "Dewpoint" here? It says 31 F, but the actual temperature is 54. During the hotter summer months, watch the dew point and when it gets above 60 or so, then your swamp cooler is not going to work very well.

Also, the closer the temperature is to the dew point…the higher the humidity. In this example we have 54 a temperature and a 31 dew point and that makes 39% humidity. If the temperature was 39 and the dew point was 39, then the humidity is 100%. It could be, say, 80 with a dew point of 39 and the humidity would be like 15 or 10%. During the summer when it is 100, the dew point might be 55 or 50 and then we have humidity of 5 or 6%. On a hot day, if the humidity is very low, then you know the dew point is low as well, and your swamp cooler is going to be more efficient.

I hope this helps.

Answered by Dan Pope Certified Broadcast Meteorologist KSL-TV

Question 2: When a swamp cooler draws air into the fan or blower, it comes in from outside and is pulled through a filter where water is drained. That is why you have to have a water hose to the swamp cooler. When this outside air is pulled through the filter, it evaporates and cools the air down toward the "dew point". Evaporation is a cooling process. That is why a swamp cooler works. The dry air is pulled through the "wet" filters and evaporated air is then drawn into the home through vents (at a lower temperature) than the outside temperature. If the air coming in from the outside is too "wet", (a higher dew point), then evaporation is not efficient and the cooling process is inhibited.

In the case of the "green house", since the air is coming in from the same outside source, the evaporation process is exactly the same as in a home. However, since a green house is made up of windows, the heat from the sun's rays come in, but the heat from inside can not escape because the sun's wavelengths are different. That is why a green house works so well during cold days (outside). But, on a hot summer day, a green house is going to get very hot if you don't vent it properly. Putting a swamp cooler over a green house will certainly cool down the air in the green house, but the heat is being held inside the windows much more efficiently than inside a home where you can cover the windows and keep the sun's rays from reaching inside the home.

So, the answer to your question is that the air coming from the swamp cooler is the same temperature if it comes into a home or a green house. But, it is going to be "hotter" in the green house because you have to push more air into the green house in order to counteract the heating effect from the sun coming through the windows.

Think of your car on a hot summer day. If you park it in the shade, you can keep it cooler than if it is parked in the sun. A car stays cooler if it is parked in a garage than outside. You don't have to work the air-conditioner as much on a cloudy day opposed to a sunny day. Same idea as with a green house and cooling it down.

Answered by Dan Pope Certified Broadcast Meteorologist KSL-TV

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