Evaporative Cooling


What is evaporative cooling? Why use it outdoors?

Swamp coolers, misting systems and misting fans ALL use evaporative / evaporation cooling.

All of these cooling method belong to a general category of products that use water and evaporation to provide cooling. Misting systems and misting fans use a water mist while swamp / desert coolers use water-soaked pads. All of these evaporative solutions are perfect for many outdoor areas, especially when conditions are hot and dry.

“Swamp cooler,” “air cooler,” and “desert cooler” are different terms for the same thing – evaporation cooling that uses water-soaked pads. The term desert cooler comes from the fact that these coolers work so well in hot, dry climates. Their use goes back long before electricity was harnessed. We use the term swamp cooler because these are often used indoors where the cooling unit might not get enough air flow. Since evaporation cooling adds moisture to the air, in an enclosed area the air can become very humid and “swampy.”

When you search for these coolers you’ll find product descriptions might use these terms interchangeably with “evaporative cooler,” or they’ll just use the term “evaporation cooler.”

How is evaporative cooling different from other cooling solutions?

An evaporative cooler is often called a cooler or air cooler to differential it from an air conditioner, which works in a different way and has different benefits and drawbacks.

Other cooling solutions do not use water. As opposed to evaporative coolers that add water to the air, air conditioners decrease the water in the air (humidity). They force special chemicals (refrigerants) to evaporate and condense in a closed system of coils and a fan blows air over the cold coils. Another cooling method – a fan uses “wind chill” (convective cooling). And shade uses a physical barrier to block the source of heat.

When and where is evaporation cooling most effective?

Evaporation cooling use the natural process of evaporation to cool the air. All that’s needed for this is some hot air and some cool water. These coolers need fairly dry conditions to provide the most cooling. The more humid the air the less cooling.

On a hot day, when warm air comes in contact with water, it makes the water evaporate. When the water evaporates, it cools the air immediately around it. (If you’ve ever felt cold when you step out of a shower or pool, it’s because of the evaporation cooling effect.)

Evaporation cooling works best in the hottest, driest part of the day.

Evaporation adds more moisture to the air. Air that already has a lot of moisture in it can’t hold much more of the cool moist air that comes from an evaporative cooler; air that is naturally dry can absorb more. Also, air that is naturally humid in the morning will be able to absorb more of this additional cool moisture as the sun rises in the sky and the air gets hotter.

Heat causes the evaporation that cools us. Evaporative coolers use the heat from the outside air to trigger that evaporation. So high temperatures with low humidity are the best conditions for a swamp cooler.

“relative humidity,” or “RH” refers to how much water the air can hold relative to how hot it is. Hotter air can hold more moisture than cooler air. A level of moisture in cool air may measure 40% RH, but as the sun rises and the air temperature increases, that same amount of moisture might be measured as 30% RH. So your swamp cooler, misting system or misting fan will all be more effective in the middle of the day when it’s hotter and drier. Perfect.

You can get the average temperature and relative humidity levels for your area at Weatherbase.com. Compare the numbers for your area with the numbers on the following chart. This chart shows how much cooling you can expect from evaporation cooling based on air temperature and relative humidity. Notice the bright blue areas that indicate the most cooling – by up to 35ºF and more.

chart: temperatures delivered by evaporation cooling
temperatures delivered by evaporation cooling

Not noted in this chart is the additional 4 – 8° F / 2 – 4.5° C of cooling from the breeze a swamp cooler or misting fan creates, or by adding an outdoor fan to a misting system. In addition to cooling from evaporation, you’ll feel some extra cooling from these methods.