How to Use Evaporative Cooling


What is evaporative cooling? Why use it outdoors?

Evaporative cooling solutions are perfect for outdoor areas when conditions are hot and dry.

Evaporation cooling use the natural process of evaporation to cool the air. All you need is some fairly dry, hot air and some water. The drier the air, the more cooling effect. You can get an idea of how much cooling you’ll feel by understanding humidity levels in your area.

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.)

Swamp coolers, misting systems and misting fans ALL use evaporative / evaporation cooling in different ways. Misting systems and misting fans use a water mist while swamp / desert coolers use water-soaked pads. Visit the pages on each cooling method for tons more information.

How is evaporative cooling different from other cooling solutions?

Other cooling solutions do not use water:

  • Air conditioners decrease humidity by drawing out the water in the air. They force special chemicals (refrigerants) to evaporate and condense in a closed system of coils and a fan blows air over the cold coils.
  • Outdoor fans use “wind chill” (convective cooling).
  • Shade uses a physical barrier to block the sun’s heat.

What are Swamp Coolers / Desert Coolers / Air Coolers?

These are different terms for the same thing – evaporation cooling that uses water-soaked pads.

  • 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.” But they are perfect for outdoor cooling because partially enclosed areas like patios have the fresh air needed!
  • The term desert cooler comes from the fact that these coolers work so well in hot, desert-y climates. Their use goes back long before electricity was harnessed.
  • Air cooler is a general term that simply means it cools the air rather than “conditioning” it the way a true air conditioner (that uses refrigerant) can.

When you search for these coolers you’ll see that product descriptions might use all these terms interchangeably. They might even call them “air conditioners.” Understanding the differences between air coolers and air conditioners can keep you from spending more on an air conditioner when all you might need is an air cooler. Also, don’t expect frosty temperatures in your humid area from a product that uses water as its main source of cooling, even it if calls itself an air conditioner.

When and where is evaporation cooling most effective?

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, but air that is dry can absorb more. Also, air that is humid in the morning can absorb more of this moisture as the sun rises in the sky because the sun’s heat evaporates some of that moisture and the air gets drier.

“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 is usually more effective in the middle of the day when it’s hotter and drier. Perfect!

One way to understand humidity in your area is to go to Compare the numbers for air temperature and % RH 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 / 19.5ºC when the humidity is 2% and the air temperature is 110ºF / 43.3ºC. Cooling effectiveness goes down as humidity goes up and temperatures go down.

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

You can feel an additional 4 – 8° F / 2 – 4.5° C cooler by adding a breeze. That might be the breeze a swamp cooler or misting fan creates, or by using an outdoor fan with a misting system. Adding shade can reduce temperatures even further.

Find out the humidity in your area

How much cooling to expect from evaporative cooling? It’s based mostly on humidity.

  • UP TO 30°F / 16.5°C or more in areas with humidity below 40%: the southwest USA like Las Vegas, NV, Phoenix, AZ, and Palm Springs, CA; Cairo, Egypt; Abadan, Iran; Baghdad, Iraq; dry inland Australia like Alice Springs.
  • UP TO 20°F / 11°C in areas with humidity between 40% and 70%: Vancouver, Canada; Los Angeles and Riverside, CA, Denver, CO, Dallas, TX USA; southern regions of Australia like Melbourne and Adelaide. Especially effective during drier periods.
  • UP TO 10° F / 5.5°C in areas with humidity above 70% that drops midday or seasonally: Tropical Australian cities like Darwin, Australia drop to 60-70% humidity in the dry season (May-October); Rio de Janeiro, Brazil; Bangkok, Thailand; Darjeeling, India. Especially effective during drier periods.
  • The southeastern USA coast – Miami, FL, New Orleans, LA may drop to 65-70% humidity in cooler winter months. These winter temps can still get up to 90ºF / 32ºC. Evaporative cooling can be helpful in these hot winter months. Since humidity tends to be too high in hot summer months, look for other cooling methods like shade, outdoor fans, and portable air conditioners.
  • CONSIDER OTHER COOLING METHODS: For areas with humidity above 80% in the hottest part of the day like Singapore, Costa Rica, Sao Paolo, Brazil, methods like shade, outdoor fans, and portable air conditioners are more helpful.

  • Houston, TX, USA : 90% morning, 55% afternoon
  • Kansas City, MO, USA: 81% morning, 55% afternoon
  • Ballarat, Victoria, AUS: 80% morning, 58% afternoon
  • Launceston, Tasmania, AUS: 79% morning, 57% afternoon

Best Cooling Methods for Areas with High Humidity

  • Kolkata, India: 85-90% throughout the day, dipping to 75-79% in hot May or June afternoons.
  • Sao Paulo, Brazil: 80-85% typical summer afternoons, down to 72-77% during the dry season (May-September).