Effective Dewpoint Range for Evaporative Coolers

Required Dew Point

• The dew point is the air temperature when the moisture in the air begins to condense. When grass is covered in dew, the dew point has been reached. In general, an evaporative cooler cools air to 10 degrees higher than the dew point. For example, when the dew point is 60 degrees, a very efficient evaporative cooler will cool the inside air to about 70 degrees. A moderately efficient evaporative cooler cools the air to about 75 degrees. An evaporative cooler doesn’t cool air well if the dew point is above 55 to 60 degrees.

Relative Humidity

• Relative humidity ranges from 0 to 100 percent. The humidity is close to 100 percent when it is raining. When the humidity is high, the dew point is close to the current air temperature. The optimum condition for an evaporative cooler with a 100-degree outside temperature is 5 to 10 percent relative humidity. When the outside temperature is 90 degrees, a 20 to 30 percent relative humidity or less is best for using an evaporative cooler. At 80 degrees, a relative humidity of under 65 percent allows for the inside air to be cooled effectively.

How to Measure the Dew Point and Relative Humidity

• A sling psychrometer is one device that accurately measures the dew point temperature and the relative humidity. The psychrometer uses a dry-bulb and a wet-bulb temperature reading. The dry-bulb is the outside temperature reading from a typical thermometer. The wet-bulb reading is from a thermometer with a wet cloth covering the thermometer. A psychrometer chart relates the dry and wet-bulb readings and gives the dew point and relative humidity.

Best Places for an Evaporative Cooler

• Deserts usually have a low dew point temperature and relative humidity. Areas in the Western United States tend to be the most suitable for evaporative coolers. These include the cities of Phoenix, Arizona, Las Vegas, Nevada and Denver. In some areas with monsoon rains in the summer, an evaporative cooler is used in conjunction with an air conditioner. When the dew point temperature and humidity are low, the evaporative cooler is used. The air conditioner cools the house at other times.