Effective Dewpoint Range for Evaporative Coolers

An evaporative cooler, also known as a swamp cooler, takes advantage of the cooling effect of evaporating water in the same way the human body does when it sweats. Evaporation is an endothermic process, which means it absorbs heat from the surrounding environment and transfers it to the water molecules, which use the energy to slip free of the bonds holding them in the liquid form and go flying around in the air. Evaporation makes the surrounding air cooler, and a fan circulates this cool air throughout the house.

One of the problems of evaporative cooling is that air can hold only a limited number of water molecules, and when this limitation is exceeded, evaporation becomes less efficient or doesn't occur at all. The capacity of the air for holding water molecules depends on two characteristics: temperature and relative humidity. Relative humidity is related to dew point, but it isn't synonymous. According to the National Weather Service, dew point is a better way of gauging how humid the air feels.

At the dew point, the air is saturated and water begins condensing out, which is a phenomenon that happens most easily in the early morning when the temperature is cool because cool air holds less water than warm air. It's defined as the temperature to which the air would have to cool for water to begin condensing and forming dew.

Another way of defining dew point is to relate it to humidity. Absolute humidity is a measure of the amount of moisture that is in the air, but relative humidity (RH), which is expressed as a percentage, compares the humidity to the maximum humidity the air can support. A relative humidity of 100 percent corresponds to the dew point, which is the temperature at which water begins condensing. If the temperature rises, the dew point stays the same, but the relative humidity — barring pressure changes and absorption of more water — will go down, because warm air can hold more water.

Temperature, relative humidity and dew point are all important quantities for an evaporative cooler, and an evaporative cooler humidity chart is a must before considering purchasing one. Evaporative coolers work best in hot, arid regions with low dew points and relative humidity, and they function poorly, if at all, in humid regions.

Unlike air conditioners, which are closed evaporative systems that provide variable cooling according to the size of the condenser pump, evaporative coolers are open systems and provide more or less standard cooling that depends on the ambient temperature and humidity. You can look up the maximum cooling any evaporative cooler can provide for any given temperature and humidity on an evaporative cooler humidity chart, such as the one provided at NewAir.

The vertical axis of the chart lists temperature in degrees Fahrenheit while the horizontal axis measures relative humidity. For any given temperature and relative humidity, the chart lists the lowest temperature a swamp cooler can produce.

To determine whether a swamp cooler is right for your home, look up the coolest temperature it will produce, given the average summer temperature and relative humidity for your region. For example, if you live in a region with a moderately high RH of around 60 percent, the cooler will lower the temperature from 95 to 87 degrees, which isn't enough to warrant buying a swamp cooler. On the other hand, in a region with low RH of 20 percent, the same cooler will lower the temperature from 95 degrees to a comfortable 74 degrees, using a fraction of the energy consumed by an air conditioner.

The dew point is the temperature (measured in degrees Fahrenheit in the United States) to which the air must cool to be completely saturated with water vapor, given the current humidity conditions. The higher the dew point, the muggier the air feels. Very low dew points in the range of 20 to 30 signify unusually low humidity, while those in the range from 60 to 70 can make the air as uncomfortable as a tropical rainforest just before a storm.

Dew point doesn't change with temperature. If the dew point is 40, it remains that way whether the outside temperature is 60 F or 100 F, so it's a more accurate and easy-to-understand way to measure humidity than relative humidity. Nevertheless, meteorologists continue to use relative humidity, mostly from habit, and that means evaporative cooler temperature charts are graduated in those units.

If you want to gauge a cooler's efficiency in terms of dew point, just keep in mind that, in general, a dew point in excess of 55 is generally a swamp cooler humidity limit. If you want to check this, you can calculate the relative humidity from the dew point and the outdoor temperature, and you can use that to look up the cooling potential of a swamp cooler on an evaporative cooler humidity chart.

If you live in a part of the country suitable for swamp cooling, you'll be able to find swamp coolers at your local home supply stores. In fact, availability of swamp coolers is one of the best ways to tell whether you should buy one, because stores won't offer them in places where they don't work and no one wants one.

The United States Geological Survey can also give you an idea of the feasibility of swamp cooling in your area, because it publishes a map showing the regions in which evaporative coolers work best. According to the map, you're in business if you live between the West coast and the Rocky Mountains, and you may be able to use swamp coolers if you live in the Central Plains or along the Canadian border west of the Great Lakes. If you live east of the Mississippi River and south of Lake Michigan, stick to air conditioners.

Microclimates exist in all parts of the country, so the data from the USGS is only advisory and not conclusive. For example, Pacific coastal communities generally experience too much humidity to make swamp coolers useful, despite being in the area designated as suitable for them. It's also good to know that portable swamp coolers are available so you can use one on days when the humidity is low and put it back in the closet when you need to turn on the air conditioner.