Kansas' Climatologist Explains What Determines Heat Index

It's not the heat, it's the humidity.

When the temperature rises and the air begins to feel heavy, talk often turns to the "heat index." But what exactly is that?

"The heat index is a combination of the temperature and relative humidity," says Mary Knapp, who is the climatologist for Kansas. "It is an attempt to indicate how we perceive the outdoor environment. The higher the temperature and relative humidity, the higher the heat index."

Because the human body cools itself through evaporation, factors that slow evaporation reduce the effectiveness of its natural cooling system. When the air's humidity is high, the body's cooling system has to work harder to maintain a desirable temperature level, explains Knapp, who heads the Weather Data Library based at Kansas State University.

"That's why you often hear the phrase, 'It's not the heat; it's the humidity,'" she says.

At 105 degrees F and 30 percent humidity, the heat index would be 114 degrees. But, at 105 degrees F and only 10 percent humidity (as might be seen in the desert Southwest), the heat index is just 100 degrees - cooler than the actual air temperature, Knapp says.

Knapp's Weather Wonders audio reports are available on the K-State Research and Extension Kansas Radio Network Web site at www.oznet.ksu.edu/radio/wxwonders.htm. Information about Kansas weather is available on the Kansas Weather Data Library Web site: www.oznet.ksu.edu/wdl/.

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