New breed of heat wave—NIU researcher: It’s not summer heat but the humidity

New breed of heat wave—NIU researcher: It’s not summer heat but the humidity


DEKALB—A study by Northern Illinois University climatologist David Changnon indicates the Chicago region is more apt now than in decades past to experience heat waves accompanied by extreme and dangerous spikes in humidity. And a familiar crop with a propensity to sweat day and night could be at the root of the problem.

“Our research findings at NIU suggest that a new, more dangerous breed of heat wave has become established in northeastern Illinois,” said Changnon, a climatologist and NIU professor of meteorology.

“Heat waves today are different than they were a half century ago because they are more frequently accompanied by extreme spikes in humidity,” Changnon said. “I strongly suspect that changes in agricultural methods—particularly in the area of corn production—are playing a major role in this by adding more water vapor to the lower atmosphere of the Upper Midwest.”

All plants transpire, that is, release water vapor into the atmosphere through their leaves. Corn is unique in that it belongs to a family of plants that transpire, or sweat, both day and night. “Stand in any cornfield, and you can feel the increased humidity,” Changnon said.

He points out that average corn yields in Illinois have increased from about 50 bushels per acre in 1950 to more than 130 bushels per acre in 2000. Planting densities climbed dramatically as well, from about 18,000 seeds per acre to nearly 30,000 seeds per acre during the 1970s, when farmers started planting crop rows closer together.

“Overall, the amount of water transpired to the atmosphere in our region is greater today than it was a half century ago just based on changes in agricultural practices and corn production,” Changnon said. “More plants and greater yields per acre imply an increased need for and use of water by corn.

“I’m not knocking the agricultural industry—corn and other crops are absolutely vital to our region, our nation and the world,” Changnon added. “These higher dew points represent a product of a complex agricultural process. We don’t want to go backward in terms of production techniques, but we need to investigate and deal with the impacts related to what appears to be a significant factor in the regional climate of the Midwest.”

Changnon increasingly began to suspect a link between agricultural production and humidity after he and two graduate student researchers—Jesse Sparks of downstate Newton and Jason Starke of suburban Buffalo Grove—studied historical trends in northeast Illinois dew-point values. The results of that study are published in the August edition of the American Meteorological Society’s Journal of Applied Meteorology. The American Meteorological society ( is the nation’s leading professional society for scientists in the atmospheric and related sciences.

The dew point is the temperature at which condensation begins, and dew-point measurements provide an indication of how much water vapor is in the air.

Initially, the study was undertaken in response to concerns of NIU’s physical plant supervisor, who saw a relationship between dew points and the efficiency of the campus cooling system. But while extreme hot and humid weather can take its toll on air-conditioning systems and increase electrical demand, it also can be deadly.

The northern Illinois heat waves of 1995 and 1999 claimed hundreds of lives. Previous studies have noted that both high air temperatures and high dew points characterized those hot spells. “The higher the dew-point value, the more difficult it is for the body to cool itself through evaporation from the skin,” Changnon said.

Changnon and his students analyzed hourly dew-point readings recorded from 1959 to 2000 at Chicago’s O’Hare Airport and Rockford Airport. The top four high dew-point frequency years were 1983, 1987, 1995 and 1999.

Analysis of the 10 most extreme heat waves in the region further showed that the number of high dew-point hours was much greater after 1980. Also, the researchers discovered that over the 42-year period they studied, three different dew-point indices at both airports showed general increases over time. Those indices include:

l Hours per summer with high dew points reaching or exceeding 75 degrees Fahrenheit.

l Summer days with at least one hour of high dew point.

l Summer days with 12 or more hours of high dew points.

“Dew points that exceed 75 degrees Fahrenheit are considered rare in most regions of the United States, the exception being the Gulf Coast,” Changnon noted.

The Gulf of Mexico does play a large role in humidity levels in northeast Illinois. Normally during the summer, tropical air masses originating in the Gulf of Mexico move into and remain in the Midwest for days or weeks at a time. Since that source of water vapor hasn’t changed over time, Changnon ruled out the Gulf as being the source for the growing frequency in high humidity levels. He also noted that during the 1995 and 1999 heat waves, the dew-point levels were greater in the Upper Midwest than in those areas between the Midwest and the Gulf Coast.

Changnon also ruled out urban heat island effect as a primary cause of humidity spikes because similar trends were identified at both the suburban O’Hare weather station and the rural Rockford site.

For transpiration to occur at levels that cause such high dew points, crops such as corn and soybeans must have access to sufficient soil moisture. Over the past 100 years, precipitation in the Midwest has increased, Changnon said.

He said the link between adequate soil moisture, increased transpiration and a greater number of high dew points was evident in the 1995 and 1999 heat events, which were preceded by average to above average precipitation across northern Illinois.

“In contrast, the heat wave of 1988 was accompanied by a drought,” Changnon said. “Corn yields dropped by nearly 50 percent. And although it was an extremely hot summer, with Chicago experiencing temperatures of 90 degrees or greater on more than 40 days, very few high dew points occurred.

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