CHICAGO (CBS) — On yet another scorcher in this sweltering summer, a group of scientists gathered beside the fountains of Millennium Park today to warn that Chicago’s heat waves are becoming more frequent and more intense.

The Union of Concerned Scientists presented its report, “Heat in the Heartland,” analyzing data going back 60 years.

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Climatologist Larry Kalkstein of the University of Miami found Chicago now has 2.6 days more of heat waves per year, on average.

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“Most of the summers are exactly the same and then, you suddenly get a summer that has 10 more days,” he said.

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It’s also hotter at night, Kalkstein said.

“There is a warmer nighttime temperature to these hot and humid and hot and dry air masses now than there were in the late 40s,” he said.

Kalkstein says Chicagoans are more vulnerable to the heat than those in normally hotter climates.

“It’s in the 70s and 80s and then all of a sudden, you get five days like this (mid to upper 90s) – that’s when you get the problem,” he said.

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A problem that he says literally can kill people. The heat wave in early July, with three days above 100, was responsible for nearly 20 deaths.