What factors contribute to Chicago’s tropical humidity levels in the summer?

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Dear Tom,
What factors contribute to Chicago’s tropical humidity levels in the summer?
— Margaret Robertson, Chicago
Dear Margaret,
The most important factor to the city’s muggy summer periods is the Gulf of Mexico. That’s when Gulf water temperatures approach 90 degrees and coastal dew points typically hover at very uncomfortable levels around 80. When Canadian and Pacific air masses control Chicago’s summer weather, it’s usually quite comfortable, but once winds turn southerly, the Gulf moisture arrives. In a typical warm season, the city logs muggy 70-degree or higher dew points on about 30 days, while it’s a daily occurrence in Florida and in the Deep South. A second and increasingly important contributor to Chicago’s mugginess is the additional moisture added to the atmosphere by evapotranspiration from the maturing corn and soybean crops that dominate the Midwest landscape.

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