Dear Tom,
I realize that heavy rain can occur at any time during the year in Chicago, but when is it most likely to occur?
—Steve Bollinger, Harvey
Dear Steve,
Mid- and late summer are the most likely times of the year for heavy rainfall events in Chicago. There are two main reasons why. First, the atmosphere is warm, so it has the potential to hold more moisture. Southerly winds transport high-dewpoint air from the Gulf of Mexico into the region frequently. This is also the peak time of the year for moisture to be given off from the Midwest’s soybean and corn crops, a process that adds considerable humidity to the air. Second, weather systems tend to become sluggish at this time of the year, and upper-level winds have a seasonal tendency to become weaker. When this happens, the clouds are able to drop large amounts of rain in any given location before moving on.
