Dear Tom,

Why does I-80 (a man-made thing) so accurately divide Chicagoland weather? It seems to be a pretty good dividing line!

Mary Carter Bradley

Dear Mary,

It sure does, and it not a coincidence. Chicago forecasts often use well-known map features like highways to describe the weather because they provide well-known references. The I-39 corridor is often used because thunderstorms first reach that area as they approach Chicagoland. While I-80 is an easily visualized dividing line, there are meteorological reasons why it often delineates changes in the weather. East-west frontal boundaries often stall out along the I-80 corridor, not because of the interstate, but because the highway coincides with the south end of Lake Michigan, a feature that strongly affects Chicago’s weather, generating significant temperature differences across the frontal boundary. In the warm season, it marks the boundary between warm air to the south and cooler lake-cooled air to the north. In winter fronts often stall out in this area and help establish the rain-snow boundary.