Dear Tom,
Why does Interstate 80 so often happen to be a division line between different kinds of weather? I have heard it mentioned in weather bulletins as the dividing line several times.
Morty Green, Bolingbrook

Dear Morty,
Interstate 80 runs east-west across northern Illinois and northwest Indiana and cuts through Joliet. It is frequently mentioned in describing the locations of weather events in the Chicago area because it is a recognizable feature and its position is well known.

Lake Michigan occasionally causes significant differences in the weather in a northeast-southwest orientation across the Chicago metropolitan area, especially during the warmer portion of the year. The dividing line between cooler air to the northeast and warmer air to the southwest occasionally stalls across Chicagoland because of the presence of Lake Michigan. Interstate 80 in south metropolitan Chicago frequently lies approximately on this dividing line.

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