Why is temperature usually higher at Midway than O’Hare?

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Dear Tom,
Why is the temperature at Midway Airport usually higher than at O’Hare International Airport, even though the two airports are only a few miles apart?
— Mark Gaffney, St. Charles
Dear Mark,
It’s true that Midway Airport usually reports higher temperatures than O’Hare International Airport. It’s because of the “urban heat island effect” of metropolitan Chicago. Metropolitan areas, especially large ones like metropolitan Chicago, have many sources of generated heat.
Buildings, heated in the winter and cooled by air conditioning during the summer, release a great deal of heat into the atmosphere. The altered land surface in the city area also contributes greatly to heat buildup: Concrete and asphalt store and release heat, especially at night. Reduced vegetation, especially trees, also contributes to city heating. Visible air pollution contributes to heating as well.

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