Why do clouds form over the tops of Chicago’s skyscrapers?

Dear Tom,
I have noticed that clouds sometimes form over the tops of Chicago’s downtown buildings. Why is that?
Orlin Sanders, Chicago
Dear Orlin,
These building-induced clouds have caught my eye as well over the years. They form here in Chicago much as orographic (mountain-induced) clouds do, but on a smaller scale, when easterly winds are blowing, temperatures are cool and the humidity is high. Buildings obstruct air flow, just as mountains do. Winds are forced to flow over or around them. Remember, the atmosphere generally cools with height. In situations in which the air is moist but not quite saturated, forcing the wind to ascend over tall buildings can be just enough to cool the air to saturation and initiate cloud development. It’s a spectacular demonstration of man-made weather and it is fascinating to observe.

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