Dear Tom,

We spotted a mirage of Chicago skyline on the south part of Lake Michigan today. Can you explain how this occurs?

—Robert Lewis

Dear Robert,

What you saw was a “superior mirage” caused by density differences within an air mass. It occurs when a layer of cold air hovering above the lake creates a temperature inversion that refracts light rays from the city back down to the colder, denser air near the lake’s surface, creating stretched images that resemble bar codes. It is sometimes called a Fata Morgana (Italian for Morgan the Fairy), a mythical character that lives in a crystal palace beneath the waves. The name comes from the fairy-like mirages that often appear over the Strait of Messina that separates Sicily and mainland Italy.