Whether you’ve seen a mirage in person or just in the movies, you’ve probably wondered what causes them. They’re mysterious optical illusions that appear in areas of high heat, and they make it look like there’s a body of water on the distant horizon. So, what gives? What atmospheric conditions cause this phenomenon?
On a hot day, desert sand or asphalt can heat up to more than 50°F above the air temperature. Sometimes this means surfaces can get close to 150°F, which is equivalent to the average temperature of a cup of hot chocolate! As this extreme heat radiates from the surface, it creates a layer of scorching air hovering just above the ground. Just how hot is this layer of air? Typically 10°F hotter than the air temperature.
When air gets this hot, it gets less dense. The density of the scorching layer of air above a hot surface is around 2% less dense than the air above it. This lower density causes passing light to speed up, sometimes as much as 2 million miles per hour! This change in speed causes the light to refract, or bend.
However, the bending light doesn’t reach the ground, but it bends back up at a slight incline. This is where things get interesting… Our brains can’t process light refracting upward. Our visual cortex interprets the illusion as a reflection rather than a refraction. That’s why a mirage is often mistaken for the reflection of a body of water!
Learn more about the phenomenon here: