Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Reader dialectical-induction asks:

Being as its pretty hot right now where I am, I was always curious, what exactly is occurring when the air is seemingly rippling on a hot day. I’ve noticed this phenomenon most often close to the pavement or anywhere where it’s really hot. Is it moisture in the air, off the pavement. What’s going on?!

Good question! This is a pretty common optical illusion to observe, especially when driving on a hot day, and it goes by many names including mirage and heat shimmer. What’s happening is actually a case of refraction, much like when a straw in a glass of water looks bent. Near the ground, the air is significantly hotter (maybe 10 degrees Celsius) than the air about a meter above the surface. Changing air’s temperature also changes its index of refraction. When a ray of light passes from the layer of cooler air into the hotter air near the ground, it encounters a lower index of refraction and will bend upward toward the higher index of refraction. From an observer’s perspective, these distorted rays look like they are coming straight from the ground, making it look as if a refracted image of the sky is the ground. The brain will often interpret this as a spurious puddle reflecting the sky. Getting closer to the mirage makes it disappear because the light bends less (relatively speaking) as the angle between the observer and mirage source increases. The rippling effect you note is typically a result of this refraction occurring through hot, moving air. (Photo credit: M. Fern)

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