Why does heat make roads appear wet in the distance? © iStock

Why does heat make roads appear wet in the distance?

It's not water to watch out for, you have to keep an eye on the inversion layer.

Asked by: Alan Dawes, Dundee

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On hot days, air just above the road can become hotter and thus less dense than air higher up. The optical properties of this “inversion layer” can then lead to light rays from the sky that would otherwise hit the road curving upwards – creating the illusion they have bounced off a reflecting pool of water on the road.


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