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Often, I'll be driving down the road on a summer day, and as I look ahead toward the horizon, I notice that the road looks like there's a puddle of water on it, or that it was somehow wet. Of course, as I get closer, the effect disappears.

I know that it is some kind of atmospheric effect. What is it called, and how does it work?

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up vote 23 down vote accepted

The phenomenon is called Mirage (EDIT: I called it Fata Morgana earlier, but a Fata Morgana is a special case of mirage that's a bit more complex). The responsible effect is the dependence of the refractive index of air on the density of air, which, in turn, depends on the temperature of the air (hot air being less dense than cold air).

Mechanism Leading to a Fata Morgana

A non-constant density leads to refraction of light. If there's a continuous gradient in the density, you get a bent curve (i) as opposed to light coming straight at you (d). Your eye does not know, of course, that the light (i) coming at it was bent, so your eye/brain continues the incoming light in a straight line (v).

This mirroring of the car (or other objects) then tricks you into thinking the road is wet, because a wet street would also lead to a reflection. In addition, the air wobbles (i.e. density fluctuations), causing the mirror image to wobble as well, which adds to the illusion of water.

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Fata Morgana is a much more complex effect. Mirage is the correct term for a simple inversion like that. – AttackingHobo May 27 '11 at 5:11
Thank you. I edited my answer accordingly – Lagerbaer May 27 '11 at 15:18
@AttackingHobo, @Lagerbaer: Particularly, the link to Wikipedia that Henry gave identifies this effect as an inferior mirage- that is, "inferior" because the image is produced underneath the actual object. It also appears that the Fata Morgana is in fact a superior mirage. – voithos May 27 '11 at 16:47
The source for my confusion was that apparently (according to Wikipedia) in Germany, Fata Morgana is the same as "Luftspiegelung" (lit air-reflection) and the sketch I posted for the inferior image was from the German Wikipedia page for Fata Morgana... So maybe the German term is more general than the English term, where, as you point out, the Fata Morgana is a special case of mirage. – Lagerbaer May 27 '11 at 16:53

Bend the light beam at the interface between a cold air mass with a mass of hot air as often seen in the mirage of the pavement. Light Bend

As we look into the hot air region we see the light comming from the colder region, the sky, as metalic surface. Highway_mirage

Physical explanation: The index of refraction of air exhibits a temperature dependence and makes the light rays bend more or less in the layer of separation of hot and cold layers

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It is a mirage: in particular it is caused by hot air near the road and less hot air above it creates a gradient in the refractive index of the air and so making a virtual image of the sky appear to be on or below the road. Air currents make this shimmer, similar to a reflection of the sky on water, hence causing the illusion of wetness.

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Fooled thirsty people lost in the desert that they were approaching water. – anna v May 27 '11 at 3:34
Imagine! Water! How thirsty has one to be to dream of water? – Georg May 27 '11 at 15:40
I vote for this answer because you correctly said that it is the sky that is seen reflected. But the mention of air currents is 'strange'. – Helder Velez May 27 '11 at 22:32
@Helder Velez: What I meant by "air currents" was minor variations in air density over a short time – Henry May 28 '11 at 12:57
sorry. Yes, there are vertical currents because hot air goes up and it is substituted by fresher air. And it contributes to a flicker effect. – Helder Velez May 28 '11 at 17:57

Mirage is an optical phenomenon very common in sunny days. It's caused by the redirection of the reflected light rays form the object, in other words, is a real physical phenomenon and not just an optical illusion. The sun light in the direction of the road gets refracted do to the temperature gradient (continuous change) of the layers of air. This refraction causes the redirection of the sun rays and finally it gets reflected (total reflection) from the layers of air near the surface of the road. This phenomenon can also be observed when the road surface is very cold and in this case the reflected images are upside down.

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