# Why in hot weather dry streets can appear to be wet?

1. Feb 26, 2005

### Steph

I'm studying interference and optics at the moment - can anyone explain why in hot weather dry streets can appear to be wet?

Thanks

2. Feb 26, 2005

### Integral

Staff Emeritus
is this homework?

The key is that the index of refraction of air is temperature dependent.

3. Feb 26, 2005

### Edgardo

Hello Steph,

directly above the street, there
is a layer of hot air, and above it, warm air.
The optical density of the hot air is smaller than that of the warm air.
Because of that, total reflectance occurs.
The street looks wet, because the light from the horizon sky doesn't
reach the street, but is directly reflected to your eye.
It's like looking into a mirror.

o_______________________o
__o___________________o
____o_______________o
______o___________o
________o_______o
__________o___o
____________o________WARM AIR
---------------------------------------------------
_____________________HOT AIR (optically thinner than above)
---------------------------------------------------
_____________________STREET

Last edited: Feb 26, 2005
4. Feb 26, 2005

### Integral

Staff Emeritus
Edgardo,
Your explanation leaves something to be desired. After responding to this question I was driving down a since straight road observing the "mirages" in the road ahead. By you explanation, since I was a significant distance down the road sitting in a pretty low car, and if the angle of reflection were equal to the angle of incidence, why was I observing the reflection of the sky and not oncoming cars on the other side of the mirage?

5. Feb 26, 2005

### Edgardo

Hello Integral,

have you seen the sky-reflection in front of the car? Then
the angles are not equal as you say. However, my explanation is simplified,
since the index of refraction changes continuously (there are not just
two layers with different optical densities, but the optical density rather changes continuously).

http://www.schremmer.de/Atmosphare/Fata_morgana_1/body_fata_morgana_1.html
In these pics the reflections also show the cars.

The car, that you were observing, was it very far away?

@Steph: This phenomenon is also called a Fata Morgana

6. Feb 27, 2005

### Steph

Thanks - that's been really interesting. I'd seen the idea of a tree appearing to be below the horizon, but I'd not linked the two together atall. I looked on google too and found lots of sites to read through - thanks.