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

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Discussion Overview

The discussion revolves around the phenomenon where dry streets appear to be wet in hot weather, exploring the optical principles behind this visual effect, particularly in the context of interference and refraction of light in varying air temperatures.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant inquires about the optical reasons behind the appearance of wet streets in hot weather.
  • Another participant suggests that the index of refraction of air is temperature dependent, which is a key factor in this phenomenon.
  • A different participant explains that the optical density of hot air is lower than that of warm air, leading to total reflectance and creating a mirror-like effect that makes the street appear wet.
  • One participant challenges the previous explanation by questioning why reflections of the sky are observed instead of other objects, given the angles of reflection and incidence.
  • A response acknowledges the complexity of the situation, noting that the index of refraction changes continuously rather than in discrete layers, and references the phenomenon as a Fata Morgana.
  • Another participant expresses interest in the topic and connects it to other optical phenomena, indicating a broader curiosity about the subject.

Areas of Agreement / Disagreement

Participants express differing views on the explanation of the phenomenon, with some agreeing on the role of temperature in affecting optical density while others challenge specific aspects of the proposed models. The discussion remains unresolved with multiple competing views present.

Contextual Notes

The discussion highlights the complexity of light behavior in varying temperatures and the limitations of simplified models in explaining the observed phenomena. There is an acknowledgment of the continuous nature of optical density changes, which complicates the understanding of reflections.

Who May Find This Useful

This discussion may be of interest to those studying optics, atmospheric phenomena, or anyone curious about visual effects related to temperature and light refraction.

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

Thanks :smile:
 
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is this homework?

The key is that the index of refraction of air is temperature dependent.
 
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_______________________[/color]o
__[/color]o___________________[/color]o
____[/color]o_______________[/color]o
______[/color]o___________[/color]o
________[/color]o_______[/color]o
__________[/color]o___[/color]o
____________[/color]o________[/color]WARM AIR[/color]
---------------------------------------------------
_____________________[/color]HOT AIR[/color] (optically thinner than above)
---------------------------------------------------
_____________________[/color]STREET[/color]
 
Last edited:
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?
 
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).

I searched on google (keyword: Fata Morgana) and found this link:
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
 
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.
 

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