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Under what conditions does the windshield of a car freeze even if the outside temperature is above freezing?

It is not clear whether this is related to the question why bridges freeze with non-freezing outside temperatures (see related topic), as I did not see an explanation in terms of radiation there. It indeed seems that electromagnetic radiation is the culprit for windshields, which would predict that this phenomenon can only occur with an open sky. Does anybody know whether this is true?

And why does ice only form on top of the windshield and not on the metal surface of the car? Is this because heat is conducted much faster on the metal surface?

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  • $\begingroup$ The windsheild doesn't substantially "freeze" more or less for normal differences around 0C. It will remain solid for practical purposes over the full range of normal windshield operating temperatures, including well below and well above freezing. Actually a windshield contains other materials than glass, but these don't substantially "freeze" or not freeze over the normal operating temperature range. Your question therefore makes little sense. $\endgroup$
    – Olin Lathrop
    Commented Feb 6, 2014 at 14:12
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    $\begingroup$ @OlinLathrop Although literally the OP is asking about the windshield freezing, it is obvious that he in fact means the frost layer formed by condensation from humid air and subsequent freezing of the condensate. $\endgroup$
    – Michiel
    Commented Feb 6, 2014 at 14:23
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    $\begingroup$ @Michiel: We shouldn't have to decide whether some other meaning than the clearly stated one is "obvious" or not. Those that can't write clear questions that ask what they mean to ask don't belong here. $\endgroup$
    – Olin Lathrop
    Commented Feb 6, 2014 at 14:43
  • $\begingroup$ @OlinLathrop - I agree with the sentiment that asking clear questions is a must, but that is an acquired skill so I would be inclined to ask the OP whether he indeed means the freezing of the windshield or whether he means the freezing of water from the air on top of the windshield. And suggest to edit the question to be clarify this issue. $\endgroup$
    – Michiel
    Commented Feb 6, 2014 at 14:52
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    $\begingroup$ Regarding the edit, what makes you think EM radiation has anything to do with the windshield freezing? Right now I don't see anything in your question that sets it apart from the bridge freezing question. $\endgroup$
    – Brandon Enright
    Commented Feb 6, 2014 at 16:59

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The outside temperature may be above the freezing point but with a clear sky radiation from the car can escape outside the atmosphere. There the temperature is very low, so energy is transferred away from the car. What happens depends upon the material. The windshield is a bad conductor of heat, so radiative energy losses are not easily compensated by conduction from other parts of the car.

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This can happen if the windshield itself has a lower temperature due to earlier freezing. This effect can also be seen in freezing rain: Air producing rain is advected over frozen ground and suddenly the traffic is in trouble.

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  • $\begingroup$ OK, but this does not clarify why windshields can freeze even if the outside temperature was never below 0 celsius. I guess it must have something to do with evaporation $\endgroup$
    – user40076
    Commented Feb 6, 2014 at 13:43
  • $\begingroup$ That is right. When the dew point is below zero degrees Celsius and the evaporation heat is removed from the system, then white frost may form and even water surfaces can freeze in shallow puddles. But this requires availbale liquid water. So in case of a wind shield, first condensation must occur, and then dry conditions with evaporation causing freezing of the remining water. $\endgroup$
    – Metalbeard
    Commented Feb 6, 2014 at 14:15

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