2/2012

Why does the windshield freeze before the side windows?

Frozen car


Along with snow and ice come frozen car windows that have to be scraped each morning. But how come the windshield is often coated with a thick layer of ice, whereas the side windows are only decorated with delicate frost patterns?

That's the question! The transfer of heat energy, or thermal conduction, between air and glass is in all probability equally strong in the windshield and the side windows. However, conduction is not the only means of heat transfer between two objects. Another phenomenon that contributes to the accumulation of frost on car windows is thermal radiation - the same process that warms your face when you're sitting by a fire. The fire exudes warmth, but you can block it out by shielding your face with even a thin sheet of paper, because paper stops thermal radiation.

Thermal radiation is electromagnetic energy, just like light, and can travel through a vacuum. For example, the insulating vacuum created between two layers of glass inside a Thermos prevents heat loss by conduction but not by radiation. This is why your Thermos has a mirrored surface on the inner surface of the outer bottle that reflects heat back into your coffee.

When a car is parked outdoors, there is usually an object, plant, building or other surface a little distance away from the side windows. This surface reflects heat back into the windows and slows down the loss of heat. As the windshield is tilted up towards the sky and space that reflect less heat back into the car, the windshield cools down faster and more ice builds up.

Why does the windshield freeze up under the open sky but not in a carport?

Exactly the same principles apply as above. The roof of the carport is a solid body directly in front of the windshield that radiates heat back into the car. This keeps the windshield warmer as opposed to a car that is left braving the elements outdoors.

 

The question was answered by Jyrki Mäkelä, who works as a lecturer in thermophysics at TUT's Department of Physics and conducts research on aerosol particles. 

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