Why do windscreens get frosty?

11 February 2007



Why is it that car front windscreens appear more prone to icing than the sides and the rear? This is even when the car is parked either way round in relation to the prevailing wind, I think.


We've had a few answers sent in for this question. Thanks to everyone for their input!

Dan in New Jersey - I think it's because over time when you're driving around you get pitting in your front windscreen from dust, water and stones. This could act as a nucleation site for ice, which encourages ice to form. Bill in Canada - I think it's probably due to moisture at the front of the car and greater air circulation at the back of the car, making for a greater differential of temperature and moisture at the front. This might make it more likely to freeze up. Thomas Coop in Germany - Ice is more likely to form on the front windscreen because it cools more quickly and to a lower temperature than the side windows. Frost normally forms during nights that are clear and calm and under such conditions, the trees and ground and many other objects cool mainly by a process called radiative cooling. That is, that they emit energy in the form of infrared radiation. The same happens to the front and side windows of your car, but the side windows cool more slowly because they are almost vertical. This allows them to pick up some of the radiation given off by trees and houses, which makes them cool more slowly. In contrast, the front windscreen points up into the clear cold sky and therefore receives very little radiation from other objects and cools more quickly. Since they cool more quickly, they are colder and frost is more likely to form on them.


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