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Why does my car windscreen defrost despite the wind chill of travelling at speed?

Steevo Barker, Stevenage

The ice disappears because the wind blows away water molecules that have evaporated or 'sublimed' from the ice, so the ice slowly shrinks in size. The molecules that escape are those with the highest energy, which means that those that are left behind have lower average energy; thus the remaining material gets cooler, but continues to shrink.

Submitted by Len Fisher

I would imagine there are

Wed, 2010-01-13 15:19
Guest

I would imagine there are several elements at work here.

Evaporation may occur due to forced convection as the air travels over the ice on the windscreen if the air temperature is higher than that of the ice. Thus as heat is transfered there will be mass transfer and the amount of ice will decrease.

I would imagine there would be some heat generated from the air due to friction as the car is travelling at speed, which may cause some evaporation, although I suspect it will be neglible.

The temperature inside the car is undoubtedly warmer than that outside and will conduct through the glass and is the dominant cause of the ice defrosting.

Even on a cold day, the amount of heat making it's way from the inside of the car to the outside of the windscreen is enough to overcome any effects of cold air passing by.

wind chill

Wed, 2009-09-02 22:15
chopin

Surely wind-chill as such is not an absolute temperature but purely a human skin sensory 'feels like' expression.

Just because the car is travelling at speed does not make the air moving over the car windscreen any colder. If the air temperature is -2c, it is still -2c when it goes past the car. The air is not any colder because it is moving faster. However, your skin would sense it was colder than it actually is because the air is moving over it thus the skin feels it. The faster and moister the air movement over your skin, the colder it feels but the air is not actually any colder than its ambient temperature.