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

Ice, ice, baby... even though it is surrounded by cold air, once a car has been de-iced and is on the road the windscreen stays clear. 

22nd July 2009
Why does my car windscreen defrost despite the wind chill of travelling at speed? (Getty)

Asked by: 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.

 


Get more fascinating Q&As from BBC Focus magazine by following @sciencefocusQA