What demists your car windscreen quicker on a cold and wet day - warm air from the heater or cold air from the aircon?

You have to heat the windscreen in order to make the water dissolve into the air again.

When the screen gets too cold, the air directly around it inside the car will also be cooled. The maximum degree of humidity of air depends on its temperature, and the cooler it gets, the less water it is able to contain. Therefore water that can not dissolve in the air will condensate - on your screen. Heat it up and it will dissolve again.