What causes the dew on my car windshield to change to ice when I turn on my wipers and the outside temp is 38 degrees?
Posted January 30, 2007
In either case, turning on the wipers resulted in spreading the droplets into a much thinner film of water with the spherical surface tension effects eliminated (allowing a surface for the otherwise supercooled water to solidify onto) and/or the barely above freezing water being rapidly cooled to just below freezing by the slightly colder window below and crossing the threshold required for it to turn to ice.
Note that when conditions are clear and calm, it is quite possible for a radiatively cooled surface to drop to 4, 6, 8, even ten degrees below the temperature of the air above. Also note that the supercooling and freezing upon contact and spreading over a subfreezing surface is a process that occurs in freezing rain situations at or near the ground, and in scenarios aloft in which clear icing occurs on aircraft.