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?

MIKE MOSS SAYS:      Robert,      Since I don't have specific measurements from that morning for air temperature at different heights, dew point, and the surface temperature of your windshield, I have to speculate a little. However, my guess would be that skies were clear and winds were calm or nearly so, and that dew had formed on your windshield while it was in the process of cooling by way of radiative heat loss. the dew apparently formed before the windshield dropped to the freezing point or below, and in fact since condensation requires a release of latent heat from the condensing water vapor to its surroundings, the formation of the dew likely slowed or briefly reversed the radiative cooling process. Once the dew was there for a little while, though, the windshield continued to cool and had probably just dropped a little below freezing when you went out to your car. By then, the dew droplets were probably a bit supercooled but had not frozen yet owing to surface tension and their semi-spherical shapes (this is more likely the case for smaller dew droplets). It is also possible that if the droplets were somewhat larger sized, that the windshield had dropped just below freezing but the droplets were just above freezing because of the warmer air (a degree or two above freezing) in contact with them on the side of the drops opposite the windshield.

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.