Many of us have invested in an ice scraper. They really come in handy on those days we walk outside to ice covering the windows. This is one area the "garage" folks have a huge advantage over those of us that keep the car exposed to the elements. Scraping ice can be a tedious chore.
The ice coating on the car may look different each time in occurs. Sometimes the ice will be shiny and feathery, sometimes it will be a frozen glaze of drops, and sometimes it will be a coating of clear ice. The three types of formation of ice on the car include white frost, frozen dew, and freezing rain (drizzle). A combination of white frost and frozen dew can occur on the same night if the dew formation process occurs when both temperature is above and below freezing.
White frost (hoar frost) occurs due to deposition. During deposition, water vapor is deposited directly to the frozen state. This ice will be feathery looking, shiny, and speckled. When the temperature and dewpoint are below freezing during the entire dew formation process, frost that develops will be of the white frost variety.
Frozen dew occurs due to condensation of water droplets that later freeze. This occurs on a night when the temperature is above freezing for the first part of the night. Dew forms, but due to overnight cooling, the temperature drops below freezing and the dew droplets freeze. Frozen dew is clearer than white frost and is not as shiny as white frost.
Freezing rain occurs as liquid water droplets freeze after they strike the car. One misconception about freezing rain is that it always, "freezes on contact". It usually does not freeze on contact but rather freezes within a minute or so of striking a below freezing surface. It takes time and latent heat release to freeze water to ice. If water froze on contact then icicles could not develop. Freezing rain that falls at near freezing temperatures or freezing rain that is heavy will take longer to freeze than light freezing rain or freezing rain at temperatures well below freezing. One easy method to determine if freezing drizzle or freezing rain was responsible for the ice on your car is to look at the distribution of the coating of ice on the car. The wind will cause one side of the car to have a greater coating of freezing rain (drizzle) than on the opposite side. White frost or frozen dew will have a more uniform covering of the car. Freezing rain will look much more like frozen dew than white frost. If temperatures have been below freezing all night and the ice is a clear glaze on the car then the ice must have been caused by freezing rain or freezing drizzle (because frozen dew requires temperatures above freezing early in the night and dew that forms when the temperature is below freezing must be white frost).
Next time you scrape ice from the car windows, think of the processes that developed the ice.