When temperatures drop below freezing and the temperature reaches the dew or frost point, the ice on the ground is termed frost or frozen dew. "Frost" can form in two ways: Either by deposition or freezing. Depositional frost is also known as white frost or hoar frost. It occurs when the dewpoint (now called the frost point) is below freezing. When this frost forms the water vapor goes directly to the solid state. Depositional frost covers the vegetation, cars, etc. with ice crystal patterns (treelike branching pattern). If the depositional frost is thick enough, it resembles a light snowfall. Frost that forms due to the freezing of liquid water is best referred to as frozen dew. Initially, both the dewpoint and temperature are above freezing when dew forms. Longwave radiational cooling gradually lowers the temperature to at or below freezing during the night. Cold air advection can also do the trick (e.g. Cold front moving through in the middle of the night after dew has formed). Once the temperature falls to freezing, the condensed dew droplets freeze. Frozen dew looks different from white frost. Frozen dew does not have the crystal patterns of white frost. White frost tends to looks whiter while frozen dew tends to look slicker and more difficult to see. Frost and frozen dew can delay people in the morning if it covers their car. Some frosts or frozen dews are much easier to scrape off the car than others. When the temperature is near freezing (29 to 32 F), the ice is fairly easy to scrap off the car windows. It is also quicker to warm up the car windows to above freezing with the defroster when temperatures are near freezing. The bonding of ice crystals is weaker in warm ice than in cold ice. Once temperatures drop into the mid-20's and below, the ice becomes more difficult to remove. It requires more "elbow grease" to remove the ice. It also takes longer to warm up the car windows to above freezing. At these temperatures ice is well bonded. Next time you witness ice in the morning, think about the processes that produced the frost or frozen dew. |