METEOROLOGIST JEFF HABY
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.