Frost: Ice formed directly from water vapor. Also known as hoar frost (See Photos)
Common Varieties: Ground frost and window frost.
How Frost Forms: The following conditions must be present for frost to form:
1) The temperature of the object e.g. window or ground must be lower than the dew point.
2) The temperature of the object must be at or below freezing.
3) The air temperature must be higher than the dew point.
When these conditions are present, the beads of dew freeze into coatings of ice.
Where Frost Forms: Frost typically forms from the edges of surfaces inward because water vapor tends to freeze at edges and corners more easily than it does at the center of flat or concave surfaces. This is why the edges of leaves often more easily become covered by frost than the centers of those leaves. Also heavy ground frosts involve moisture evaporating from under damp soil.
Because cool air is denser and heavier than warmer air, this air tends to sink into low spots such as valleys. Consequently, exposed valleys (exposed to radiational cooling) are far more prone to occurrences of frost than hilltops.
How the Surfaces Cool: The window is cooled by losing heat to the cooler outside air. The ground and plants are cooled by nocturnal radiational cooling (where the day's heat escapes during the nighttime). For maximum nocturnal radiation cooling to occur skies must be mainly or completely clear and the breeze must be light to calm. Cloud cover tends to hold in the heat like a blanket and strong winds mix the air keeping the temperature at the surface and air relatively similar.
Rime: Consists of supercooled fog droplets that freeze onto exposed objects such as trees, wires, and other objects.
Sources: David M. Ludlum, National Audubon Society Field Guide to North American Weather, New York: Alfred A. Knopf, July 1994 and "Focus: Fog Season," The New York Times, September 30, 2000, p.D14.
