When you wake up with wonder, to a fairy tale world where tree branches, fences and wires are magically covered with an icy, airy, white fringe it may not be just frost. It could be a beautiful presentation by winter called hoar frost, a light covering of ice on objects when it is very cold.
Hoar frost is defined as: a covering of minute ice needles, formed from the atmosphere at night upon the ground and exposed objects when they have cooled by radiation below the dew point, and when the dew point is below the freezing point. www.dictionary.com
It often forms on a clear cold night and in the morning everything is covered by a white icy covering. This icy coat comes when ice crystals are deposited on the ground and other objects when heat rises, allowing the ground and other surfaces to become colder than the air. Hoar frost is also referred to as radiation frost.
There are several types of hoar frost but only two of them are commonly seen. Air hoar frost is an airy, maybe fuzzy covering of ice needles on tree branches, bushes and fences. Surface hoar frost is a fern like covering that forms on top of snow or other frozen surfaces.
Hoar frost is not dew that forms at temperatures above freezing. The frozen dew presents itself in a smoother, clear covering over grasses and trees and other objects. Hoar frost is a fuzzy ice covering of tiny ice needles. It can look like fringe hanging from the branches and twigs on bare trees and shrubs.
Cold air is denser than warm air and can form pools at ground level sometimes backing up against walls and fences. These areas often are areas where fog regularly forms. They are also referred to as areas of “frost pockets”. Hoar frost often forms in these locations.
In many areas a temperature inversion may occur when extremely cold air hugs the ground and warmer air rises. This provides a climate that is suitable to hoar frost forming.
Denver, Colorado experiences temperature inversions every year. It is a common enough occurrence to give rise to a local joke in the mountain communities just west of Denver. They call themselves the “banana belt” when it is 50 degrees at 8000 feet and 20 degrees in Denver at 5200 feet. Hoar frost may form during these inversions. It is usually seen in low-lying areas along creeks and valleys where fog forms from time to time.
The next time a frost covers the areas around your home examine it closely, if it is a bit furry looking, it is probably hoar frost.