Last week the area was blanketed with a layer of frost that turned the world into a winter wonderland. The question of the day: is it hoar frost or rime ice? The difference between the two lies in the weather pattern that encourages the formation. According to CBS 3 meteorologist Dave Anderson, what we saw last week was hoar frost.
The word “hoar” comes from an Old English adjective that means “showing signs of old age.” In this context it refers to the frost that forms on trees and such that makes them appear to have white hair.
Hoar frost forms on cold, clear nights when heat radiates out to the open air faster than it can be replaced from nearby sources such as wind or warm objects.
Hoar frost is a deposit of ice crystals on objects exposed to the air like trees, grass or fence posts. According to the Weather Channel, in order to produce any frost you need water vapor in the air over cold ground with a surface dew point of at least 32 degrees.
When these water vapor molecules contact a subfreezing surface, such as a tree branch, they go directly from a gas to a solid. This process is called deposition and leads to the coating of tiny ice crystals.
In order for hoar frost to appear, the air mass must be much more moist. With more moisture in the air, the interlocking crystal patterns of frost become more intricate and much larger, building up to a greater depth on tree branches, signs, fences — anything.
Kanabec County Times readers submitted their images of the phenomenon (shown above).
Rime Frost differs from hoar frost in that it is denser and harder. Another difference is rime ice is formed by freezing fog or cloud droplets. The small “supercooled” water droplets remain liquid until they come in contact with any obstacle and freeze instantaneously.