What is Hoarfrost and How is it Formed?
Hoarfrost, or white frost, refers to white, needle-like ice crystals that are deposited onto tree branches, grasses, fences and other exposed objects when the temperature drops quickly and moisture condenses from the air and freezes. The name “hoar” originates from an Old English word meaning "showing signs of old age;" in this case, it refers to the frost that makes trees and bushes look like fuzzy white hair.
To produce any frost, you need water vapor in the air over cold ground with a surface dew point at least as cold as 32 degrees, according to weatherchannel.com. When these water vapor molecules come in contact with a subfreezing surface, such as a blade of grass, they jump directly from the gas state to a solid state, a process known as “deposition,” leading to a coating of tiny ice crystals.
Hoarfrost gets a boost when a very moist air mass is in place. One or more days of freezing fog (fog with air temperatures of 32 degrees or colder) is a perfect scenario, according to weatherchannel.com. 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 and fences.
When conditions are right, Springbrook Prairie Nature Preserve in Naperville is a wonderful place to see hoarfrost, where the fuzzy white crystals cover huge expanses of tall grasses and winter-dried wildflowers. Get out early in the morning to catch hoarfrost before it melts in the sun or rising temperature or is blown off vegetation by wind.