This week's winner is Jeremy Nelson
I know that there are two types of frost, hoarfrost and regular frost. Regular frost is the type that we scrape off our windshields and hoarfrost if a feathery type of frost. I'm wondering what causes hoarfrost and why it is feathery in appearance?? I've also noticed that hoarfrost appears more often when it is foggy, is this a coincidence???
Condensation and sublimation require a suitable surface. Frost forms easily on grass and windows when the temperature reaches the frost point temperature. Conditions favorable for the occurrence of frost include calm wind, clear sky, long nights and a surface that is a good emitter of terrestrial radiation. When ice particle form, the shape they take on is a function of the environmental temperature and vapor pressure that they grow in.
The Glossary of Meteorology (edited by Ralph E. Huschke and published by the American Meteorological Society) defines hoarfrost as:
Hoarfrost-- (Commonly called frost; also called white frost, crystalline frost, hoar). A deposit of interlocking ice crystals formed by direct sublimation on objects, usually those of smaller diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wire, poles, etc.
Thus, hoarfrost and frost are, at least technically, the same condition. Jeremy's observations that indicate there are different kinds of frost. His observations are correct. Back to the Glossary for more definitions.
Amorphous frost--Hoarfrost which possesses no apparent simple crystalline structure; opposite of crystalline frost.
Crystalline frost-- Hoarfrost that exhibits a relatively simple macroscopic crystalline structure; to be distinguished from amorphous frost. Nakaya classifies the crystalline frosts into five forms: (a) needle, (b) feather-like, (c) plate, (d) cup and (e) dendritic. Such ice forms are typically developed as a result of sublimation at temperatures well below 0C; the degree of supersaturation and temperature controlling the form.
Jeremy was observing a type of crystalline frost and comparing it to amorphous frost, which may be more common on his windshield.
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