Rime frost and its cousin hoar frost have dazzled observers with their beauty since ancient times. In his book Μετεωρολογικά, Meteorologia, Aristotle noted
"Both dew and hoar-frost are found when the sky is clear and there is no wind. For the vapour could not be raised unless the sky were clear, and if a wind were blowing it could not condense." (347a26-28)
The term hoar frost derives from hoary which means gray or white in medieval English of the 14th century, and was used as adjective for a person with a white beard and hair. Hoar frost forms when water vapor in moist air comes in contact with objects that are well below freezing, like trees and grass during freezing night in winter, often with starry skies above. Here the water vapor, a gas, changes its state directly into a solid, the ice on the trees and surface objects, and we have frost.
The term rime frost derives from the 12th century, old Norse term hrīm or rimfrost. Conditions for rime frost are less likely in Greece or in England than for hoar frost, but occur regularly in snow covered northern landscapes like ours. Rime frost can form when layers of fog or mist develop in moist air over snowfields during cold nights with temperatures below freezing. The fog consists of tiny water droplets, cooled below freezing and suspended in the cold air.
When these supercooled water droplets come into contact with a freezing surface, such as a branch or grass, then the droplets of water, a liquid, change state from liquid to solid state and form ice on these objects. Since the droplets are so tiny, the result is beautiful feathery ice crystals of almost magical appearance.
Being both a meteorologist and historian of science by training, I take pleasure in tracing how people have always been fascinated with special weather phenomena and have tried to describe and explain them. We are grateful to Steve Sentoff for providing the photos.
Gisela Kutzbach and contributors