What are hoarfrost and rime?
These two wintertime icy phenomena can be beautiful to look at, but one can cause problems on aircraft and ski slopes.
While winter often brings challenges like slippery ice, snow shoveling and hazardous travel, it also gifts us with stunning natural spectacles. One such wintry phenomenon is hoarfrost, where delicate white crystals transform trees and other objects into an icy wonderland.
A video from Hibbing, Minnesota, on Dec. 31, 2024, depicted the icy needles clinging to the branches of a tree. This video shows one of several ways that water vapor can freeze on objects to present a beautiful display of nature.
The term "hoarfrost" is derived from the Old English meaning of frost resembling an old man's beard.
The fence around a tennis court is covered with hoarfrost on 24 November 2020, Bavaria, Halblech, Germany. (Photo by Karl-Josef Hildenbrand/picture alliance via Getty Images)
While it creates picturesque landscapes that captivate the imagination, it's also a phenomenon rooted in fascinating meteorological processes. From a scientific standpoint, hoarfrost, like regular frost, forms when moisture freezes as ice crystals on an object. It often appears as soft needles of frost that are easily brushed away.
"The key for hoarfrost to form is there must be an influx of moisture into the area while the air remains sufficiently cold," according to AccuWeather Senior Meteorologist Dan Kottlowski.
Hoarfrost in State College, PA on Dec. 6, 2015. (AccuWeather/Jesse Ferrell)
That's different than rime, sometimes called rime ice.
Rime forms when tiny, near-freezing water droplets, usually from thick fog and clouds, attach to the surface of a below-freezing object and turn into ice immediately on contact. Rime builds up in the direction of the wind and is more like accumulated freezing dew rather than thick frost. Because it's heavy and sticks to an object, rime can be hard to remove.
In extreme cases, rime can build up over a few days and weigh down trees, power lines and communication towers to the point of causing damage to them, and can be a problem for aircraft.
In this Tuesday, March 10, 2015 photo, rime ice extends horizontally from a metal pole at the summit of Mt. Washington, in New Hampshire. Rime ice forms in the direction of the wind driven fog that often blows across the summit at hurricane force. (AP Photo/Robert F. Bukaty)
Rime also occurs over high ridges where clouds accompany extended periods of below-freezing air, such as on top of Mount Washington in New Hampshire, which frequently experiences the phenomenon. Ski slopes are another place where rime forms often -- and can cause problems with ski lifts.
Rime that has adhered to frozen bushes on Roan Mountain, Tennessee on Feb. 21, 2025 (Tom Niziol)
As you navigate the icy roads and shovel snow this winter, take a moment to marvel at these frosty formations when they develop. It's meteorology at its best!
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