What's the Deal with Hoarfrost and Rime Ice?

Jan 03, 2019
As you're out enjoying the beauty of the outdoors during these winter months, have you noticed any trees or shrubs with a layer of what looks like frost or ice? What is this phenomenon, and how does it happen?


by Jana Hemphill
                     

As you’re outside skiing, snowboarding, and snowshoeing, have you noticed any trees with a layer of what looks like frost or ice on their trunks or needles? What is this phenomenon and how does it happen? Depending on the situation, you could be looking at hoarfrost, rime ice, or even glaze!

 

Hoarfrost on pine needles. Photo: Kris Kristovich.
Hoarfrost on pine needles. Photo: Kris Kristovich.
Hoarfrost: Hoarfrost (which can also be spelled hoar frost) occurs when a gas turns to a solid, skipping the liquid phase. It forms when the temperature of an object is lower than the frost point of the surrounding air. It’s usually composed of interlocking ice crystals, which gives it a feathery appearance. Hoarfrost typically forms on objects with a small diameter, such as wires, thin tree branches, and plant stems. Hoarfrost also typically likes calm air in order to form, although a slight breeze will cause hoarfrost to form on only the downwind side of objects.

 

Surface Hoar: When hoar frost occurs on the ground, it’s called surface hoar. This mainly happens when there is humid air that hits ground that is cooler than the surrounding air. It also typically has a feathery appearance.

 

A beautiful icicle with hoarfrost. Photo: Kris Kristovich.
A beautiful icicle with hoarfrost. Photo: Kris Kristovich.
Rime Ice: Rime ice occurs when a liquid turns to a solid. It forms when cloud or fog droplets hit an object, which causes the droplets to then freeze very rapidly. Rime ice generally has an icy, more solid appearance. Rime ice also tends to form on only one side of an object. Since places like Mt. Bachelor can be shrouded in fog or clouds throughout the winter, you can catch a view of rime ice on ski lift towers and at the top of the Summit Express.

 

Glaze: Glaze occurs when rain, drizzle, or fog freezes onto an object, creating a film of super-cooled water. Glaze is more dense, harder, and more transparent than either rime ice or hoar front. It is also known as black ice.

 

The next time you head outside, keep a lookout for the beauty of hoarfrost and rime ice, and watch out for any glaze!

 


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