Hoar Frost vs. Rime Ice

Few PEI scenes are more beautiful than frosty winter mornings like the one we had last week. Under certain conditions, ice crystals coat trees, twigs, and fence lines, turning the landscape into something magical. We call this ‘hoar frost’, but there are actually two very different types of ice that cause this phenomenon. Let’s take a look!

‘Hoar’ is an old word for feathery white, like a beard. It’s an apt description for the delicate, feathery ice crystals that form true hoar frost (left photo). Regular frost – the sort that concerns gardeners in spring and fall – forms when moisture in the air condenses on a surface and then freezes. Hoar frost skips that condensation step: when moisture in the air meets a surface that is already below freezing, it goes straight from gas (water vapour) to solid (ice crystals). Water vapour continues to contact those ice crystals, and the frost grows. Hoar frost forms in cold weather under clear skies and calm winds.

The frost we saw last week is commonly mistaken for hoar frost but is a different type of ice, called rime. Rime ice forms under cloud cover, usually in foggy conditions. In this case, the moisture in the air is a super-cooled liquid, such as freezing fog: its temperature is below zero, but it is still in liquid form. When that moisture comes into contact with something solid – trees, fences, or your car – it turns to ice.

Hoar frost is delicate, can be easily brushed away, and disappears quickly. Rime ice is heavy and can build up on trees and power lines, weighing them down. Hoar frost is decorative and doesn’t cause any problems. Rime ice may be beautiful but, when it accumulates, it can damage trees, cause power outages, and make roads slippery. And, as my fellow pilots know, rime is dangerous to aviation and a good thing to avoid!

Last week’s rime ice was beautiful and didn’t build up enough to be hazardous. Cool natural processes like hoar frost and rime ice are all parts of PEI Untamed!