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Rime ice vs. hoar frost

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The freezing fog resulting in a dazzling display of scenery across the area this morning as the trees were coated with ice.

Michael Floy - Bluffton

Was this hoar frost or rime ice? That is a big question we have been receiving. The difference between the two has to do with the physical phase change of water.

Rime Ice

Rime ice is caused by a phase change from supercooled liquid water to ice. I explained what supercooled water is in a blog post last night. You can read it here.

Many clouds (and the fog last night) are composed of liquid water droplets that exist in below freezing temperatures. Temperatures were well below freezing last night which meant that when these supercooled liquid water drops made contact with solid objects below freezing, they froze to them.

So the frosty trees this morning were the result of rime ice via dense freezing fog. Freezing fog is a primary cause of rime ice.

Faith Sonksen - Waterloo

Rime ice can be hard or soft and will generally depend on the wind strength. A stronger wind will result in hard rime which appears thicker and more compact to soft rime. Soft rime may appear more feathery and fragile and resembles hoar frost.

Jessica Ricchio - Oelwein

Hoar Frost

Hoar frost is caused by a phase change from gaseous water vapor directly into ice when the air reaches the frost point via cooling. It is similar to dew but when temperatures are below freezing. It is not liquid dew that freezes. This type of frost normally happens on a cold, clear, and calm night in the presence of sufficient moisture.
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Brandon Libby


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