Atmosphere And Weather

A Guide to the different Forms of Frost



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Jack Frost is not usually a welcome visitor, and, frost is not a word that people necessarily like. The first frost of the season is a harbinger of winter and things to come. If you’ve ever had tomatoes on the vine at this time, you know it’s not an encouraging prediction.

Frost is water vapor that forms on solid surfaces on or near the ground. We see it every year in colder climates, on our car windows, on  the grass and trees. When the air is saturated with moisture, and surfaces are cooled below the surrounding air, we get frost.

The amount and type of frost is determined by the time of day, the temperature, the amount of moisture, and, even the wind direction.

Since cooler air lies closer to the ground, on calm days, in low lying areas, “frost pockets” form.

There are different forms of frost, and, the first one, window, can form some of the most beautiful designs in nature. Window frost forms when the air outside of the window is below freezing and the inside of the window has high moisture. Some of the most beautiful designs are a result of this frost.

Hoarfrost is caused on cold nights when objects lose their heat, and ice crystals form on their surfaces. There are several types of hoarfrost.

1. Air hoar frost causes the beautiful white world, where the frost forms on objects above the surface, wires, fences, trees, and roofs.

2. Surface hoar forms on already frozen surfaces.

3. Crevasse hoar are crystals forming in crevasses where water vapor may have collected.

4. Depth hoar forms inside dry snow, beneath the surface.

Surface hoar can actually cause a serious problem in areas prone to avalanches. Since surface hoar forms on top of the snow, other snowfalls will cover it. The frost within the snow layers are formed from crystals which are weak and unstable, and will weaken the snow bank.

Advection frost is caused from moist wind blowing across objects, and can happen at any time of the day.

Rime can form quickly, and does not have the beautiful ice crystal designs as other frost. It is more of an icy layer that forms during a cold wind. Hard rime and soft rime are both formed when an icy fog cloud freezes on objects near the surface. Soft rime has a more feathery and delicate appearance.

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