Science behind frost formation

QUAD CITIES, Iowa/Ill. (KWQC) - During the late fall, winter and early spring, frost is a common occurrence in the Quad Cities.

Frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers or fans.

This happens most often on grass and plants close to the ground, but how does it form?

Believe it or not, it doesn’t have to be 32 degrees or colder for frost to form.

Frost generally forms on a night with a clear sky and light wind. The air temperature is measured about five feet off the ground. Remember, cold air sinks, so a lot of times the ground temperature is several degrees cooler.

If there is enough moisture present in the air, water molecules will touch the surface, such as grass, and freezing on contact forming ice crystals, or frost.

If conditions are right, a phenomenon known as hoar frost can form.

Hoar frost occurs when water vapor is deposited on solid surfaces and freezes on contact, taking on the appearance of a feather shape, and usually happens on a night with light wind.

Rime ice may look similar, but is formed a bit different.

It typically forms in fog, when small liquid droplets, known as super cooled water droplets, freeze on contact. Super cooled water droplets are droplets in a liquid state, even when the temperatures is below 32°.

The ice freeze to solid surfaces, and grow toward the prevailing wind.

This happens a lot during “freezing fog” and can lead to slick road conditions, similar to what happened on Interstate 80 near Iowa City Jan. 8 when a 16 vehicle pileup happened due to the roads being 100% ice covered. Two people died and several others were injured.

Previous Weather Word Wednesday segments on Quad Cities Today at 11:

Radiational Cooling

Lake Effect Snow

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