How does frost form?

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How does frost form?

heavy frost on burr oak trunk and branches

Heavy frost on Burr Oak tree in Wisconsin

Frost on objects is just water vapor in the air that has condensed as ice onto a surface. Frost forms on objects close to the ground, such as blades of grass. At night, a blade of grass loses energy by emitting a non-lethal kind of radiation, but it absorbs energy emitted by surrounding objects. Under clear nighttime skies, objects near the ground emit more radiation than they receive from the sky, and so a blade of grass cools due to the net energy loss. Once a grass blade gets cold enough, frost will form on it.

Overnight cooling of air near the ground causes morning frost on grass and car windshields. Frost will only form on a surface that is at or below freezing temperature. The observed air temperature may be above 32 F, since observations are taken at about four feet above the ground, where it can be warmer.

If you get outside early in the morning, you may notice that frost forms in open fields but not under trees. Trees emit more radiation toward the ground than does the clear sky, and so grass under a tree loses less energy than grass in an open field. Grass in an open field cools faster and reaches the frost point first.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.