Dear Tom,
I have seen frost on garage roofs at sunrise when temperatures are 38 or 39 degrees. What gives?
–Robert Wolfson
Dear Robert,
Frost forms when the shallow layer of air at the ground (or on any other surface such as a roof) cools below its saturation temperature (and below freezing) by contact with a cold surface that itself has cooled to a subfreezing temperature. Water vapor in the air condenses directly, in ice-crystal form, onto the cold surface and frost forms. Under clear, calm nighttime conditions, surfaces, like roofs or even blades of grass, radiate a great deal of heat and their temperature drops sharply. The cold-air layer can be very shallow, often only a fraction of an inch. Temperatures can be cooler because the National Weather Service readings are taken by sensors a few feet above the ground and sheltered from radiational cooling.
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Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or: asktomwhy@wgntv.com
Weather Report is prepared by the WGN-TV Weather Center, where Tom Skilling is chief meteorologist. His forecasts can be seen Monday through Friday on WGN News at 11 a.m., 5 p.m. and 9 p.m.
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