Dear Tom,
On Wednesday I woke up to a beautiful frosted world. If this is hoarfrost, how does it happen?
Sue Paulin, Rochelle, Ill.
Dear Sue,
What you witnessed a few mornings ago was indeed hoarfrost–the white deposit of ice crystals on small diameter objects. This phenomenon occurs when atmospheric moisture sublimates directly on surfaces as ice without changing into liquid water in between. Typically, hoarfrost is deposited during clear, cold, calm nights, and its brilliance enhanced hours later in morning sunlight.
Hoarfrost is not that unusual in rural areas of northern Illinois where calm, clear nights are often 10 to 15(degrees) colder than their urban counterparts. Veteran weather observer Frank Wachowski reports hoarfrost in Chicago is rare, occurring only about twice every five years at Midway airport.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
