Dear Tom,
Forecasters often use the terms “partly cloudy” and “partly sunny” in their forecasts. Instead of saying “partly cloudy,” why not say “partly sunny,” which sounds more cheery?
Francisco Perez, Chicago
Dear Francisco,
Your questions surface quite often and, in fact, cloud coverage terms (and all other terms) that are used in weather forecasts are regulated by precise definitions. The meanings of the terms that you mentioned, partly cloudy and partly sunny, differ. Here are definitions for several commonly used cloud-coverage terms:
Clear, sunny: Coverage by opaque clouds is from 0.0-0.2 of the sky; partly cloudy, mostly sunny: from 0.2-0.5 coverage; mostly cloudy, partly sunny: 0.5-0.8; cloudy: 0.8-1.0. Note that partly cloudy and mostly sunny are equivalent terms, as are partly sunny and mostly cloudy.
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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.
