Dear Tom,
What is the difference between partly cloudy and partly sunny? Is it a matter of how many clouds there are in the sky?
Jack Gilliam, West Chicago, Ill.
Dear Jack,
As we use the terms partly sunny and partly cloudy on the Tribune weather page, they are for all intents and purposes interchangeable.
The definitions of sky coverage terms are based on the amount of the sky that is covered by opaque clouds. Very thin clouds that do not block sunlight are ignored.
Precise definitions of the terms do not exist and their use is somewhat arbitrary, even among weather forecasters. But, here are the sky coverage criteria we apply when we use these terms here on the weather page: Clear, sunny: no clouds or less than 0.1 sky coverage; mostly sunny: 0.1 to 0.2 coverage; partly cloudy/partly sunny: 0.3 to 0.8 coverage; mostly cloudy: 0.8 to 0.9 coverage, overcast/cloudy: Virtually 100% cloud covered.
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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.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
