The question is often raised by the public "What is the difference between partly cloudy and partly sunny?" At first glance, this question appears to be similar to describing a glass as being part empty or part full - in other words, the difference depends upon the eye of the beholder. The National Weather Service often uses the terms "partly cloudy" and "partly sunny" in the wording of a public forecast. While these terms may appear somewhat vague, many of the National Weather Service Offices use a particular set of criteria based upon the sky condition or cloud coverage when preparing the public forecast text.
The sky cover represents the total fraction of the local sky hemisphere (or celestial dome) above a surface based observer that is covered or hidden by clouds. The evaluation is made with respect to the entire sky above the local unobstructed horizon. If distinct cloud layers are observed, the fractional amounts for each layer are added to form a total sky cover. Usually the sky cover is expressed to either the nearest tenths (1/10th) or the nearest eighths (1/8th or oktas).
The following descriptions are used in public forecasts:
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|
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| Cloudy
Mostly cloudy, or Considerable cloudiness Partly cloudy or Partly sunny Mostly clear or Mostly sunny Fair |
9/10 to 10/10 of the sky covered by clouds
7/10 to 8/10 3/10 to 6/10 1/10 to 3/10 Less than 4/10 cloud cover, no precipitation Generally pleasant weather conditions |
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Clear
Few Scattered Broken Overcast |
0/8 of sky covered by clouds
1/8 through 2/8 of sky covered by clouds 3/8 through 4/8 of sky is covered by clouds 5/8 through 7/8 of sky is cloud covered 8/8 of sky is cloud covered |
Thus the term "fair" would cover cases of CLR, FEW, or SCT. Other equivalents
would be approximately, "mostly clear" - FEW, "partly cloudy" - SCT, "mostly
cloudy" - BKN, and "cloudy" - OVC.
You can make daily observations of the sky and determine
the sky cover for yourself. No more sophisticated equipment is needed than
your eyes. If possible, view the entire sky hemisphere above you from an
unobstructed location. Estimate the total fraction of the entire sky above
you that is covered by all cloud types and at all levels. If no clouds
are present, the sky is clear. With only a little practice, you should
be able to judge a broken cloud deck from scattered clouds, and ultimately,
answer the question "Is it partly sunny or partly cloudy?"
REFERENCE:
Thompson, S. and A.F. Kapela, 1996: Media guide to National Weather Service Terminology. National Weather Service. NWS Forecast Office, Milwaukee/Sullivan, WI. 21 pp. (Also appears as http://www.crh.noaa.gov/mkx/media. htm)
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.