Wednesday, 19 October 2016


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:

SKY CONDITION (from NWS, Media Guide to National Weather Service Terminology, 1996)

Sky Condition

Cloud Coverage

Mostly cloudy, or Considerable cloudiness
Partly cloudy or Partly sunny
Mostly clear or Mostly sunny

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

When communicating with other meteorologists and with the aviation industry, a slightly different code is used. This code is used in plotting of the station model on the surface weather maps. According to observational practice, the following terminology and codes are used to report the cloud cover.

SKY COVER CODE (as of 1 July 1996)



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?"


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.

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Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2016, The American Meteorological Society.