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What is the difference between "partly cloudy" and "partly sunny" in a weather report?

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The expression partly sunny was brought to you by the same folks who brought you comfort station and sanitary engineer. As a technical meteorological term, partly sunny doesn't exist. So while you might assume that a partly sunny sky should be clearer than a partly cloudy one, the two terms signify the same condition. You have merely encountered a weathercaster who prefers to see the glass as half full rather than half empty.

Actually, most of the meteorological terms that seem vague and arbitrary have precise meanings. The degree of cloudiness is measured by the National Weather Service and described according to the following scales:

Percentage of Cloud Cover	Term
0-30	clear
31-70	partly cloudy
71-99	cloudy
100	overcast

Where does "fair" weather fit into this spectrum? Fair weather generally refers to any day with less than a 50 percent cloud cover (thus even some "partly cloudy" days could also be "fair"). But even a cloudy day can be termed fair if the cover consists largely of transparent clouds. On days when a profusion of thin cirrus clouds hangs high in the sky but does not block the sun, it is more descriptive to call it a fair day than a partly cloudy one, since one thick cloud formation can screen more sunshine than many willowy cirrus formations.

You might also have heard the aviation descriptions of cloud cover used in weather forecasts. Here's what they mean:

Percentage of CloudCover	Term
0-9	clear
10-50	scattered clouds
51-89	broken sky
90-99	cloudy
100	overcast

Not many people know what the weather service means when it forecasts that there is a "chance" of rain. Precipitation probabilities expressed in vague adjectives also have precise meaning:

Chance of Precipitation	National Weather Service Term
0-20%	no mention of precipitation is made
21-50%	"chance" of precipitation
51-79%	precipitation "likely"
80-100%	will not hedge with adjective:"snow," "rain," etc.

How does the National Weather Service determine the daily cloud cover in the space age? Do they send up weather balloons? Satellites? Not quite. They send a meteorologist to the roof of a building in a relatively isolated area (airports are usually used in big cities) and have him or her look up at the sky and make a well-informed but very human guess. Imponderables. Copyright © by David Feldman. Reprinted by permission of HarperCollins Publishers, Inc. All rights reserved. Available now wherever books are sold.