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Partly Cloudy
Thu Oct 23, 2003
Listen in RealAudio 
Hi, I'm Bryan Yeaton and this is The Weather Notebook. When you hear the forecast for a sunny
day, you assume you will see the sun. But what about those other terms, vague and mysterious,
like Partly Sunny and Mostly Cloudy? Today, as a public service, we will do our best to clear
the matter up.
By National Weather Service guidelines, these terms have very specific usages, however, your
local forecaster may or may not buy into that system. The terms refer to what is known as
"opaque sky cover," meaning clouds through which higher clouds or sky cannot be seen. "Clear,"
or "sunny" skiesthose two terms are synonomouscontains up to 10 percent opaque sky cover,
while mostly clear or mostly sunny falls between 10 to 30 percent. "Partly sunny" and "partly
cloudy" have the same definition in the Weather Service glossarya 30 to 60 percent cover. So,
if you thought that "partly sunny meant a brighter day than "partly cloudy," sorry. Mostly
cloudy," also known as "considerable cloudiness," is 60 to 90 percent. Anything over that, and
it's just plain "cloudy."
There is a slightly different formula and lexicography if the cloud cover isn't completely
opaque. This would mean that sky and/or higher clouds are visible through a lower layer of
thin clouds. Under this system, an "overcast" sky has a 91 to 100 percent cover, while 51 to
90 percent would be considered "scattered." "Broken clouds" control 10 to 50 percent of the
sky, and any less is considered "clear."
You can check out the sky cover chart and other weather terms from the Weather Service's
Denver office, by linking from our website, www.weathernotebook.org.
The Weather Notebook is produced The Mount Washington Observatory. Major funding comes from
The National Science Foundation, and Subaru of America.
Today's Links
Glossary:
http://www.crh.noaa.gov/cgi-bin-den/showProduct.pl?title=Glossary&product=appendix10.txt&backt
o=1
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