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Partly Cloudy
Wed Oct 19, 2005
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/dvn/ScienceandEducation/glossary.htm
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