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Overcast or overcast weather, as defined by the World Meteorological Organization, is the meteorology condition of obscuring at least 95% of the sky. However, the total cloud cover must not be entirely due to obscuring phenomena near the surface, such as fog.
Overcast, written as "OVC" in the METAR observation, is reported when the cloud cover is observed to equal eight (eighths). An overcast sky may be explicitly identified as thin (mostly transparent), but otherwise considered opaque—which always constitutes a ceiling in aviation meteorology.
Sometimes clouds can be of different colors such as black or white, but overcast usually refers to darker skies. Oxford English Dictionary, 3rd ed. (website), s.v. “overcast," 2a and 2b. http://www.oed.com/view/Entry/134377#eid32922408 (Accessed September 7, 2016). In some cases, it can be characterized by almost zero distinction of borders of clouds. Or the sky may be covered by a single type of cloud, such as Stratus cloud, turning the whole sky into a dull white color.
Periods of overcast weather can range from a few hours to several days. Overcast weather can also affect people suffering from seasonal affective disorder.
The same weather when observed from above may be referred to as undercast, generally by pilots reporting in-flight weather conditions.
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