Overcast or overcast weather, as defined by the World Meteorological Organization, is the meteorological condition of clouds obscuring all of the sky by at least 95 percent. However, the total sky cover must not be entirely due to surface-based obscuring phenomena such as fog.
Overcast, written as "OVC" in the METAR observation, is reported when the cloud cover is observed to equal eight oktas (eighths). An overcast sky may be explicitly identified as thin (predominantly transparent) but otherwise is considered opaque which always constitutes a ceiling in aviation meteorology.
Sometimes clouds can be different colors such as black or white, but overcast usually refers to darker skies. In some cases, it can be impossible to see distinct borders of clouds or the sky may be covered by a single type of cloud, such as stratus and the whole sky will be a dull white.
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, might be referred to as (an) undercast.
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- "Aerodrome Weather Report" (pdf). World Meteorological Organization. Retrieved 30 August 2016.
- "Overcast". American Meteorological Society. Retrieved 30 August 2016.
- World Meteorological Organization. "Overcast". Eumetcal. Retrieved 30 August 2016.
- Oxford English Dictionary, 3rd ed. (website), s.v. “overcast," 2a and 2b. http://www.oed.com/view/Entry/134377#eid32922408 (Accessed September 7, 2016).
- "Undercast". American Meteorological Society. Retrieved 30 August 2016.
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