Overcast or overcast weather, as defined by the

World Meteorological Organization The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics. The WMO originated from the Inter ...

, is the

meteorological Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...

condition of

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

s obscuring at least 95% of the sky. However, the total

cloud cover Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud c ...

must not be entirely due to obscuring phenomena near the surface, such as

fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...

. Overcast, written as "OVC" in the

METAR METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting. Raw METAR is the most common fo ...

observation, is reported when the cloud cover is observed to equal eight

okta In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at any given location such as a weather station. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging fr ...

s (eighths). An overcast sky may be explicitly identified as thin (mostly transparent), but otherwise is considered

opaque Opacity or opaque may refer to: * Impediments to (especially, visible) light: ** Opacities, absorption coefficients ** Opacity (optics), property or degree of blocking the transmission of light * Metaphors derived from literal optics: ** In lingui ...

, which always constitutes a

ceiling A ceiling is an overhead interior surface that covers the upper limits of a room. It is not generally considered a structural element, but a finished surface concealing the underside of the roof structure or the floor of a story above. Ceilings ...

in aviation meteorology. Sometimes clouds can be 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 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 Seasonal affective disorder (SAD) is a mood disorder subset, in which people who have normal mental health throughout most of the year exhibit depressive symptoms at the same time each year. Common symptoms include sleeping too much, having li ...

. The same weather when observed from above may be referred to as undercast, generally by pilots reporting in-flight weather conditions.

References

Clouds {{Cloud-stub fr:État du ciel

See also

References