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Overcast or overcast weather, as defined by the
Overcast or overcast weather, as defined by the World Meteorological Organization
The World Meteorological Organization (WMO) is a List of specialized agencies of the United Nations, specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology an ...
, is the meteorological
Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agriculture ...
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 influenc ...
.
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 highly standardize ...
observation, is reported when the cloud cover is observed to equal eight okta
In meteorology, an okta is a scale 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 fro ...
s (eighths). An overcast sky may be explicitly identified as thin (mostly transparent), but otherwise considered opaque
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shie ...
—which always constitutes a ceiling
A ceiling is an overhead interior roof 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 can ...
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
Stratus may refer to:
Weather
*Stratus cloud, a cloud type
**Nimbostratus cloud, a cloud type
**Stratocumulus cloud, a cloud type
**Altostratus cloud, a cloud type
** Altostratus undulatus cloud, a cloud type
**Cirrostratus cloud, a cloud type
Mu ...
, 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
Seasonal affective disorder (SAD) is a mood disorder subset in which people who typically have normal mental health throughout most of the year exhibit depressive symptoms at the same time each year. It is commonly, but not always, associated w ...
.
The same weather when observed from above may be referred to as undercast, generally by pilots reporting in-flight weather conditions.
See also
*Diffuse reflection
Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ''ideal'' dif ...
**Forward scatter
Forward scattering is the deflection of waves by small angles so that they continue to move in close to the same direction as before the scattering. It can occur with all types of waves, for instance light, ultraviolet radiation, X-rays as well as ...
**Light scattering by particles
Light scattering by particles is the process by which small particles (e.g. ice crystals, dust, atmospheric particulates, cosmic dust, and blood cells) light scattering, scatter light causing optical phenomena such as the Rayleigh scattering, blue ...
*Diffuser (optics)
In optics, a diffuser (also called a light diffuser or optical diffuser) is any material that diffuses or scatters light in some manner to transmit soft light. Diffused light can be easily obtained by reflecting light from a white surface, wh ...
*Gloom
Gloom is a low level of light which is so dim that there are physiological and psychological effects. Human vision at this level becomes monochrome and has lessened clarity.
Optical and psychological effects
Light conditions may be considered g ...
**June Gloom
June Gloom is a mainly Southern California term for a weather pattern that results in cloud cover, cloudy, overcast skies with cool temperatures during the late spring and early summer. While the marine layer is most common in the month of June ...
*Hard and soft light
Hard means something that is difficult to do. It may also refer to:
* Hardness, resistance of physical materials to deformation or fracture
* Hard water, water with high mineral content
Arts and entertainment
* ''Hard'' (TV series), a French T ...
*Photon diffusion
Photon diffusion is a situation where photons travel through a material without being absorbed, but rather undergoing repeated scattering events which change the direction of their path. The path of any given photon is then effectively a random wal ...
References
Clouds {{Cloud-stub fr:État du ciel