In meteorology

Meteorology

Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

, an okta is a unit of measurement

Units of measurement

A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

 used to describe cloud cover

Cloud cover

Cloud cover refers to the fraction of the sky obscured by clouds when observed from a particular location...

. Sky

Sky

The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...

 conditions are estimated in terms of how many eighths of the sky are obscured by cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

, ranging from completely clear, 0 oktas, through to completely overcast

Overcast

Overcast or overcast weather, as defined by the World Meteorological Organization, is the meteorological condition of clouds obscuring all of the sky. Overcast, written as "OVC" in the METAR observation, is reported when the cloud cover is observed to equal eight oktas .Sometimes clouds can be...

, 8 oktas. In addition in the SYNOP

SYNOP

SYNOP is a numerical code used for reporting weather observations made by manned and automated weather stations. SYNOP reports are typically sent every six hours on shortwave using RTTY...

 code there is cloud cover indicator '9' indicating that the sky is obscured, usually due to dense fog

Fog

Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

 or heavy snow

Snow

Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

.

Although relatively straightforward to measure (for instance, by using a mirror), because it estimates cloud cover simply in terms of the area

Area

Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...

 of the sky obscured by clouds, it does not account for cloud type or thickness. This limits its use for estimating cloud albedo

Cloud albedo

Cloud albedo is a measure of the albedo of a cloud - higher values mean that the cloud reflects more solar radiation, or more radiation is transmitted....

 or surface solar radiation receipt.