In meteorology, an okta is a unit of measurement used to describe cloud cover. Sky conditions are estimated in terms of how many eighths of the sky are obscured by cloud, ranging from completely clear, 0 oktas, through to completely overcast, 8 oktas. In addition in the SYNOP code there is cloud cover indicator '9' indicating that the sky is obscured, usually due to dense fog or heavy snow. Satellite image of Hurricane Hugo with a polar low visible at the top of the image. ... The former Weights and Measures office in Middlesex, England. ... Cloud cover is the amount of the sky obscured by clouds when observed at a particular location. ... A typical daytime sky. ... Cumulus mediocris clouds, as seen from a plane window. ... SYNOP (surface synoptic observations) is a numerical code (called FM-12 by WMO) used for reporting marine weather observations made by manned and automated weather stations. ... Evening fog obscures Londons Tower Bridge from passers by. ... Animation of snowcover changing with the seasons Trees covered with snow Snow covering a leaf. ...

Although relatively straightforward to measure (for instance, by using a mirror^[1]), because it estimates cloud cover simply in terms of the area of the sky obscured by clouds, it does not account for cloud type or thickness. This limits its use for estimating cloud albedo or surface solar radiation receipt. Area is a physical quantity expressing the size of a part of a surface. ... Clouds (from above) Simply put, clouds form when the dewpoint of water is reached in the precence of condensation nuclei in the troposphere. ... Cloud albedo is a measure of the reflectivity of a cloud - higher values mean that the cloud can reflect more solar radiation. ... Solar irradiance spectrum at top of atmosphere. ...

References

1. ^ Measuring cloud cover, BBC Weather Centre