What is Cloud cover?

Cloud cover 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 cover is correlated to the sunshine duration as the least cloudy locales are the sunniest ones while the cloudiest areas are the least sunny places, as clouds can block sunlight, especially on sunrise and sunset where sunlight is already limited.

The global cloud cover averages around 0.68 when analyzing clouds with optical depth larger than 0.1. This value is lower when considering clouds with an optical depth larger than 2, and higher when counting subvisible cirrus clouds.