Cloud cover, also known as cloudiness, cloudage, or cloud amount, refers to the fraction of the sky obscured by clouds when observed from a particular location. 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.
Okta is the usual unit of measurement of the cloud cover. The global cloud cover averages around 0.68 when analyzing clouds with optical depth larger than 0.1. This value is lower (0.56) when considering clouds with an optical depth larger than 2, and higher when counting subvisible cirrus clouds.
On average, about 52% of Earth is cloud-covered at any moment. On a continental scale, it can be noticed based upon a long-term satellite recording of cloudiness data that on a year-mean basis, Europe, North America, South America and Asia are dominated by cloudy skies. On the other hand, Africa, the Middle East and Australia are dominated by clear skies. On a regional scale, it can be also worthy of note that some extensive areas of Earth experience cloudy conditions virtually all time such as South America’s Amazon Rainforest while other ones experience clear-sky conditions virtually all time such as Africa’s Sahara Desert
Although clouds can exist within a wide range of altitudes, typical cloud cover has a base at approximately 4,000m and extends up to an altitude of about 5,000m. Clouds height can vary depending on latitude; with cloud cover in polar latitudes being slightly lower and in tropical regions the cloud cover may extend up to 8,000m.