Cloud cover
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. Okta is the usual unit of 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. 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. Role in the climate system. Clouds play multiple critical roles in the climate system. In particular, being bright objects in the visible part of the solar spectrum, they efficiently reflect light to space and thus contribute to the cooling of the planet. Cloud cover thus plays an important role in the energetic balance of the atmosphere and a variation of it is a consequence of and to the climate change expected by recent studies. Variability. Cloud cover values only vary by 0.03 from year to year, whereas the local, day to day variability in cloud amount  typically rises to 0.3 over the globe. Most data sets agree on the fact that the land is covered by 0.10–0.15 less cloud than the oceans. Lastly, there is a latitudinal variation in the cloud cover, such that around 20°N there are regions with 0.10 less cloudiness than the global mean. Almost the same variation (0.15 instead of 0.10) is found 20°S. On the other hand, in the storm regions of the Southern Hemisphere midlatitudes were found to have with 0.15–0.25 more cloudiness than the global mean at 60°S. On average, about 67% 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. Altitude of typical cloud cover. 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.