After more than decades of satellite observations of the stratosphere, scientists from NASA have calculated that nearly 67% of our planet's surface is covered by clouds at any given moment.
Scientists created a special map, which clearly shows that the Earth's cloud cover is situated above the oceans and larger water basins almost year round. Barely 10% of the water areas ever see sunlight, shows the massive study.
The MODIS (Moderate Resolution Imaging Spectroradiometer) map was in the making by NASA experts in the 2002-2015 year period. Thanks to it we can clearly see that there are 3 main cloud rings on the Earth's surface. One of them is narrow, covering the equator, while the other 2 are about 60° broader and cover the 2 middle latitudes.
The middle ring, located around the equator, is formed by specific atmospheric factors and phenomena, summarily called Hadley cells. They are characterized by the effects of winds with warm humid tropical air coming together, rising to an altitude of about 8 mi (13 km) and heading north and south near the 30° latitudes.
Having risen miles above the Earth, the warm air cools quickly, helping form storm clouds after the vapor condenses. Then the cold dry air falls down to about the 30° latitudes.
The cloud rings over temperate zones form after the warm air, risen above the surface, collides with polar currents originating from the Arctic Ocean. This process is described as atmospheric circulation Ferrel cells.
Ferrel cells are atmospheric winds created by the upward flow of polar air currents at about 60° latitude and downward flow of dry cool air by Hadley cells at about 30° latitude.
The few regions where the sky is almost always clear are the Sahara desert, Arabian Peninsula, South Africa and Australia. There, the air falls instead of rising, thus not allowing clouds to form.