Global cloud cover

The importance of the cloud cover of the Earth is troubling. Satellite observations and photographs of astronauts show that clouds dominate the sky from Earth.
From water vapor to huge cloud structures, the sky is obscured average ≈67%, this amazing figure is the result of a decade of observations and shows that the surface of the Earth is generally covered by clouds.
Cloud cover is particularly important over the oceans, only 10% of the sky is completely clear of clouds at some point over the oceans.
While on land, 30% of sky are completely blue.
The cloudiness of Earth is unequivocal on this world map redrawn from data collected by onboard scientific observation instruments on satellites EOS program (Earth Observing System) NASA, whose Aqua then Terra then Aura satellite launched in 2004 and still operational in 2015.
The various instruments, spectrometers, radiometers, sounders and sensors, take a complete picture of the Earth every 1 or 2 days. They are designed to provide comprehensive measures, such as variations in cloud cover or the radiation balance.
This redrawn map from the Aqua satellite data and its main instrument MODIS, shows an average cover of all observations between July 2002 and April 2015. The colors range from dark blue (no clouds), light blue (some clouds) and white (frequent clouds). The interpretation of these data is difficult because this average in the distribution of clouds, not illustrate, exhaustively every day of the period or seasonal variations or the altitude of the clouds or the presence or absence of several layers of clouds. However, it informs us properly on very cloudy areas compared to the very sunny regions seen by MODIS (Moderate Resolution Imaging Spectroradiometer).
The sensitivity of this instrument is somewhat different depending on the environments (over the ocean over the coast, over the desert or over the surfaces of vegetated land).
For example, MODIS better detect clouds on the dark surfaces of oceans and forests, than over shiny surfaces of ice. Similarly, cirrus are more difficult to detect by sensors that thick layers of cumulus.

What do we see in this picture redrawing the average cloud cover of the Earth?
There are three "bands" where the skies of Earth are often cloudy.
The first cover band is a narrow equatorial band, crossing the Pacific, South America, Atlantic, Africa and Indonesia. This is due to the warm and moist air from the equatorial zone, which collides with the colder air of the tropics. In this thermal exchange, the hot and humid air rises, cools and condenses into clouds producing thunderstorms in an area known as the convergence intertropical zone name (ITCZ).
The other two cloud bands are located in the middle latitudes 60 degrees North and 60 degrees South. This is due to thermal shock between the polar air circulation and air circulation at mid-latitudes, which rises, cools and condenses into clouds. Conversely, descending air inhibits cloud formation. Thus, between 15 and 30 degrees north and south of the equator, the descending air prevents cloud formation and promotes the deserts that can be guessed on this map, at these latitudes.
We also note a tendency to cloud formation off the west coast of continents, particularly off the coast of South America, off the coast of Africa and off the coast of North America.
This is due to surface ocean water is pushed back to the west away from the western edge of the continent owing to the rotation of the Earth on its axis. In this process called upwelling the cold depths water up and replaces the hot water of the surface, which cools the air above the water. The moist sea air cools, the water vapor condenses into water droplets to form low clouds called stratocumulus. Stratocumulus are the most common clouds in the world, they cover about one fifth of the surface of the Earth.
In some places on earth, the shape of the landscape favors deserts. Indeed the mountain ranges stop the air streams, so the rains tend to rush on the slopes. While on the other side of the mountain barrier leeward occur deserts. For example the desert of the Tibetan plateau to the north of the Himalayan mountains, the Death Valley in south-eastern California to east of the Sierra Nevada and the Atacama Desert in America South located east of the Andes.