67% is the percentage of Earth’s surface covered by clouds at any given time. Cloud cover is especially dominant over the oceans where less than 10 per cent of the sky is completely cloud free at any one time. On average 30 per cent of land surface is cloud free. NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) map shows cloud free areas appearing dark blue; module cloud cover appearing light blue; and frequently overcast areas are white.
The cloud band near the equator is caused by large scale atmospheric circulation patterns known as Hadley cells which dominate the tropics.
Hadley cells are atmospheric circulations defined by the convergence of winds containing warm moist tropical air near the equator which rises to altitudes of 10 to 15 kilometres and moves pole wards to about 30 degrees latitude north and south of the equator.
As this warm, moist air rises and cools at higher altitudes, it loses its capacity to hold water vapour which condenses into clouds producing regular thunderstorms.
The cold dry air then sinks back to the surface at about 30 degrees latitude.
Clouds also commonly form in the middle latitudes 60 degrees north and south of the equator where polar and mid-latitude Ferrel circulation cells collide, which pushes air upwards, fuelling the formation of large-scale frontal systems that dominate weather patterns.
Ferrel cells are an atmospheric eddy formed by the upward movement of polar air currents at about 60 degrees latitude and the downward flow of cool dry air from the Hadley cycle at about 30 degrees latitude.
Picture Credit : Google
