News in Science

Two-thirds of Earth is covered in clouds

Blue and white planet Forget Earth as a pale blue marble, or perhaps a water planet, decades of satellite observations show Earth is actually dominated by clouds.

A new global cloud map based on nearly a decade of NASA satellite data have found that about 67 per cent of Earth's surface is covered by clouds at any given time.

NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) map shows an average of cloud observations collected between July 2002 and April 2015.

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 only 30 per cent of land surface is cloud free.

The MODIS map shows three main cloud bands dominating the skies, a narrow strip near the equator and two wider strips in mid-latitudes at about 60 degrees.

Cloud free areas appear dark blue; moderate cloud cover appears light blue; and frequently overcast areas are coloured 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.

Related: Our beautiful planet: the blue marble in 2015

Hosted by Stuart Gary, StarStuff takes us on a weekly journey across the universe. StarStuff reports on the latest news and discoveries in science, with a special focus on astronomy, space sciences and cosmology.



