Edition: U.S. / Global

Environment

One Year of Clouds Covering the Earth

At any moment, about 60 percent of the earth is covered by clouds, which have a huge influence on the climate. An animated map showing a year of cloud cover suggests the outlines of continents because land and ocean features influence cloud patterns. Related Article »

April 15, 2011

April 23, 2011

May 1, 2011

May 9, 2011

May 17, 2011

May 25, 2011

June 2, 2011

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July 4, 2011

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July 20, 2011

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Aug. 5, 2011

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Oct. 8, 2011

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Nov. 1, 2011

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Nov. 25, 2011

Dec. 3, 2011

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Jan. 1, 2012

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Jan. 17, 2012

Jan. 25, 2012

Feb. 2, 2012

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March 6, 2012

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April 15, 2012

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50%

100%

AUSTRALIA

ASIA

EUROPE

SOUTH

AMERICA

NORTH

AMERICA

8-day average cloud cover

AFRICA

Radiated heat from land, oceans and low clouds is held near the earth’s surface by greenhouse gases but must eventually escape to space, as shown above. Areas with more cirrus cloud cover tend to emit less heat to space.

Reflected sunlight from low clouds, ice and other bright surfaces returns some of the sun’s energy back into space. The oceans and other dark surfaces reflect less light and are warmed by absorbing sunlight.

Incoming sunlight varies with location and with the seasons. The above map shows the average sunlight falling on the earth’s surface in March, as measured in watts per square meter.

Cirrus clouds are high, thin and cold. Many of these wispy clouds allow sunlight to pass through easily, but they tend to trap rising heat. A future increase in cirrus clouds would probably warm the planet, while a decrease would most likely cool it.

0

213

425

W/m2

85

218

350

W/m2

0%

15%

30%

cirrus

cover

0

275

550

W/m2

EARTH

LOW CLOUDS

Outgoing light

reflected by

clouds and

surface

Light absorbed

by surface and

radiated as heat

Incoming

sunlight

Outgoing

heat

Heat trapped

by cirrus clouds

Light absorbed

by atmosphere

CIRRUS CLOUDS

Global Energy Balance To maintain a stable temperature, the earth must balance the incoming energy of sunlight with the outgoing energy of reflected light and radiated heat. Clouds alter the balance in several ways, with an overall cooling effect in today’s climate. Humans are adding greenhouse gases to the atmosphere, causing warming near the surface, and a big question is how clouds will react.