Cloud Fraction

In addition to their role as rain- and snow-makers in Earth's water cycle, clouds play a major part in Earth's energy budget—the balance of energy that enters and leaves the climate system. Clouds may have a warming or cooling influence depending on their altitude, type, and when they form. Clouds reflect sunlight back into space, which causes cooling. But they can also absorb heat that radiates from the Earth's surface, preventing it from freely escaping to space. One of the biggest sources of uncertainty in computer models that predict future climate is how clouds influence the climate system and how their role might change as the climate warms.

These maps show what fraction of an area was cloudy on average each month. The measurements were collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. Colors range from blue (no clouds) to white (totally cloudy). Like a digital camera, MODIS collects information in gridded boxes, or pixels. Cloud fraction is the portion of each pixel that is covered by clouds. Colors range from blue (no clouds) to white (totally cloudy).

From month to month, a band of clouds girdles the equator. This band of persistent clouds is called the Intertropical Convergence Zone, the place where the easterly trade winds in the Northern and Southern Hemispheres meet. The meeting of the winds pushes warm, moist air high into the atmosphere. The air expands and cools, and the water vapor condenses into clouds and rain. The cloud band shifts slightly north and south of the equator with the seasons. In tropical countries, this shifting of the Intertropical Convergence Zone is what causes rainy and dry seasons.

Another frequently cloudy place is the Southern Ocean. Although there is not as much evaporation in the high latitudes as in the tropics, the air is cold. The colder the air, the more readily any water vapor in the air will condense into clouds.

View, download, or analyze more of these data from NASA Earth Observations (NEO):

Cloud Fraction