Earth's climate, including its average surface temperature, depends on the balance between incoming and outgoing energy. Energy comes in to the system when sunlight penetrates the top of the atmosphere. Energy goes out in two ways: reflection by clouds, aerosols, or the Earth's surface; and thermal radiation — heat emitted by the surface and the atmosphere, including clouds. The balance between incoming and outgoing energy is called net flux. Clouds can influence net flux in many ways. They reflecting incoming sunlight, and they also absorb heat energy emitted by the surface of the Earth.
The cloud maps show what fraction of an area was cloudy on average for the month. The measurements were collected by the MODIS sensors on NASA's Terra and Aqua satellites. Colors range from blue (no clouds) to white (totally cloudy).
The map of net flux shows monthly changes in the balance of incoming and outgoing energy on Earth as measured by the Clouds and the Earth Radiant Energy System (CERES) sensor on NASA's Terra and Aqua satellites. Places where the amounts of incoming and outgoing energy were in balance are white. Places where more energy was coming in than going out (energy surplus) are orange. Places where less energy was coming in than going out (energy deficit) are purple.
While net flux shows a very obvious seasonal pattern, global cloudiness is much more variable from month to month. The part of the planet where net flux is positive (that is, more energy coming in than going out) shifts north of the equator beginning with the March equinox and south of the equator with the September equinox, coinciding with spring and summer in the respective hemispheres. A broad band of clouds around the equator shifts slightly north and south with the seasons, but across the rest of the globe, seasonal cloud patterns are less obvious.
One connection between net flux and cloudiness that is shown by these maps is the impact on net flux of marine-layer clouds that often form off the west coasts of continents. Clouds reflect much more incoming sunlight than ocean waters, and this difference in brightness is very obvious at certain times of year. For example, from September through December, when the area exposed to direct sunlight is increasing in the Southern Hemisphere, patches of ocean to the west of South America and Africa are persistently cloudy. These cloudy patches are mirrored by pockets of yellow (more neutral net flux) surrounded by red (strongly positive net flux).
View, download, or analyze more of these data from NASA Earth Observations (NEO):