Science Briefs

Cloud Climatology

Global Distribution and Character of Clouds

The new global datasets show that clouds typically cover more than 60 percent of the planet, some 10 percent more than had been thought. Oceans are significantly cloudier than continents. Roughly 67 percent of the sky over water is cloudy, and more than half of that area is densely overcast. Just about half of the total land area is usually covered with clouds and only 15 percent is thickly blanketed. Conversely, almost a third of the continental surface, but only 8 percent of the ocean surface, is unbroken blue sky. Clouds on average are about 19°C (35°F) colder than the surface is, and they reflect between 20 and 30 percent more sunlight. But far more interesting than such averages is how widely clouds can vary with location, with time of day, with changing weather and with season.

Cloud over the ocean, for instance, are quite different from clouds over land. The tops of ocean clouds are generally about a kilometer lower than the tops of clouds over land, and ocean clouds reflect about 10 percent less sunlight. Above the oceans at low latitudes, moreover, morning clouds are more common than afternoon clouds, and the early ones are the most reflective of the day. Over land there are more clouds, with higher reflectivity, in the afternoon.

Clouds also vary with distance from the equator. The cloudiest regions are the tropics and the temperate zones; the subtropics and the polar regions have between 10 and 20 percent less cover. Tropical cloud tops are substantially higher, extending between one and two kilometers higher than cloud tops in the midlatitudes and more than two kilometers higher than the clouds over the subtropics and the poles. High-latitude clouds have been found to be almost twice as reflective as other clouds.

Any attempt to explain such variations must take into account the kinds of clouds common to a given region. Consider storm clouds. In the tropics exceptionally large thunderheads often form, extending from the surface to an altitude of between twelve and fifteen kilometers. Similar storm clouds occur in areas of low pressure over temperate regions, but their tops only reach altitudes of between seven and ten kilometers. Elsewhere thunderheads are virtually absent.

Meteorologists have long associated greater cloud cover, higher cloud tops and denser, more reflective clouds with regions of more vigorous storms. Both the tropics and the low-pressure areas at midlatitudes are regions of severe weather. The frequency and strength of storms are also related to such climatic factors as average wind speed and direction, temperature, humidity, sunlight and topography. By comparing satellite observations of cloud variations with meteorological data, it may be possible to establish correlations between these conditions and the cooling and heating properties of clouds.