Science Briefs

Cloud Climatology

How Clouds Form and Travel

A cloud is formed when surface water evaporates and then condenses on microscopic airborne particles — dust, sea salt and bits of organic matter. Between evaporation and condensation, water vapor is carried along by winds from warm, moist regions to cooler, drier ones. Because the atmosphere, except for clouds , is nearly transparent to solar radiation, the surface absorbs 70 percent of the total solar heat taken up by the earth-atmosphere system, making the air warmer near the surface than it is at high altitudes. Because sunlight strikes the planet most directly near the equator, the tropics are warmer than the polar regions.

Both temperature gradients — the temperature variations from low to high altitudes and from low to high latitudes — are intensified by the effects of water vapor on radiative heating and cooling and by the transformations of water from liquid or solid into vapor and back. The wavelengths of incoming solar radiation are between 200 and 3,000 nm (nm = nanometer, one billionth of a meter). Water vapor is essentially transparent at those wavelengths; it lets virtually all the energy in. The warmed surface radiates away the absorbed energy as thermal radiation with wavelengths between 3,000 and 100,000 nm, which, unlike the incoming radiation, is absorbed by water vapor. The absorption of most of the outgoing thermal radiation by vapor creates most of Earth's natural greenhouse effect — an effect that is being amplified by pollution. Without the atmospheric water vapor Earth's surface would be, on average, about 31°C (55°F) colder than it is now, and the differences in temperature between high and low altitudes and between the poles and equator would be smaller.

Since cold air is denser than warm air, temperature differences give rise to atmospheric motions, and those motions work to eliminate the differences. Winds move warm, moist air upward and poleward from the tropical surface and move cold, dry air downward and toward the equator from higher altitudes and latitudes. The contrasts in heating, together with the winds, also drive ocean currents, which help reduce temperature differences between the equator and the poles even more.

Some of the water evaporated from the surface (primarily from the oceans) condenses into clouds and eventually falls as rain or snow. These transformations not only redistribute water but also play an important role in global heat transport. When surface water evaporates, the heat required to change liquid water into vapor is absorbed from the surface and carried along with the vapor into the air. When water vapor condenses into a cloud and falls as rain, it releases that heat, known as latent heat, into the air.

Atmospheric scientists have learned a great deal in the past several decades about how clouds form and travel in Earth's radiation-driven system, but until recently investigations were motivated primarily by a desire to make more accurate daily weather forecasts. Cloud patterns and motions were useful for what they revealed about winds, humidity and precipitation. In the past ten years attempts to forecast the weather further ahead (one to several weeks into the future) have focused attention on how clouds might influence Earth's radiation balance. Investigators now realize that traditional computer models of global climate have taken a rather simplistic view of clouds and their effects, partly because detailed global descriptions of clouds have been lacking.