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The study of clouds, where they occur, and their characteristics, play a key role in the understanding of climate change. Low, thick clouds primarily reflect solar radiation and cool the surface of the Earth. High, thin clouds primarily transmit incoming solar radiation; at the same time, they trap some of the outgoing infrared radiation emitted by the Earth and radiate it back downward, thereby warming the surface of the Earth. Whether a given cloud will heat or cool the surface depends on several factors, including the cloud's altitude, its size, and the make-up of the particles that form the cloud. The balance between the cooling and warming actions of clouds is very close although, overall, averaging the effects of all the clouds around the globe, cooling predominates.
The Earth's climate system constantly adjusts in a way that tends toward maintaining a balance between the energy that reaches the Earth from the sun and the energy that goes from Earth back out to space. Scientists refer to this as Earth's "radiation budget." The components of the Earth system that are important to the radiation budget are the planet's surface, atmosphere, and clouds. The energy coming from the sun to the Earth's surface is called solar energy. Most of it is in the form of radiation from the "visible" wavelengths, i.e., those responsible for the light detected by our eyes. Visible radiation and radiation with shorter wavelengths, such as ultraviolet radiation are labeled "shortwave." Both the amount of energy and the wavelengths at which energy is emitted by any system are controlled by the average temperature of the system's radiating surfaces, plus the emission properties. The temperature of the sun's radiating surface, or photosphere, is more than 5500°C (9900°F). However, not all of the sun's energy comes to Earth. The sun's energy is emitted in all directions, with only a small fraction being in the direction of the Earth.
Energy goes back to space from the Earth system in two ways: reflection and emission. Part of the solar energy that comes to Earth is reflected back out to space in the same, short wavelengths in which it came to Earth. The fraction of solar energy that is reflected back to space is called the albedo. Different parts of the Earth have different albedos. For example, ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the sun's energy. Deserts, ice, and clouds, however, have high albedos; they reflect a large portion of the sun's energy. Over the whole surface of the Earth, about 30 percent of incoming solar energy is reflected back to space. Because a cloud usually has a higher albedo than the surface beneath it, the cloud reflects more shortwave radiation back to space than the surface would in the absence of the cloud, thus leaving less solar energy available to heat the surface and atmosphere. Hence, this "cloud albedo forcing," taken by itself, tends to cause a cooling or "negative forcing" of the Earth's climate.
Another part of the energy going to space from the Earth is the electromagnetic radiation emitted by the Earth. The solar radiation absorbed by the Earth causes the planet to heat up until it is emitting as much energy back into space as it absorbs from the sun. Because the Earth is absorbing only a tiny fraction of the sun's energy, it remains cooler than the sun, and therefore emits much less radiation. Most of this emitted radiation is at longer wavelengths than solar radiation. Unlike solar radiation, which is mostly at wavelengths visible to the human eye, the Earth's longwave radiation is mostly at infrared wavelengths, which are invisible to the human eye. When a cloud absorbs longwave radiation emitted by the Earth's surface, the cloud reemits a portion of the energy to outer space and a portion back toward the surface. The intensity of the emission from a cloud varies directly as its temperature and also depends upon several other factors, such as the cloud's thickness and the makeup of the particles that form the cloud. The top of the cloud is usually colder than the Earth's surface. Hence, if a cloud is introduced into a previously clear sky, the cold cloud top will reduce the longwave emission to space, and (disregarding the cloud albedo forcing for the moment) energy will be trapped beneath the cloud top. This trapped energy will increase the temperature of the Earth's surface and atmosphere until the longwave emission to space once again balances the incoming absorbed shortwave radiation. This process is called "cloud greenhouse forcing" and, taken by itself, tends to cause a heating or "positive forcing" of the Earth's climate. Usually, the higher a cloud is in the atmosphere, the colder is its upper surface and the greater is its cloud greenhouse forcing.
If the Earth had no atmosphere, a surface temperature far below freezing would produce enough emitted radiation to balance the absorbed solar energy. But the atmosphere warms the planet and makes Earth more livable. Clear air is largely transparent to incoming shortwave solar radiation and, hence, transmits it to the Earth's surface. However, a significant fraction of the longwave radiation emitted by the surface is absorbed by trace gases in the air. This heats the air and causes it to radiate energy both out to space and back toward the Earth's surface. The energy emitted back to the surface causes it to heat up more, which then results in greater emission from the surface. This heating effect of air on the surface, called the atmospheric greenhouse effect, is due mainly to water vapor in the air, but also is enhanced by carbon dioxide, methane, and other infrared-absorbing trace gases.
In addition to the warming effect of clear air, clouds in the atmosphere help to moderate the Earth's temperature. The balance of the opposing cloud albedo and cloud greenhouse forcings determines whether a certain cloud type will add to the air's natural warming of the Earth's surface or produce a cooling effect. As explained below, the high thin cirrus clouds tend to enhance the heating effect, and low thick stratocumulus clouds have the opposite effect, while deep convective clouds are neutral. The overall effect of all clouds together is that the Earth's surface is cooler than it would be if the atmosphere had no clouds.
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