On a clear night the temperature drops rapidly as the ground surface loses by radiation the heat it absorbed during the day. Cloudy nights are much warmer. In spring and fall, meteorologists use their forecasts of cloudiness to predict whether there will be an overnight frost. Clouds that shade and therefore cool the surface by day prevent it from cooling by night.
Clouds reflect and absorb the heat that is radiated from the surface and they reradiate some of the heat they absorb. This is how they warm the air below the cloud. Incoming solar radiation is predominantly at short wavelengths, but outgoing terrestrial radiation is at long wavelengths (see "Radiation from the Sun and from the Earth" on pages 128-134). The difference in wavelength affects the way clouds reflect and absorb radiation.
Water droplets and ice crystals reflect shortwave radiation very efficiently. That is why clouds shine so brightly. They do not reflect all of the sunlight, of course. If they did, the ground below would be in total darkness. Some of the radiation passes through the cloud. As the radiation passes through the cloud, liquid droplets, ice crystals, and water vapor absorb a proportion of it. Water absorbs shortwave radiation very inefficiently. How much the water vapor and cloud particles absorb and reflect depends on the distance light must travel in passing through the cloud. This is known as the optical thickness of the cloud. The greater the optical thickness, the greater is the likelihood that radiation will strike a droplet, crystal, or molecule, and be reflected or absorbed by it. Clouds are believed to absorb between 1 percent and 10 percent of the radiation passing through them. Cirriform clouds absorb the least (about 1 percent) and cumulus and cumulonimbus absorb the most (10 percent). The average is about 5 percent.
Clouds are much less efficient at reflecting radiant heat—radiation at near-infrared wavelengths (see the sidebar "Solar spectrum" on page 133)—and much more efficient at absorbing it. The proportion of the outgoing radiation the cloud absorbs depends on its optical thickness. If the cloud is more than about 3,300 feet (1 km) thick, it will absorb all of the outgoing radiation.
This does not mean the heat remains forever locked inside the cloud. As soon as the cloud droplets and ice crystals absorb energy, they begin radiating it in all directions. Some of this radiation is directed downward and warms the lower parts of the cloud and the ground surface beneath the cloud. Some is directed upward and it collides with and transfers energy to particles at a higher level. These are colder, so although the additional energy raises their temperature, they nevertheless remain cooler than the particles below. The heat leaving the planet is proportional to the surface temperature (see the sidebar "Blackbody radiation" on page 129), and if a cloud covers the surface, the relevant temperature is that at the top of the cloud. If the top of a cloud is very cold it will radiate very little energy into space, regardless of the temperature lower down.
Low clouds that remain close to the surface temperature throughout their depth have an overall cooling effect on the atmosphere, because they radiate energy into space. Clouds with high tops, where the temperature is very low, have a warming effect, because little of the heat that they absorb is lost from the cloud top.
Clouds reflect incoming radiation. This has a cooling effect on the climate. They also absorb radiation, which has a warming effect. They absorb heat radiated from the surface. This has a cooling effect if the clouds are low and warm, but a warming effect if they extend to a great height. Clearly, their effects are complicated, but fortunately scientists have been able to cut through the confusion and simply measure what happens. They have used satellites to measure the amount of radiation falling on cloud-covered regions of the Earth and the amount leaving from the cloud tops. They found that over the world as a whole, clouds reduce the amount of radiation reaching the surface by 48 watts per square meter (W m-2) and reduce the amount of radiation leaving the Earth by 31 W m-2. The difference, of 17 W m-2, is the amount by which clouds cool the Earth.
Clouds form by the condensation of water vapor. This releases latent heat (see the sidebar "Latent heat and dew point" on page 32), warming the surrounding air. Cloud formation therefore warms the air. This affects conditions inside the cloud itself, but it has no wider climatic effect. This is because before very long all the water that condenses to form the cloud evaporates once more, absorbing precisely the same amount of latent heat from the surrounding air that it released when it condensed.
Was this article helpful?