The Nature of Terrestrial Radiation

In addition to solar radiation reflected back to space, our planet also loses the energy via thermal emission. The emission from the Earth's surface corresponds to the emission of a blackbody at 288 K. Because the thermal emission strongly depends on temperature and Earth's temperatures are much smaller than those of the Sun, electromagnetic radiation emitted by the Earth's surface-atmosphere system occurs at the longer wavelengths, with its maximum at about 10 versus 0.5 mm for the Sun. About 99% of the radiant energy emitted by the Earth's surface and atmosphere is found in the band of 4-100 mm. This radiation is called terrestrial radiation. Other commonly used names are longwave radiation and thermal IR radiation. Emission at wavelengths larger than 100 mm is very small and practically irrelevant for the global energy budget, although it is actively used in various remote sensing applications.

Most land and water surfaces are very efficient emitters at thermal IR wavelengths. Their emissivities, the degree to which an object behaves like a perfect blackbody, are between ^0.9 and 1. The longwave radiation emitted from the surface is absorbed and re-emitted by gases, clouds, and, to a lesser extent, aerosols throughout the atmosphere.

The atmospheric gases that can absorb IR radiation and are important to the radiation balance are water vapor, carbon dioxide, ozone, methane (CH4), and nitrous oxide (N2O), with H2O being the chief IR absorber. Collectively, they all are called the greenhouse gases because they trap the fraction of the radiant energy emitted by the surface which otherwise will escape to space. Gases absorb IR radiation selectively, that is, they absorb not all but certain wavelength bands in the IR. Some bands are more important to the global radiation balance than others, depending on their position in the surface emission spectrum.

The cloud-free atmosphere is the most transparent to IR radiation between about 8 and 12 mm, the so-called atmospheric window. Practically all energy emitted by the surface in this band escapes to space. Outside of this band, the atmosphere is largely opaque.

Clouds have a profound effect on longwave radiation. In general, they work in the same way as greenhouse gases do: they trap the emission from the surface and re-emit some energy back to the system. Efficiency of clouds to absorb and emit longwave radiation depends on several factors including their amount and makeup (water drops or ice crystals) as well as height of cloud tops. Most water clouds emit as blackbodies. Vertically extended cloud systems with the high tops at temperatures much lower than the surface temperature emit little longwave radiation to space while they absorb essentially all surface emission. However, these clouds have offsetting effects on solar and terrestrial radiation in terms of the energy balance of the planet. By reducing the thermal emission they contribute to a heating, but they result in a cooling by reducing the amount of absorbed solar radiation due to a generally higher albedo than the underlying surface. Unlike water clouds, cirrus (ice) clouds are partially transparent to thermal IR radiation, and their albedo at the short-wave band is smaller than that of water clouds. As a result, cirrus and water clouds can cause an opposite effect. Satellite observations indicate that overall on the global mean, clouds reduce the radiant energy of the planet.

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