The atmospheric radiation field

The theory presented in this book applies to the lower 50 km of the Earth's atmosphere, that is to the troposphere and to the stratosphere. In this part of the atmosphere the so-called local thermodynamic equilibrium is observed.

In general, the condition of thermodynamic equilibrium is described by the state of matter and radiation inside a constant temperature enclosure. The radiation inside the enclosure is known as black body radiation. The conditions describing thermodynamic equilibrium were first formulated by Kirchhoff (1882). He stated that within the enclosure the radiation field is:

1. isotropic and unpolarized;

2. independent of the nature and shape of the cavity walls;

3. dependent only on the temperature.

The existence of local thermodynamic equilibrium in the atmosphere implies that a local temperature can be assigned everywhere. In this case the thermal radiation emitted by each atmospheric layer can be described by Planck's radiation law. This results in a relatively simple treatment of the thermal radiation transport in the lower sections of the atmosphere.

Kirchhoff's and Planck's laws, fundamental in radiative transfer theory, will be described in the following chapters. While the derivation of Planck's law requires a detailed microscopic picture, Kirchhoff's law may be obtained by using purely thermodynamic arguments.