It is important to understand the basics of the Planck emission curves that feature in the interpretation of emission spectra of the Earth. This section provides the Planck equation and an example for a black body at a temperature of 288 K [15 C] which is the mean temperature of the Earth's surface.

This is the Planck equation for radiant exitance or radiance from a black body at an absolute temperature *T*, *h* is Planck's constant, *c* is the speed of light, *k* is Boltzmanns's constant and *v *is the wavenumber of the radiation. The graph shows a plot of radiance against wavenumber for a black body at 288 K [15°C], the overall mean temperature of the Earth's surface.

The units of radiance are Watts per square metre per steradian [solid angle] per wavenumber. The area under the Planck curve multiplied by p is the emitted energy in Watts per square metre. In the 288 K case this is 389.6 Watts per square metre, typical of the Earth's surface.

Some real and simulated emission spectra are shown in subsequent pages.

The area under the curve and similar curves forms the basis of the Stefan-Boltzmann equation that has been referred to previously.