Our planet is heated by absorbing solar radiation and cooled by emitting infrared radiation. There must be a balance between the incoming and outgoing radiation, otherwise the mean temperature of Earth will change. This model calculates the temperature that results in a balance, for a simple model of the planet.

The model in its simplest form neglects the atmosphere. Earth’s temperature is then only governed by the magnitude of solar radiation and the fraction of solar radiation that is reflected. This fraction is called the planet’s albedo, A.

You can turn on an approximation of the greenhouse effect. How effective the atmosphere is to emit radiation is called the emissivity, ε. The fraction of emission from the surface that reaches space is 1-ε.

Finally, you can also turn on that the atmosphere absorbs a fraction of the solar radiation passing it. This is called the absorptivity, α.

Some details: The magnitude of solar radiation outside of the atmosphere is called the solar constant, S. Averaged over all positions on Earth and day/night, the average solar radiation entering the atmosphere is S/4. The absorptivity in the model is the one-way attenuation. All the absorption is assumed to take place above where the planet reflects solar radiation (in real life this would partly be above the clouds). The surface and the atmosphere emitt according to the Stefan-Boltzmann law. An emissivity of 1 is assumed for the surface.