Two-stream approximation

Two stream approximation of the radiative transfer equation - approximation of the radiative transfer equation in which radiation is propagating in only two discrete direction.

This approximation captures essence of the radiative transport in light scattering atmosphere. Two stream approximation is commonly used in parameterizations of radiative transport in global circulation models and in weather forecasting models such as WRF. There is suprisingly large number of applications of the two stream approximations incuding variants such as Kubelka-Munk approximation. The two stream approximation is the simplest approximation which can be used to explain common observation inexplicable by single-scattering arguments such as the brightness and color of the clear sky, the brightness of clouds, the whiteness of a glass of milk, the darkening of sand upon wetting.