Two-stream approximation of the radiative transfer equation is an approximation of the radiative transfer equation in which radiation is propagating in only two discrete directions.
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 a surprisingly large number of applications of the two-stream approximations, including 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. The two-stream approximation comes in many variants, including Eddington approximation, Modified Eddington, Quadrature, Hemispheric constant models. Modern mathematical description of the two-stream approximation is given in several books.