Two-stream approximation

A two-stream approximation replaces the angular distribution of radiation by upward and downward representative intensities or hemispheric fluxes, with a specified angular closure. Angular path factors can be absorbed into an effective optical depth, producing exponential attenuation in that coordinate. Different closures change numerical emission depths and diffusivity factors. Fluxes and directional radiances must remain distinguished, and a prescribed effective optical depth need not equal literal vertical absorption optical depth.