= 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.
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