Scattering asymmetry parameter ($g$)

The scattering asymmetry parameter is the mean scattering-angle cosine, $g=(4\pi {)}^{-1}\int P\cos \Theta d\Omega$ in the $4\pi$ normalization. Forward/backward symmetry gives $g=0$; positive $g$ indicates a net forward preference. In a two-stream transport approximation, the reduced scattering optical depth is $(1-g)\tau$.