Angularly resolved molecular arrival flux
= Angularly resolved molecular arrival flux
For isotropic velocity directions and normalized speed density $f(v)$, the number arriving at a plane per unit area and time is $d\Gamma=\tfrac12nf(v)v\cos\theta\sin\theta\,d\theta\,dv$, over the incoming hemisphere. The cosine weights the normal speed, and the sine comes from solid angle.