Sometimes referred to as the inertial parameter; it is an index of the impactability of an aerosol particle. It is defined by the equation: \(St = 2\ \tau \ (V_{\rm{t}}- \nu_{\rm{t}})\ D_{\rm{p}}\) where \(D_{\rm{p}}\) is the diameter of a small drop, \(V_{\rm{t}}- \nu_{\rm{t}}\) is the difference in fall velocities of the drop and aerosol particles and \(\tau \) is the characteristic relaxation time of a particle.