Stokes number, St
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 τ ( V t − v t ) D p where D p is the diameter of a small drop, V t − v t is the difference in fall velocities of the drop and aerosol particles and τ is the characteristic relaxation time of a particle.
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2216