The kinematic viscosity is the mean free path times the thermal molecular velocity (comparable to the speed of sound). Therefore, the viscous scale is much larger than the mean free path as long as the velocity difference on the viscous scale is much smaller than the speed of sound. This is practically always the case for turbulence in liquids and gases. For example, in atmosheric turbulence, the external scale L can be in hundreds of meteres with velocity v(L) in tens of meters per second, but the viscous scale l is usually less than a millimiter so that