From Glossary of Meteorology


(Also called internal friction.) The transport of mass motion momentum solely by the random motions of individual molecules not moving together in coherent groups.

Viscosity is a consequence of gradients in velocity fields in fluids. Sometimes described as fluid friction because velocity gradients in fluids are damped as a consequence of viscosity.
See viscous force, stress tensor, dynamic viscosity, kinematic viscosity, Newtonian friction law, Navier– Stokes equations, eddy viscosity.