Methods for estimating gas viscosity
Viscosity - Estimation methods for fluids

When a gas undergoes shear stress resulting in a certain ensemble motion of the molecules, each molecule will have an ensemble velocity vector in addition to its own random velocity vector. Close to the source where the stress is applied, the ensemble velocity vector is high, but as the molecules move away from the source, they are slowed down as a result of random molecular collisions. This exchange of random molecular momentum due to molecular collisions is the main cause of gas viscosity.