Viscosity in gases is due to the transfer of
momentum between neighbouring layers of gas. (In liquids it is mainly due to the molecular
attraction between adjacent layers. Energy is needed to drag one layer over another against the
force of attraction.) The viscosity of a gas is given by:
where σ is the effective diameter of a molecule.
It can be seen that the
viscosity of a gas is directly proportional to the average velocity of the gas molecules. Since the
average velocity of the molecules is related to the temperature of the gas, the viscosity of a gas
increases with increasing temperature. This is explained by the greater rate of transfer of
momentum between layers in the gas at higher temperatures.