Sutherland’s law, or Sutherland’s formula, is an approximation for how the viscosity of gases depends on the temperature. This law is based on an idealized intermolecular-force potential and reads (Ref. 5)

where Sμ is an effective temperature called the Sutherland constant. Each gas has its own Sutherland constant. Equation 5-9 is strictly valid only for single-component gases at low pressure. It does, however, work well for air because air is mainly composed of nitrogen and oxygen, which have very similar properties. Parameter values for some common gases are given in Table 5-2 (Ref. 5).

The SI unit for μ0 in Table 5-2 is N·s/m2. The SI unit for T0 and Sμ is kelvin (K).

Sutherland’s law can also be formulated for thermal conductivity (Ref. 5):

Values for k0, T0, and Sk for some common gases are given in Table 5-3(Ref. 5).

The unit for k0 in Table 5-3 is W/(m·K). The unit for T0 and Sk is kelvin (K).