. The conservation of this property is often expressed as a consequence of Ertel's theorem.
where Pe is the Peclet number, Re the Reynolds number, the kinematic viscosity, and the thermal diffusivity. When Pr = 1, the viscous time scale is equal to the time scale of thermal diffusion, and similarity exists between viscous dissipation and thermal diffusion. The Prandtl number is equal to about 0.7 for air, and is about 13 at 0 C and 7 at 20 C for water. See Kraus and Businger (1994), p. 33.