He joined the British Meteorological Office in 1939 and was asked to study turbulent diffusion in the atmospheric boundary layer ( the first few hundred metres of atmosphere above the earth's surface ).
12.
In real fluids, however, molecular and turbulent diffusion always cause internal heating / cooling even in absence of external heating / cooling, as long as the temperature of the fluid considered is non-uniform.
13.
Thus, this approach is used to evaluate the position and velocity of particles relative to their neighbors and environment, and approximates the random concentrations and velocities associated with turbulent diffusion in the statistics of their motion.
14.
Under various external conditions such as directional flow speed ( wind ) and environmental conditions, the variances and diffusivities of turbulent diffusion are measured and used to calculate a good estimate of concentrations at a specific point from a source.
15.
This turbulent diffusion coefficient is defined in a phenomenological sense, by analogy with the molecular diffusivities, but it does not have a true physical meaning, being dependent on the flow conditions, and not a property of the fluid itself.
16.
Therefore, because of internal diabatic heating / cooling by molecular / turbulent diffusion, the overall heating experienced by a stratified fluid always occurs at greater pressure than the total cooling, even if the external cooling / heating occur at the same pressure.
17.
As is well-known, molecular and turbulent diffusion tends to relax the system toward thermodynamic equilibrium, i . e ., toward an isothermal state, which for a statically stable fluid, will warm up the fluid at high pressure, and cool it down at low pressure.
