Countergradient flow is explained in terms of a nonlocal flux-gradient
relationship, obtained through solution of the widely-used BGK or ''r
elaxation time'' model kinetic equation in one dimension for a layered
source in an atmosphere with stationary turbulence. It is also pointe
d out that the classical formula derived by Taylor [Proc. London Math.
Soc. Ser. A 20, 196 (1921)] for dispersion from a point source is ful
ly recoverable from BGK theory.