Taylor's hypothesis
From Glossary of Meteorology
An assumption that advection contributed by turbulent circulations themselves is small and that therefore the advection of a field of turbulence past a fixed point can be taken to be entirely due to the mean flow; also known as the Taylor "frozen turbulence" hypothesis.
It only holds if the relative turbulence intensity is small; that is, 
, where U is the mean velocity and u the eddy velocity. Then the substitution t = x/U is a good approximation.
