Fossil turbulence is the name given to the three-dimensional microstructure of temperature, humidity, or some other scalar property that is advected by the mean flow. (The first use of the expression seems to be attributable to Woods, but the physical phenomenon that it represents has been recognized for some time by fluid dynamicists.) The fossil turbulence was produced by a turbulent field that has since decayed; its scalar fluctuations occur at an intensity and on a scale that cannot be accounted for by the existing velocity field.
The relevance to this colloquium of fossil turbulence in the atmosphere is that it may produce appreciable radar backscattering, provided that it contains significant three-dimensional refractive-index structure on a scale equal to half the radar wavelength (typically 5–50 cm). Similarly, fossil turbulence may be of importance in radio forward-scattering, although the relevant scale sizes here are larger (typically 3 meters or more) and depend on radio wavelength and scattering angle. Fossil turbulence, if it exists in the atmosphere, will also affect line-of-sight radio propagation and the propagation of optical and acoustical waves, its effects depending on the propagation geometry and the frequency of the propagating waves.