From AMS Glossary
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In radar, a scattering mechanism proposed to explain certain kinds of clear- air echoes observed by UHF and VHF radars.
Such echoes are observed by vertically pointing radars operating at wavelengths of about 1 m and longer. They are in the form of thin, horizontal layers that exhibit strong aspect sensitivity, in the sense that the reflectivity for a vertical beam is greater than that for off-vertical beams. They are thought to be explained by partial reflections from thin layers containing sharp vertical gradients of refractivity. The layers have vertical extents that are comparable to or less than a wavelength and horizontal extents that are as large as the width of the first Fresnel zone, namely, (zλ)½, where z is the altitude and λ is the radar wavelength. Echoes explained by Fresnel reflection have longer coherence times than those explained by Bragg scattering from beam-filling echoes and are more in the nature of specular reflections. Sometimes a distinction is made between Fresnel reflection and Fresnel scattering. The term scattering is used if there are several or many thin reflective layers in the pulse volume; reflection is reserved for the situation of only one layer in the pulse volume.
Röttger, J., and M. F. Larsen 1990. UHF/VHF radar techniques for atmospheric research and wind profiler applications. Radar in Meteorology. D. Atlas, Ed., American Meteorological Society, . 241–242.