From Glossary of Meteorology
A radar echo returned from a region of the atmosphere with no apparent meteorological scatterers such as clouds or precipitation. Clear-air echoes are caused by either 1) solid target returns from birds, insects, dust, or other particulate matter, or 2) spatial fluctuations of refractivity with scale sizes on the order of or smaller than the radar wavelength.
Two examples of the latter type of echo are Bragg scattering from atmospheric turbulence and Fresnel reflection or specular reflection from layers with sharp gradients in refractivity. Insects, birds, and particulates are the principal cause of clear-air echoes observed with radars having wavelengths of 3 cm or less, whereas high-powered radars operating at wavelengths of 10 cm or greater routinely detect backscattered signals from strong refractive index fluctuations.