# Radar reflectivity

From AMS Glossary

## radar reflectivity

In general, a measure of the efficiency of a radar target in intercepting and returning radio energy. It depends upon the size, shape, aspect, and dielectric properties of the target.

It includes not only the effects of reflection but also scattering and diffraction. In particular, the radar reflectivity of a meteorological target depends upon such factors as 1) the number of hydrometeors per unit volume; 2) the sizes of the hydrometeors; 3) the physical state of the hydrometeors (ice or water); 4) the shape or shapes of the individual elements of the group; and 5) if asymmetrical, their aspect with respect to the radar. The radar reflectivity η has dimensions of area per unit volume (e.g., cm where where λ is the wavelength, where

^{2}m^{-3}, or, more commonly, cm^{-1}or m^{-1}) and is defined by*N*_{i}is the number of hydrometeors per unit volume with backscattering cross section σ_{i}and the summation is over all the hydrometeors in a unit volume. For spherical hydrometeors small enough compared with the wavelength for the Rayleigh scattering approximation to be valid, the radar cross section is related to particle size by*D*the diameter of the hydrometeor, and*K*a dielectric factor defined by*m*is the complex index of refraction of the hydrometeor.*See*radar equation,**radar reflectivity**