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., cm²m^-3, or, more commonly, cm^-1 or m^-1) and is defined by

where 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

where λ is the wavelength, D the diameter of the hydrometeor, and K a dielectric factor defined by

where m is the complex index of refraction of the hydrometeor.
See radar equation, radar reflectivity

(Coined word for radio detection and ranging.) An electronic instrument used for the detection and ranging of distant objects of such composition that they scatter or reflect radio energy.