Attenuation of electromagnetic waves propagating through precipitation. Depending on the wavelength of the radiation, the attenuation is accounted for by some combination of absorption and scattering by the precipitation particles. The relative importance of scattering tends to increase as the wavelength becomes shorter. For radar, the specific attenuation Y (dB km⁻¹) due to rain is described by empirical power relations of the form Y = aR^b, where R (mm h⁻¹) is the rainfall rate, and a and b are empirical constants that depend on wavelength and temperature. The specific attenuation of snow is less than that of rain, and for wavelengths of 10 cm and longer is usually negligible. For lidar, the precipitation attenuation is approximately proportional to the cross-sectional area of the precipitation particles per unit volume. See rain attenuation; Compare cloud attenuation.