1. (Also called extinction, especially in reference to optical frequencies. ) A general term used to denote a decrease in signal strength in transmission from one point to another. For the propagation of electromagnetic waves through a medium, attenuation is caused by absorption and scattering. The volume attenuation coefficient (m−1) of such a medium is the fractional reduction of radiance per unit pathlength. In radar usage, the specific attenuation is the fractional reduction in power density per unit pathlength as plane-wave radiation propagates through a medium, usually expressed in decibels per kilometer. Attenuation ordinarily does not refer to the inverse-square falloff of irradiance or power density with range that is simply a consequence of beam divergence. See extinction coefficient. 2. The scattering and absorption of radiant energy by clouds or the atmosphere that decreases the radiation received by satellite sensors.