A measure of the depletion of monochromatic radiance passing through some medium in a constant direction. The volume extinction coefficient equals the fractional depletion due to absorption and scattering when monochromatic radiance passes unit distance through a medium in a constant direction. Units are inverse length or km⁻¹. Values depend critically on the medium being traversed, as well as the wavelength. For example, in a Rayleigh atmosphere the volume extinction coefficient at a wavelength of 0. 5 μm is less than 0. 02 km⁻¹, whereas in a wet cloud it may exceed 100 km⁻¹ at the same wavelength.