]> The ratio of reflected flux density to incident flux density, referenced to some surface. Albedo The optical depth due to extinction by the aerosol component of the atmosphere. Aerosol Optical Depth The extinction coefficient for a particular substance is a measure of how well it scatters and absorbs electromagnetic radiation (EM waves). If the EM wave can pass through very easily, the material has a low extinction coefficient. Conversely, if the radiation hardly penetrates the material, but rather quickly becomes extinct within it, the extinction coefficient is high. The extinction coefficient for a particular substance is a measure of how well it scatters and absorbs electromagnetic radiation (EM waves). If the EM wave can pass through very easily, the material has a low extinction coefficient. Conversely, if the radiation hardly penetrates the material, but rather quickly becomes extinct within it, the extinction coefficient is high. The optical thickness measured vertically above some given altitude. Optical depth is dimensionless and may be used to specify many different radiative characteristics of the atmosphere. The vertical integral of the density of absorbers between two altitudes; used mainly in determining the transmission through an absorbing gas. Radar cross section (RCS) describes the extent to which an object reflects an incident electromagnetic wave. It is a measure of the strength of the radar signal backscattered from a target object for a given incident wave power.. A measure of the extinction due to scattering of monochromatic radiation as it traverses a medium containing scattering particles. The Secchi disk is a device used to measure water transparency in open waters of lakes, bays, and the ocean. A pattern is drawn or painted onto a card or acrylic, mounted on a pole or line, and lowered slowly in the water. The depth at which the pattern on the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the Secchi depth and is related to water turbidity. The effect of (primarily) aerosols, through their total optical depth, in reducing the transmission of direct solar radiation to the surface below that through a purely molecular atmosphere. Turbidity is the cloudiness or haziness of a fluid, or of air, caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The greatest distance in a given direction at which it is just possible to see and identify with the unaided eye scattering of waves (photons) that reverse the direction of propagation Attenuation of a beam of light by an optically homogeneous (transparent) medium.