]> Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength. The energy difference between the absorbed and emitted photons ends up as molecular vibrations or heat. Usually the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range, but this depends on the absorbance curve and Stokes shift of the particular fluorophore. Fluorescence is named after the mineral fluorite, composed of calcium fluoride, which often exhibits this phenomenon. The process by which a chemical species undergoes a chemical change as the result of the absorption of a photon of light energy. Radiation radiative process The radiance per unit wavelength or wavenumber interval. The electromagnetic (EM) spectrum is the range of all possible electromagnetic radiation. The electromagnetic spectrum (usually just spectrum) of an object is the characteristic distribution of electromagnetic radiation from that object. A radiometric term for the rate at which radiant energy in a set of directions confined to a unit solid angle around a particular direction is transferred across unit area of a surface (real or imaginary) projected onto this direction. Infrequently, any energy propagated by a physical quantity governed by a wave equation. Radiant energy per unit time passing some specified area from one side. The ratio of reflected flux density to incident flux density, referenced to some surface. Albedo In Raman spectroscopy, the depolarization ratio is the intensity ratio between the perpendicular component and the parallel component of the Raman scattered light. In radiation, the net flux of radiation into or out of a system. As a consequence of radiative forcing there must be some change to the nonradiative energy states of the system. 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 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.. 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 Attenuation of a beam of light by an optically homogeneous (transparent) medium. Line-of-sight propagation refers to electromagnetic radiation or electromagnetic waves travelling in a straight line. The rays or waves are deviated or reflected by obstructions and cannot travel over the horizon or behind obstacles. Beyond that, material disperses the rays respectively the energy of the waves. In U.S. weather observing practice, the designation for the sky cover when the sky is completely hidden by surface-based obscuring phenomena.