The reduction with distance from the source of the intensity of an acoustic or an electromagnetic signal propagating through the atmosphere caused by interaction of the signal with gaseous constituents of the atmosphere, aerosols, or hydrometeors. In general, scattering and absorption account for attenuation. For sound, absorption is usually more important than scattering; it depends on temperature and humidity, and generally increases with increasing acoustic frequency. The main atmospheric constituents that absorb radar energy are oxygen, water vapor, and liquid hydrometeors. Absorption is often neglected at wavelengths of 10 cm and longer, but becomes increasingly important at shorter wavelengths. The contribution of scattering to radar attenuation also increases with decreasing wavelength. For lidar, scattering by molecules, aerosols, and hydrometeors dominates the attenuation, although gaseous absorption is significant at certain spectral bands and is exploited by differential absorption lidar (DIAL) to measure molecular concentrations.