level H is the scale height of the atmosphere z is the height in question The optical depth of a plane parallel cloud layer is given bywhere: Qe is the extinction efficiency L is the liquid water path H is the geometrical thickness N is the concentration of droplets ρl is the density of liquid water So, with a fixed depth and total liquid water path, . Astronomy In astronomy, the photosphere of a star is defined as the surface where its optical depth is 2/3. This means that each photon emitted at the photosphere suffers an average of less than one scattering before it reaches the observer. At the temperature at optical depth 2/3, the energy emitted by the star (the original derivation is for the Sun) matches the observed total energy emitted. Note that the optical depth of a given medium will be different for different colors (wavelengths) of light. For planetary rings, the optical depth is the (negative logarithm of the) proportion of light blocked by the ring when it lies between the source and the observer. This is usually obtained by observation of stellar occultations. See also Air mass (astronomy) Absorptance Actinometer Aerosol Angstrom exponent Attenuation coefficient Beer–Lambert law Pyranometer Radiative transfer Sun photometer Transparency and translucency References External links Optical depth equations Scattering, absorption and radiative transfer (optics) Visibility Spectroscopy