text: In atmospheric sciences, one often refers to the optical depth of the atmosphere as corresponding to the vertical path from Earths surface to outer space; at other times the optical path is from the observers altitude to outer space. The optical depth for a slant path is , where τ′ refers to a vertical path, m is called the relative airmass, and for a plane-parallel atmosphere it is determined as  where θ is the zenith angle corresponding to the given path. Therefore,The optical depth of the atmosphere can be divided into several components, ascribed to Rayleigh scattering, aerosols, and gaseous absorption. The optical depth of the atmosphere can be measured with a Sun photometer.
  The optical depth with respect to the height within the atmosphere is given by and it follows that the total atmospheric optical depth is given by
  In both equations:
  * k is the absorption coefficient
  * w is the mixing ratio
  * ρ is the density of air at sea 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:
  * Q is the extinction efficiency
  * L is the liquid water path
  * H is the geometrical thickness
  * N is the concentration of droplets
  * ρ is the density of liquid water
  So, with a fixed depth and total liquid water path, .