The Bouguer–Lambert law may be applied to describe the attenuation of solar or stellar radiation as it travels through the atmosphere. In this case, there is scattering of radiation as well as absorption. The optical depth for a slant path is τ ′ = mτ, where τ refers to a vertical path, m is called the relative airmass, and for a plane-parallel atmosphere it is determined as m = sec θ where θ is the zenith angle corresponding to the given path. The Bouguer-Lambert law for the atmosphere is usually written T = exp ⁡ ⁡ ( − − m ( τ τ a + τ τ g + τ τ R S + τ τ N O 2 + τ τ w + τ τ O 3 + τ τ r + ⋯ ⋯ ) ) , where each τ x is the optical depth whose subscript identifies the source of the absorption or scattering it describes: