When a parallel, monochromatic beam of light traveling in some specific direction encounters a medium of finite extent, a certain amount of the flux will be absorbed and a certain amount will be scattered into other angles. The rate at which energy is taken out of the beam by absorption and scattering can be characterized in terms of coefficients with dimensions of area, which are known as cross sections. The term absorption cross section is often used to include both the portion due to scattering and that due to true absorption (loss of the photon into another form of energy, such as heat). For atmospheric gases, this total absorption cross section is defined by the Beer’s law expression:

$$ I={I}_0\; \exp \left(-\sigma nl\right) $$

where I_{0} and I are the incident and transmitted light intensities, respectively, σ is the absorption cross section (cm^{2} molecule^{−1}), n is the molecular density, and l is the pathlength in cm.