m is the optical mass or airmass factor, a term approximately equal (for small and moderate values of θ) to ⁠ 1 cos ⁡ ⁡ θ θ , ⁠ where θ is the observed object's zenith angle (the angle measured from the direction perpendicular to the Earth's surface at the observation site). This equation can be used to retrieve τ a, the aerosol optical thickness, which is necessary for the correction of satellite images and also important in accounting for the role of aerosols in climate.