About: Chapman function

A Chapman function describes the integration of atmospheric absorption along a slant path on a spherical earth, relative to the vertical case. It applies to any quantity with a concentration decreasing exponentially with increasing altitude. To a first approximation, valid at small zenith angles, the Chapman function for optical absorption is equal to where z is the zenith angle and sec denotes the secant function. The Chapman function is named after Sydney Chapman, who introduced the function in 1931.