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 for 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

${\displaystyle \sec(z),\ }$

where z is the zenith angle and sec denotes the secant function.

The Chapman function is named after Sydney Chapman.