 Strong-line limit

A regime where optical-depth-one transmission occurs in the far wings of a strongly absorbing line. With a uniformly mixed absorber, $\Delta ={\Delta }_{s}p/p_s$ and $C=qS{p}_s/(2\pi g{\Delta }_s)\gg 1$, the radiative-wing optical depth is $\tau \simeq qS{\Delta }_s{p}^2/[2\pi g{p}_s(\nu -{\nu }_0{)}^2]$. This condition does not make pressure broadening valid in the arbitrarily rarefied upper atmosphere.