Curve of growth
From Glossary of Meteorology
curve of growth
For an isolated, gaseous absorption band, the relation between integrated absorption by the gas and its amount.
That is, if
where κ is the (volumetric) absorption coefficient of the gas, x is the path length through it, and the range of integration over frequency ν is sufficient to include essentially all absorption within the band, then W as a function of x is the curve of growth. This term, which Goody describes as "curious," is a relic from astronomical spectroscopy.
Goody, R. M. 1964. Atmospheric Radiation. Vol. 1, . p. 125.