A study was made of the total absorption of five water vapor bands between 1µ and 7 µ, using a multiple-reflection absorption cell in which optical path length, water vapor density, and total pressure could be varied independently. Absorption was determined as a function of these parameters. For the range of concentrations studied it was found that the fractional absorption of an entire band can be represented by an error-function formula [Equation] where t=½β(πω)<sup>½</sup>(P<sub>T</sub>P<sub>P</sub>/760)<sup>½</sup>. Here ω is the water vapor concentration, P<sub>T</sub> the total pressure, and P<sub>P</sub> the partial pressure of the water vapor. The quantity t in this formula is very nearly proportional to the fourth root of the pressure. Values of the “generalized absorption coefficient” β are determined for the five bands.
JOHN N. HOWARD and ROBERT M. CHAPMAN, "The Pressure Dependence of the Absorption by Entire Bands of Water Vapor in the Near Infrared," J. Opt. Soc. Am. 42, 423-424 (1952)