A theoretical and experimental investigation of the molecular absorption process in air has been carried out with particular reference to the variation of Napier frequency (fmax) with water‐vapor concentration (h′). It turns out that the relationship between these two quantities is a quadratic only at very low humidities and that, over the rest of the humidity range, the general relationship derived here is best approximated by an equation of the formfmax = const h′x, where 1