The Absorption of Microwaves by Uncondensed Water Vapor

Abstract

The absorption due to uncondensed water vapor in the short microwave region is computed by means of quantum mechanics. The attenuation is attributed to two causes: (a) a single line $\lambda =1.35\mathrm{}$ cm, and (b) the combined residual effect of all the other lines, whose wave-lengths are too short for resonance. There is a sharp peak in the absorption due to (a) at 1.35 cm, amounting to about 0.2 db/km per gram of ${\mathrm{H}}_2$O per cubic meter. The absorption caused by (b) is inversely proportional to the square of the wave-length. The theory is compared with existing microwave data on damp air, on pure water vapor at low pressures, and on steam. From these data one can determine the precise value of the resonance frequency in (a) and the line-breadth. Until recently these constants could only be roughly estimated from infra-red measurements. On the whole the theory and experiment agree satisfactorily, except that the attenuation due to the residual effect (b) is apparently about four times as large as predicted. Possible causes of this discrepancy are speculated upon—perhaps the Lorentz model of infinitely sharp collisions which we use is too simple. Finally a curve is ïncluded of the predicted absorption in the millimeter region, where water vapor is much more opaque than at centimeter wave-lengths, and new resonances come into play.