The continuum absorption by H2O has several characteristics that are common throughout the windows in the infrared and millimeter-wave regions. Values of the continuum absorption coefficient calculated on the basis of simple, widely used line shapes may differ greatly from observed values in the windows between strong absorption lines. The temperature dependence of this absorption is also not predictable from present day understanding of line shapes or of dieters, which may also contribute. The shapes of self-broadened H2O lines are quite different from those of N2-broadened lines, and the difference increases with increasing distance from the centers of the lines. Data obtained from laboratory samples and from atmospheric paths are presented to compare the various windows in the infrared and millimeter regions.
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The microwave absorption between 100 and 300 Gc/s by H
O vapor and its mixtures with N
, CO
, and O
has been measured at room temperature for intermediate and atmospheric pressures. Near the absorption lines the line shape parameters are close to those measured at low pressures by Rusk. Between the lines, very large absorption coefficients were observed, particularly for self-broadening and for broadening by CO
. This increased absorption is ascribed to collision-induced polarization of the molecules involved. Empirical formulas are given for the calculation of line shapes. These formulas require the use of four line-shape parameters for each mixture--two of these are for the line centers, the other two for the wings.
