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 water vapor absorption line resulting from the rotational transition 5-1-6-5 has been investigated experimentally. Radiation is fed into an air-filled cubical copper cavity 8 ft. on an edge. Strings of thermocouples with alternate junctions coated with a "lossy" material are placed at random in the cavity. The e.m.f. of these thermocouples is proportional to the Q of the cavity and its contents. With the total pressure inside the cavity at one atmosphere, the partial pressure of the water vapor is varied from 1 mm to 55 mm of Hg. A measurement of the change in e.m.f. with humidity yields a value for the losses in the water vapor, provided the Q of the cavity is known. This quantity may be determined from additional measurements taken with an aperture opened in the side of the cavity. The wave-length range between 0.7 cm and 1.7 cm has been explored. Results indicate a peak at ν̃=0.744±0.005 cm-1, corresponding to a wave-length λ=1.34 cm. The absorption line is broadened as the water vapor density is increased. At very low density, the half-width of the curve (half-width at half-height) is 0.087±0.01 cm-1, while the corresponding value for a density of 50 gram/meter3 is 0.107±0.01 cm-1. The cross section for a water-water collision must be nearly 5 times that for a water-air collision to account for this change in half-width with vapor density. The attenuation at the peak is 0.025 db per kilometer for 1 gram of water vapor per cubic meter.
