Infrared absorption by the water vapor continuum near 1200 cm-1 has been measured with a lead-tin-telluride diode laser over a 40.5-m optical path. The measurements were made as a function of temperature from 333 K to 473 K; thus, they overlap and extend previous measurements made at temperatures between 293 K and 388 K. Over the entire temperature range studied here, the continuum extinction coefficient increases quadratically with water-vapor partial pressure as expected for the relatively high partial pressures used in these measurements. At temperatures below 398 K. our measured extinction coefficients agree well with previously reported data. At higher temperatures, however, the extinction coefficient is almost independent of temperature and is substantially larger than predicted by empirical formulas. Values of the self-broadening coefficient for water vapor have been extracted from the experimental data, and a possible interpretation of the results involving both dimer and line-broadening effects is presented.
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We have carried out a detailed analysis of several long pathlength transmission measurements in the 8–12-µm atmospheric window in order to determine the extinction coefficient due to the water vapor continuum. Our results indicate that three modifications to the current LOWTRAN atmospheric transmission model are required. The first two corrections involve an improved fit to the pure water vapor continuum absorption together with the elimination of the atmospheric broadened continuum term. Finally, and most critically, a strong measured temperature dependence must be included in the water vapor continuum absorption coefficient. For pathlengths ranging from 10 km to 50 km, failure to incorporate these corrections can lead to errors in the computed transmission ranging from factors of 2 to more than 10,000.
