Table 138 423 



ABSORPTION OF RADIATION BY WATER VAPOR, 10-25,* 



There is an unfortunate lack of suitable laboratory data on the absorption spectrum of 

 water vapor. For many years the work of Hettner * on the infrared absorption spectrum 

 of steam at atmospheric pressure had been widely used in meteorology. In 1932 Weber 

 and Randall 2 redetermined the percentage absorption (not absorption coefficients) for 

 steam (10-16/u) and for saturated water vapor at "room temperature" (16— 25/a). The 

 latter authors found smaller absorptions and a more complex spectrum than did Hettner, 

 though the data are not strictly comparable, owing to different laboratory techniques and 

 to the higher resolutions used by Weber and Randall. However, the primary purpose of 

 their research was to determine the positions and the relative intensities of the various 

 absorption lines, and the data given are insufficient for the accurate determination of the 

 absorption coefficients. The element in doubt is the path length of water vapor employed, 

 which depends on the "room temperature" appropriate to the tabulated absorptions and 

 on the length of the absorption cell used, although it seems probable that the latter was 

 3 meters. 



The confusion arising from the failure to specify the temperature is apparent in the 

 literature. Ramanathan and Ramdas 3 computed the absorption coefficients assuming the 

 room temperature to be 26.3 C C, a value obtained from the record of another experiment 

 described in Weber and Randall's article. Also, Wexler 4 points out that owing to a 

 typographical error in the original article, 2 the absorption lines from 21.39/w. to 22.55^ 

 were ascribed to steam rather than to saturated vapor at room temperature ; therefore 

 the absorption coefficients computed by Ramanathan and Ramdas are much too low in 

 this region. Wexler, 4 through correspondence with Professor Randall, obtained the value 

 22.5 °C. for the room temperature and attempted to correct the Ramanathan and Ramdas 

 data accordingly; but Wexler assumed that they had used 30 C C. in their computations 

 rather than 26.3 °C. Although this was pointed out in a later article, 6 the earlier data 

 presented by Wexler have found their way into meteorological literature,* as have the 

 data of Ramanathan and Ramdas.' 



Elsasser 8 discusses the concept of the absorption coefficient in the case of overlapping 

 absorption lines, as occurs in the water vapor spectrum. 



The Weber and Randall data are given below. 



Note. — There has been considerable recent research on the problem of water-vapor absorption, but the 

 results were not available in time to be included here. See, for example: Chapman, R. M., Howard, 

 J. N., and Miller, V. A., Atmospheric transmission of infrared, Summary report. Ohio State Univ. 

 Res. Found., Columbus, June 30, 1949. Drummeter, L. F., and Strong, J., Infrared absorption of water 

 vapor at 1.8 microns. Johns Hopkins Univ. Rep., Baltimore, 1949. 



1 Hettner, G., Ann. d. Physik, vol. 55, p. 476, 1917. 



2 Weber, L. R., and Randall, H. M., Phys. Rev., vol. 40, p. 835, 1932. 

 'Ramanathan, K. R., and Ramdas, L. A., Proc. Ind. Acad. Sci., vol. 1A, p. 822, 1935. 

 * Wexler, H., Month. Weath. Rev., vol. 64, p. 122, 1936. 



6 Wexler, H., Month. Weath. Rev., vol. 65, p. 102, 1937. 



6 See Schnaidt, F., Gerl. Beitr. Geophys., vol. 56, p. 230, 1939. 



7 See Brunt, D., Physical and dynamical meteorology, p. 117, Cambridge, f941. 



8 Elsasser, W. M., Harvard Meteorological Studies No. 6,*p. 35, Milton, 1940. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



