446 



Table 157 

 ABSORPTION OF RADIATION BY SEA WATER 



Utterback 1 has made observations of the extinction coefficient of typical oceanic waters, 

 defined in the same manner as the absorption coefficient k of Table 142. Sverdrup, John- 

 son, and Fleming 2 have summarized Utterback's observations as follows : 



He has made numerous observations in the shallow waters near islands in the inner 

 part of Juan de Fuca Strait and at four stations in the open oceanic waters off the coast 

 of Washington, and these can be considered typical of coastal and oceanic water, respec- 

 tively. Table 157 contains the absorption coefficients of pure water at the wave lengths 

 used by Utterback, the minimum, average, and maximum average, and maximum coeffi- 

 cients observed in oceanic water, and the minimum, average, and maximum coefficients 

 observed in coastal water. The minimum and maximum coefficients have all been com- 

 puted from the four lowest and the four highest values in each group. 



Type of water 



Pure water (from Table 156) 



{lowest 

 average 

 highest 



Wave length * — /i 



0.46 0.48 0.515 0.53 0.565 0.60 0.66 



cm.- 1 cm. -1 cm. -1 cm. -1 cm.- 1 cm. -1 cm. -1 



.00015 .00015 .00018 .00021 .00033 .00125 .00280 



.00038 .00026 .00035 .00038 .00074 .00199 



.00086 .00076 .00078 .00084 .00108 .00272 



.00160 .00154 .00143 .00140 .00167 .00333 



flowest .00224 .00230 .00192 .00169 .00375 .00477 



Coastal water^ average .00362 .00334 .00276 .00269 .00437 .00623 



[highest .00510 .00454 .00398 .00348 .00489 .00760 



* It should be understood that the wave length actually stands for a spectral band of finite width. 



1 Utterback, C. L., Cons. Perm. Intern. l'Explor. de la Mer, Rapp. et Proc.-Verb., vol. 101, pt. 2, 

 No. 4, 15 pp., 1936. 



2 Sverdrup, H. U., Johnson, M. W., and Fleming, R. H., The oceans, p. 84, copyright 1942 by 

 Prentice-Hall, Inc., New York. 



Table 158 



SCATTERING AREA COEFFICIENTS FOR WATER DROPS IN AIR 



Table 158 gives values of the scattering area coefficient A', as a function of the parameter 

 a = 2irr/X, where r is the radius of the scattering particle (sphere) and X the wave length 

 of the incident light in the medium surrounding the sphere. All data and the following 

 discussion are from the work of Houghton and Chalker. 1 



The scattering area coefficient K, is a measure of the total light scattered by the sphere 

 regardless of the state of polarization of the incident light. (The angular distribution of 

 the scattered light, which is not considered here, is dependent on the polarization of the 

 incident light.) The total light scattered by a number of spheres is equal to the sum of 

 the portions scattered by the individual spheres so long as the spheres are far enough apart 

 to insure that the scattering is incoherent. This requires a mean narticle spacing large 

 compared to the wave length. This condition is met by all natural aerosols and by all 

 stable aerosols. The scattering area coefficient is most useful in determining the trans- 

 mission of a parallel beam of light through an aerosol or other colloid. The values com- 

 puted here are for nonabsorbing spheres of index of refraction 4/3 as compared to the 

 index of refraction of the medium. These conditions were selected specifically for the case 

 of water drops in air and for the spectral interval within which the index of water is 

 substantially 4/3 and the absorption is negligible (roughly from the near ultraviolet to 

 the very near infrared). 



The transmission of a parallel beam through such an aerosol is given by: 



Tranmission = E/E = e~ z ™ r2K , z 



(1) 



where E is the flux density of the incident parallel beam, E is the flux density of the 

 parallel beam after passing a distance Z through the aerosol, n is the number of spheres 

 of radius r per unit volume of the medium and K, is the appropriate scattering area co- 

 efficient. The summation is taken over all sizes of spheres present. Equation (1) holds 

 for a single wave length. If the incident beam is not monochromatic, equation (1) must 

 be integrated over the spectral interval involved. 



The computations for Table 158 are based on the equations developed by Mie 2 from 

 Maxwell's electromagnetic theory ; for a concise theoretical treatment see Stratton. 3 The 



1 Houghton, H. G., and Chalker, W. R., Journ. Opt. Soc. Amer., vol. 39, p. 955, 1949. 



2 Mie, G., Ann. Phys., vol. 25, p. 277, 1908. 



3 Stratton, J. A., Electromagnetic theory, p. 563, McGraw-Hill Book Co., N. Y., 1941. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



