312 REFRACTION AND REFRACTIVE INDEX MODELS 



and H^, are the scale heights of D and W, respectively. This particular 

 form has been found useful by Katz [6], in his derivation of the potential 

 refractive modulus, and by Zhevankin and Troitskii [7] in their treatment 

 of atmospheric absorption. It would be well for the reader to recall 

 that scale height, as used in this study, is merely the height at which the 

 value of the atmospheric property has decreased to 1/e of its surface value. 

 Typical values of Do, Wq, and A^o are listed for arctic, temperate, and 

 tropical locations in table 8.1. It is seen that the contribution of W to 

 the total value of A^ is nearly negligible in the arctic but becomes greater 

 as one passes from temperate to tropical climates. There is, of course, 

 generally an inverse correlation between the magnitude of D and W since, 

 at sea level, where P '^ 1,000 mbar, the low arctic temperature increases 

 the D term and, combined with low atmospheric water vapor capacity, 

 decreases the wet term. Conversely, the higher temi^eratures of the 

 temperate and tropical climates depress the D term and provide a greater 

 water vapor capacity with the result that W may have a sizable contribu- 

 tion to the total A^. 



Table 8.1. Typical average values of the dry and wet components of N 



8.1.2. N Structure in the I.C.A.O. Atmosphere 



One may examine A^ structure in a standard atmosphere as a guide to 

 its general distribution in the free atmosphere. On this basis the I.C.A.O. 

 standard atmosphere [8] (fig. 8. 1) was examined. The conditions specified 

 for this atmosphere are an approximately exponential pressure decrease 

 with respect to height and a linear temperature decrease from ground level 

 to the tropopause. It is evident, then, that in this atmosphere D de- 

 creases in an exponential fashion with height. 



When these data are converted into refractive index and plotted on 

 semilogarithmic paper, as on figure 8.2. both D and W are seen to display 

 an approximately exi^onential distribution from the surface to the tropo- 

 pause. This conclusion is based upon the observation that the distribu- 

 tion is nearly linear as one would expect if one inverted the function 



y = A exp i-h/c) (8.4) 



