20 



RADIO REFRACTIVE INDEX OF AIR 



characteristic diagram of the meteorologist; even for that process one 

 must first calculate the potential values of both temperature and water 

 vapor pressure. The relative merits of the various units are summarized 

 in table 1.9. 



1.6. Conclusions 



The above discussion has emphasized the following points: 



(a) The differences in constants in the expression for A'' are small com- 

 pared with the error inherent in the formula. 



(b) The refractive index is effectively nondispersive for frequencies 

 below, say, 50 Gc/s. 



(c) The choice of atmospheric model for A'^, and concomitant units, 

 depends upon the application at hand. 



(d) The atmosphere, on the average, yields an exponential d stribution 

 of A^" wath height. 



(e) The use of an exponential model facilitates the preparation of 

 climatic maps of A'' and makes clear the effect of non-normal A^ structure 

 upon the bending of radio rays. 



Table 1.9. Cotnparison of various units iised in radio meteorology 



Unit 



B(ft) = .V(/i) + (/i/4a)10i', 



M(h)=N(h) + (h/a)W, 



N(,{h) = N(h) c\p(h/-.0). 



.■nh)=N(h) 

 +313|l-exp(-/!/7.0)], 



.(A)^^X 



6/,ooo+^-^» 



Oradient 



dBjh) _dN{h) 

 dh " dh 



+ (l/4a)106, 



dM(h)_dNih) 

 dh " dh 



+ (l/a)10«, 



dA'o(ft) dN(h) ( h 



dh 





dA(h) _ dN(h) 313 

 dh ~ dh ^ 1 

 exp(-ft/7.0), 



d0(fe) _ d<i>(h) de 

 dh de dh 



d<j>{h) deo. 



+ 



deo dh 



Referred 

 to: 



Surface. 



Surface. 



Sea-level. 



Surface. 



1000 mbar 



Advantages 



Vertical distribution 

 identifies "4/3 earth" 

 conditions; absolute 

 value and gradient of n 

 easily recovered. 



Trapping or ducting 

 layers identified by 

 {dM/dh)^0, absolute 

 value and gradient of 

 n easily recovered. 



Removes effects of both 

 height and station ele- 

 vation; facilitates map- 

 ping since N{h) is 

 easily recovered. 



Removes effects of height 

 n and gradient easily 

 recovered. 



Removes the effects of 

 height and station ele- 

 vation, may be de- 

 rived from character- 

 istic meteorological 

 diagram. 



Disadvantages 



Over-corrects 

 N(h ) above 

 the first few 

 kilometers. 



Grossly over- 

 corrects N{h) 

 above the 

 trapping 

 layer. 



Gradient on n 

 not easily re- 

 covered. 



Does not re- 

 move the ef- 

 fect of station 

 elevation. 



Neither n or 

 dn/dh may 

 be recovered 

 from </){ft); 

 must calcu- 

 late both 

 from basic p, 

 T, and e data 



