4 RADIO REFRACTIVE INDEX OF AIR 



be assumed to be approximately unity for air. Since 



n = \/l + (/xe - 1) , 

 one may employ the approximation 



and obtain the familiar form of practical application^ 



N = (n - 1) 10*' = K, ^ + K, -- + K,jr^-\-K,^ (1.10) 

 where A'l, A'o, A'3, and A'4 are constants. 



1.3. Constants in the Equation for N 



A survey of the various deterniinations of Ai, A2, and A3 was recently 

 made by Smith and Weintraub [4] to arrive at a set of "best values" to 

 represent the mean of previous independent determinations. In radio 

 work one is interested in propagation through the troposphere, therefore, 

 the composition of air should be taken to include an average amount of 

 carbon dioxide. However, laboratory measurements usually are made 

 on C02-free air due to variable concentrations of CO2 in the laboratory. 

 Hence Smith and Weintraub have adjusted the the values of e — 1 

 originally published for C02-free air by raising them to 0.02 percent to 

 correspond to a 0.03 percent CO2 content. It should be noted that this 

 correction of 0.02 percent is essentially of academic interest, since as 

 shall be seen, the final equation for A'^ will be considered accurate to 0.5 

 percent. The value of 0.02 percent was obtained by noting that the 

 value of dielectric constant for C02-free air, e', in the expression 



e - 1 = — 2^ 



could be utilized for applications in the real atmosphere by expressing 

 total atmospheric pressure, P t, as 



Pt = Pd-{- Pc 



5 Henceforth, A^ will be used to denote {n — 1)10^ and will not be used again for 

 Avogadro's number. This slight inconsistency was adopted to maintain notational 

 agreement with both Debye's early work and the later work in radio meteorology. 



