6 RADIO REFRACTIVE INDEX OF AIR 



The statistical mean of the dielectric constant is then converted to 

 refractive index. The constant Ki is evaluated from 



N = Kv 



T ' 



(1.11) 



which stems from (1.10) when e = 0. Setting N = 288.04, Pa = 1013.25 

 mbar, T = 273 °K and solving for K^: then^ 



Ki = 77.607 ± 0.13 



'K 



mbar ' 



(1.12) 



The constants K2 and K^ have been evaluated from a survey of deter- 

 minations of the dielectric constant of water vapor in the microwave 

 region by Birnbaum and Chatterjee [7]. These values, determined from 

 the data of table 1.2, are 



and 



K2 = 71.6 ± 8.5 °K/mbar, 

 K, = (3.747 ± 0.031) 10'(°K)Vmbar 



(1.13) 

 (1.14) 



and were obtained as the weighted means of the various determinations, 

 the weights being taken inversely proportional to the square of the prob- 

 able errors. Although Birnbaum and Chatterjee considered only those 

 data that would allow a determination of Ko and Kz over a wide range 

 of temperature, they conclude their results are in satisfactory agreement 

 with those of Stranathan [8] at 1 Mc/s, Phillips [9] at 3000 Mc/s, Grain 

 [10] at 9000 Mc/s, and Essen and Froome [11] at 24,000 Mc/s. 



Table 1.2. Values of the constants K2 and Kz used by Birnbaum and Chatterjee 



* The development here follows that of Smith and Weintraub. Grain's [10] de- 

 termination of the dielectric constant of dry air (e = 1.000572) indicates that Ki may 

 be somewhat lower than the "best value" of Smith and Weintraub, since his value, 

 Ki = 77.10, is about 40 standard deviations lower than that of Smith and Weintraub. 



