CONSTANTS IN EQUATION FOR A^ 

 The full equation for the refractive index is now 



^ + 72 I + 3.75 X 10^ jr. 



N = 11. Q ^ + 72 ^ + 3.75 X 10' 7^ , (1.15) 



after reducing the values to three figures where significant. These are 

 the constants recommended by Smith and Weintraub to yield an overall 

 accuracy of ± 0.5 percent in A^.^ Equation (1.15) may be simplified by 

 assuming Pt = P = Pd + e and obtaining 



N = 11.6 I - 5.6 1^ + 3.75 X lO' |^ . (1.16) 



For practical work in radio-meteorological studies, (1.16) may be sim- 

 plified to the two-term expression 



I + 3.73 X 10' f. 



N = 77.6 ~ + 3.73 X lO' ^ , (1.17) 



which yields values of A^^ within 0.02 percent of those obtained by (1.16) 

 for the temperature range of —50 to 40 °C; i.e., a maximum error of 0.1 

 A'' unit, and a standard error of 0.5 percent of about 4.5 A'^ units. 



This is accomplished by dividing the second and third terms of (1.16) 

 by e/T and solving for the composite constant, K^, in relation 



2:^^-5.6 = 1, (1.18) 



which, for T = 273 °K, results in: 



^5 = 3.73 X 10'(°KVmbar). (1.19) 



Equation (1.17) is commonly Avritten as 



iV = ^(p + ^). (1.20) 



Although this last equation is in widespread use by radio scientists 

 throughout the world, it is by no means adopted on an international scale. 



^ The values of K2 and Kz given in (1.13) and (1.14) are those of Birnbaum and 

 Chatterjee as reported by Smith and Weintraub. A consideration of the ratio of 

 Ko/Ki shows the Smith-Weintraub value to be about 0.2 percent greater than that 

 obtained from the original data, indicating an arithmetic conversion error. This 

 error is, however, totally absorbed in the rounding to three significant figures in 

 (1.15). 



