16 RADIO REFRACTIVE INDEX OF AIR 



particularly true of climatological data such as 5-year averages for given 

 months. In the United States the expression [26] 



-AN = 7.32 exp (0.005577iV.), (1.36) 



TlnAN . T's = 0.93, 



where r denotes the standard correlation coefficient, has been found, while 

 in Germany [27] the expression 



-AN = 9.30 exp (0.004:5Q5Ns), (1.37) 



^InAiV • Ns — 0.6, 



has been reported. A recent article by Lane [28] indicates that a similar 

 correlation, rin^" 'Ns= 0.85, is found over the British Isles. Although 

 the slopes and intercepts of this relation between AA'^ and Ng obviously 

 vary with climate, one concludes, for the purposes of the present discus- 

 sion, that the observational evidence corroborates the assumption of an 

 average exponential decrease of A^ with height. The general application 

 of the exponential model nmst await considerable work upon the part of 

 the radio climatologists. 



This has naturally led to the definition of a new unit that adds a correc- 

 tion factor to account for this exponential decrease [29, 30]. This new 

 unit. A, is given by 



A(h) = Nih) + A^.[l - exp {-h/H)] (1.38) 



It is i)ossible to calculate a theoretical value of the scale of N by 

 as.suming a distribution of water vapor. This is, however, quite a 

 complex procedure. Futhermore, the value obtained depends upon the 

 model of the water vapor distribution, and no definite conclusion can be 

 justified considering the extreme variation of water vapor concentration 

 with season, geographic location, and height above the earth's surface. A 

 convenient and simple alternative is to adopt a value for H from the 

 average (n — 1) variation with height in the free atmosphere. Several 

 such values of H were determined by reference to the National Advisory 

 Committee for Aeronautics (NACA) standard atmosphere and recent 

 climatological studies of atmospheric refractive index structure [26]. It 

 is seen in table 1.8 that H varies from 6.56 to 7.63 km in the NACA 

 standard atmosphere, depending on the value of the relative humidity 

 assumed. The value of // = 7.01 km for SO jiercent relative humidity is 



