PRESENTATION OF N DATA 



17 



in close agreement with H = 6.95 km obtained from climatological 

 studies of (n — 1). 



Table 1.8. Determination of the effective scale height, H, for the radio refractive index 



*The value of H = 7. km was arbitrarily adopted for use in the present discus- 

 sion. Note that this value is smaller than the usual scale height near sea level of 

 8. km for the density distribution of the air. Although the value H is quite arbi- 

 trary, it is evident from table 1. 8 that H should lie between 6. 5 and 7. 5 km. 



A comparison of B, M, N, and .4 units is shown on figure 1.1 for arctic, 

 temperate, and tropical climates. It is quite evident that both M and B 

 over-correct the profile to produce increasing values with height while 

 the A unit tends to yield a constant value at heights in excess of 2 km. 

 It is noted, however, that important departures of the Nik) profile from 

 normal within the first few kilometers are emphasized by both the A and 

 B profiles. 



In radio meteorology, as in other branches of meteorology, it is often 

 convenient to express the potential value of the refractive index referred 

 to some standard pressure level. In practice this is found by adding to 

 the values at any height the product of the iV lapse rate and the height. 

 This has already been done in the case of the B unit, while the A unit 

 simply adds the total decrease of N in an exponential atmosphere from 

 the surface to the height under consideration. Yet another approach is 

 to replace the values of pressure, temperature, and vapor pressure in the 

 expression for A^ with their values at some desired pressure level. This 

 unit, called the potential refractive modulus, (/>, is then defined by 



77.(5 



Po + 4810 -} 



(1.39) 



where d is the potential temperature and eo the potential partial pressure 

 of water vapor, both referred to the reference pressure Po. This formula 

 is that of Katz [31] with the Smith-Weintraub (constants. 

 The potential temperature is defined as 



d = T{Po/p)' 



(1.40) 



