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N CLIMATOLOGY 



Figure 4.L Mean Ns, August 0200 local time. 



where z is the akitude, po the average sea level density of moist air and 

 H the average scale height between zero and z. It is useful to introduce 

 the concept of an effective scale height, H*, for the average variation of 

 refractive index in the atmosphere. Many studies have shown that the 

 average refractive index variation with height is quite well represented, 

 to a first approximation, by a formula similar to (4.3) [5, 6, 7]. It is 

 possible to calculate a theoretical value of this effective scale height using 

 a distribution of water vapor. This is, however, quite a complex pro- 

 cedure. Furthermore, 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 NACA standard 

 atmosphere [8] and recent climatological studies of atmospheric refractive 

 index structure [9]. 



It is seen from table 1.8, chapter 1, that H* varies from 6.56 to 7.63 

 km in the NACA standard atmosphere, depending on the value of rela- 

 tive humidity assumed. The value of H* = 6.95 km obtained from 

 chmatological studies of (n - 1) variations over the first kilometer above 



