108 N CLIMATOLOGY 



At the time the present study was initiated it was felt that Ns should 

 be reduced to sea level by at least the dry term correction factor as in 

 (4.6). The absence of published work on models of A^ structure in the 

 free atmosphere encouraged the decision to rest on prudence and adopt 

 this dry term correction factor. Since that time several effective ex- 

 ponential models of the free atmosphere have been demonstrated [17, 9]. 

 In future work a smaller value of H* on the order of 7.0, which corresponds 

 to the N decay in the free atmosphere, will be adopted. The adoption 

 of any value of H* between 6.5 and 7.5 km would have reduced the range 

 of A^'o values on figure 4.4 by no more than 2 A'' units. Since this reduction 

 in range is more than an order of magnitude less than the reduction of 

 (4.6) used to obtain No, it appears that the basic advantage of the method 

 has been realized. 



A map of A^'o, such as that of figure 4.4, which represents a large con- 

 tinental area, may easily be compared and merged with maps for ocean 

 areas. This would be more difficult with N s, since, for example, the 

 strong gradient over California mainly represents the rapid altitude 

 variations of the Sierra Nevada Mountains. It has also been demon- 

 strated that A^'o is a better indicator of tropospheric storms and air 

 masses than N s when considered on a synoptic or "daily weather map" 

 basis [19]. 



Also (4.6) aids in comparing air properties as a function of altitude at 

 the same position. The variations of No will represent the local depar- 

 tures between this quantity and the value in a standard atmosphere and 

 will show the perturbations in the structure of the atmosphere produced 

 by fronts, air masses, and other synoptic features. Although any value of 

 H* between 6.5 and 7.5 km will remove the gross altitude dependence of 

 the refractive index, the choice of value of H* within this range could de- 

 pend on the application. The synoptic application, which is discussed in 

 detail in chapter 5, would be best served by a scale height near 8.0 km, 

 whereas the objectives of the climatic chart usage would best be met by 

 a scale height near 6.5 km. This seeming paradox is easily understood in 

 terms of the physical interpretation of the various scale heights. For 

 example, H* = 8.0 km is typical of a perfectly dry atmosphere and its use 

 results in emphasis of humidity differences between air masses, whereas 

 H* = 6.5 km corresponds to a saturated atmosphere and minimizes 

 moisture differences. Thus it would appear that eventually one might 

 use a value of H* as indicated by the application at hand. As a practical 

 matter, however, H* = 7.0 km appears to reach a desirable compromise 

 between the objectives of the two preceding examples. 



4.2.7. Conclusions 



With the above critical appraisal in mind, the salient conclusions of the 

 present study are : 



