Chapter 4. N Climatology 



4.1. Introduction 



The contents of this chapter include a study of the surface variation of 

 the radio refractive index on a worldwide scale in terms of a reduced-to- 

 sea-level form of the index that gives a significantly more accurate descrip- 

 tion of refractive index variations than the nonreduced form. 



The mean vertical structure of the refractive index parameter in the 

 troposphere over central North America is presented, again in terms of 

 a reduced-to-sea-level form of A^. 



A climatological treatment of the phenomenon of the atmospheric duct, 

 or waveguide, and associated fading regions is also presented. The 

 chapter is concluded with a discussion of refraction of radio waves in 

 various air masses. It is demonstrated that refraction differences within 

 air masses arise from departures of refractive index structure from normal. 



4.2. Radio-Refractive-Index Climate Near the Ground 



4.2.1. Introduction 



The radio refractive index of air, w, is a function of atmospheric pres- 

 sure, temperature, and humidity, thus combining in one parameter three 

 of the normal meteorological elements used to specify climate. In the 

 following sections we will examine the variability of n during different 

 seasons of the year and in differing climatic regions. The systematic 

 dependence of n upon station elevation will make it necessary to consider 

 a method of expressing n in terms of an equivalent sea-level value in 

 order to see more clearly the actual climatic differences of the various 

 parts of the world. After a consideration of the n climate of the world, 

 the application of this information to such practical problems as the 

 prediction of radio field strength and the refraction of radio waves will be 

 discussed. 



4.2.2. Presentation of Basic Data 



Near the surface of the earth, for VHF and UHF frequencies, n is a 

 number of the order of 1.0003. Since, for air, n never exceeds unity by 



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