INTRODUCTION 163 



With a 200-m-thick duct : 



166 m < loss region < 18 + 200 m, or 

 166 m < loss region < 218 m. 

 For mean ducting conditions with a 100-m-thick duct: 



96 m < loss region < 103 m. 

 With a 200-m-thick duct: 



187 m < loss region < 218 m. 

 For minimum ducting conditions with a 100-m-thick duct: 



99 m < loss region < 103 m, 

 with a 200-m-thick duct : 



199.3 m < loss region < 218 m. 



4.6. Air Mass Refractive Properties 



4.6.1. Introduction 



The foregoing material on refractive index climatology concerned the 

 geographical variation of mean A'' (or A^'o) over the surface of the earth 

 and the three-dimensional distribution of mean A'^ over North America. 

 This section develops another aspect of the climatological variation of the 

 refractive index: the mean profile of A^ in various air masses. The 

 meteorologist defines air masses as bodies of air in the troposphere having 

 approximately homogeneous character at the surface. An air mass is de- 

 scribed in terms of its origin, as, for example, polar continental or tropical 

 maritime. 



Profiles of A'^ and values of bending of radio rays are given in terms of 

 departures from normal for various air masses. 



Recent studies have led to the evaluation of radar elevation angle 

 errors in different climates and air masses [43, 44]. In general, these 

 studies did not emphasize the relation between air mass refractive index 

 structure and the refraction of radio waves within the air mass. For 

 example, mean angular bendings for radio rays passing completely through 



