1. Introduction 



There has been a need in radio engineering 

 for a method whereby the radio refractivity, 

 N, at some height, z, or the gradient of N with 

 respect to height, dN/dz, could be accurately 

 estimated for any world location during any 

 season of the year. Previous studies have at- 

 tempted to solve this problem by determination 

 of the value of surface refractivity, N s , and the 

 use of an exponential decay with height [Bean 

 et al., 1960a] 1 , or by presenting seasonal means 

 and distributions of N at fixed pressure levels in 

 the atmosphere for various radiosonde stations 

 [U. S. Navy, 1955-59; Michaelis and Gossard, 

 1958]. However, these results did not provide 

 any means whereby N(z) could be obtained at 

 any arbitrary location, i.e., places for which 

 meteorological measurements were not avail- 

 able. In addition, specific information on the 

 gradient of the refractive index has not been 

 available previously on a worldwide basis, espe- 

 cially for the very important layers of the at- 

 mosphere at or near the surface where the pres- 

 ence of superref ractive or ducting gradients can 

 produce anomalous propagation of microwaves. 



This atlas presents maps, charts, and discus- 



1 Literature references on page 26. 



sions of the worldwide variations in the radio 

 refractive index. With the aid of this atlas, 

 estimates of the following parameters may be 

 readily determined for any part of the world : 

 the refractivity at any height, N(z) ; the average 

 gradient of N over the first kilometer above the 

 surface, AN ; and the gradient of N in the lowest 

 layer of the atmosphere (with emphasis on sub- 

 refractive, superrefractive, and ducting layers 

 and the probability of trapping of radio waves 

 by ducting layers). The world distribution of 

 N(z) is presented in the form of a three-part 

 exponential model, with separate exponential 

 terms for the water-vapor term, the density 

 term in the troposphere, and the density term 

 in the stratosphere; the parameters used to 

 represent this N(z) model are the reduced-to- 

 sea-level values of surface water vapor and den- 

 sity terms, the scale heights of the three ex- 

 ponential terms, and the transition height 

 between the tropospheric and stratospheric 

 density exponentials. Seasonal maps of mean 

 tropopause heights, which were obtained in the 

 course of the data reduction required for the 

 three-part exponential model, are also presented. 



