DATA AND REDUCTION TECHNIQUES 109 



(a) The radio refractive index varies in a systematic fashion with 

 climate and different climates may be identified by the range and mean 

 values of the refractive index. 



(b) It is 4 or 5 times more accurate to estimate the station value of the 

 index from charts of the reduced-to-sea-level index than from charts of 

 the station value. This improved accuracy results from using a method 

 that allows height dependence to be accurately taken into account. 



(c) Identically equipped tropospheric communications systems might 

 be expected to vary as much as 30 dB in monthly mean signal level in 

 different climatic regions, and the annual range of monthly mean field 

 strength could be as high as 20 dB in the Sudan of Africa and as low as 

 to 6 dB in the high plains of the western United States. 



4.3. On the Average Atmospheric Radio Refractive 

 Index Structure Over North America 



4.3.1. Introduction 



As has been already noted, the radio refractive index of the atmosphere 

 combines three of the meteorological elements normally used to specify the 

 state of the atmosphere on either a synoptic or a climatological basis. 

 This fact has led to its being used as a synoptic tool [19, 20, 21, 22, 23] and 

 as a measure of climatic characteristics [7, 24, 25, 26, 27]. 



The present treatment is concerned with the degree to which the average 

 A^ structure in the vertical direction reflects the gross differences in climate 

 over the North American continent. Diurnal and seasonal range graphs 

 of N at the earth's surface also shed light upon climatic characteristics. 



4.3.2. Meteorological Data and Reduction Techniques 



The basic data used in this study are the significant level data of the 

 radiosonde observations from the 18 weather stations shown on figure 4.15 

 for the 5-year period 1952-57. These observations were converted by 

 means of (1.20) to radio refractivity, N. 



The significant level data were collected for the values of N at the 

 earth's surface and within height increments centered upon 0.25, 0.5, 0.75, 

 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 km. Each value of N was referenced to the 

 center of its height increment by use of the average atmosphere, 



N{h) = Aoexp [-h/1.0\, (4.9) 



which has been shown to be a reasonable model for the decrease of the 

 refractive index with height for the United States. By use of this ex- 

 ponential model, the gross height dependence of N within each height 



