6 A WORLD ATLAS OF ATMOSPHERIC RADIO REFRACTIVITY 



very dependent upon temperatures because of 

 the larger water content possible at high tem- 

 peratures. There are two exceptions : 



(a) Large interior deserts, where moun- 

 tains block the normal moisture flow, tend to 

 have low wet-term values relative to their tem- 

 peratures. 



(b) In August, when the monsoonal mois- 

 ture is trapped south of the Himalayas, India 

 shows unusually high wet-term values. 



(6) The intersection height, z t , is closely re- 

 lated to the height of the tropopause, but in 

 areas where an isothermal layer precedes the 

 stratospheric increase of temperature, the D 2 



2 t 6 B 10 \2 14 16 IB 20 22 2A 26 28 30 



Z - km 



FIGURE 2. Three-part exponential fit to mean N-profile: Dakar. 



curve may intersect the Z), curve as much as 2 

 km below the tropopause heights given in fig- 

 ures D-l through D-4 (see appendix D) . 



3.3. Reliability of Contours of N(z) Maps 



Although radiosonde stations are the only 

 worldwide source of upper - air meteorological 

 data, many areas of the world had few, if any, 

 radiosonde reporting stations before 1957. As 

 a result of the IGY, many new stations were 

 established, especially in the lower latitudes ; 

 however, radiosonde data are still not available 



for a number of large areas, such as Brazil, 

 China, and the Indian Ocean. High latitudes 

 also show a noticeable sparsity of upper-air 

 data; fortunately, there is a fairly small and 

 uniform transition in most parameters at these 

 far-south and far-north latitudes. Even in the 

 U.S.A., where the first radiosonde network was 

 established in 1938, radiosonde stations are still 

 several hundred miles apart. 



The maps were hand-contoured by interpola- 

 tion between the widely-spaced plotted data 

 points, using a technique similar to that used 

 in the analysis of synoptic weather maps. Each 



