72 A WORLD ATLAS OF ATMOSPHERIC RADIO REFRACTIVITY 



30 60 90 120 



150 180 



180 150 



120 90 60 30 



60 90 



Figure C-17. Percent of time gradient < — 100 (N/fcm): August. 



FIGURE C-18. Percent of superrefractive layers thicker than 100 m: August. 



