GROUND-BASED ATMOSPHERIC DUCTS 



145 



With these reservations in mind, the present study has shown: 



(a) Ducts occur no more than 15 percent of the time. 



(b) The annual cycle of the incidence of ducts is reversed for the arctic 

 and tropical stations studied. The arctic station has a wintertime maxi- 

 mum and the tropic station a summertime maximum. The temperate 

 station has a summertime maximum incidence of less than 5 percent. 



(c) The maximum initial elevation angle is limited to 5.8 mrad with 

 a mean value of about 3 mrad. 



(d) The steepest gradient of A^ observed is —420 A'^ units per kilometer. 



(e) The maximum thickness of observed ducts is such as to trap radio 

 waves of 1000 Mc/s and above at all locations for at least 5 percent of the 

 observed ducts. 



(f) Ducts in the arctic appear to be associated with temperature in- 

 versions at ground temperatures of —25 °C or less; temperate zone, with 

 the common radiation inversion and accompanying humidity lapse; 

 tropics, with a moderate temperature and humidity lapse for temperatures 

 of about 30 °C. 



450 



400 



350 



300 



250 



Q 200 

 E 



•i 150 



100 



50 



0.1 0.2 0.5 I 2 5 10 20 30 40 50 60 70 80 90 95 98 99 995 99S 999 

 Percentage of Observations That Equal or Exceed the Ordinate Value 

 Figure 4.40. Maximum ground-based duct thickness. 



