140 N CLIMATOLOGY 



4.4.5. Description of Observed Ground- Based 

 Atmospheric Ducts 



Approximately three years of radiosonde data typical of an arctic 

 climate (Fairbanks, Alaska), a temperate climate (Washington, B.C.), 

 and a tropical maritime climate (Swan Island, W.I.) were examined by 

 means of a digital computer for the occurrence of ducts during the months 

 of February, May, August, and November. The percentage occurrence 

 of ducts is shown on figure 4.36. The maximum occurrences of 13.8 

 percent for August at Swan Island and 9.2 percent for Fairbanks in 

 February are significantly greater than the values observed at other times 

 of the year. The Washington data display a summertime maximum of 

 4.6 percent. These data indicate that the temperate zone maximum 

 incidence is about one-half the wintertime maximum incidence in the 

 arctic, and about one-third of the summertime tropical maximum. 



The range of observed values of 0p is shown in figure 4.37. The mean 

 value calculated for each month as well as the maximum and minimum 

 values of dp observed for the limiting cases are given for each month and 

 location. The mean value of the angle of penetration under these condi- 

 tions is between 2 and 3 mrad and appears to be independent of climate. 

 The maximum value of dp observed during ducting is 5.8 mrad. 



The refractivity gradients observed during ducting are given on figure 

 4.38. The maximum gradient of 420 N units per kilometer was observed 

 during February at Fairbanks, Alaska. The mean values of N gradient 

 appear to follow a slight climatic trend from a high value of 230 N units 

 per kilometer at Fairbanks to a value of 190 A^ units per kilometer at 

 Swan Island. 



Another property of radio ducts is their thickness, which is given in 

 figure 4.39. Again there is observed a slight climatic trend as the median 

 thickness increases from 66 m at Fairbanks to 106 m at Swan Island. 

 These values of thickness correspond to the gradients given in figure 4.38. 

 One can then obtain, by linear extrapolation, the thickness at which the 

 gradient is equal to — 1/a; i.e., the height corresponding to the gradient 

 just sufficient to trap the ray at do = 0. These values, shown on figure 

 4.40, display an increase in the median thickness of about 25 percent for 

 Swan Island, 100 percent for Washington, and 200 percent for Fairbanks, 

 which results in a reversal of the climatic trend of the observed thickness 

 between Fairbanks and Swan Island. This increase in height emphasized 

 the preceding conclusion. Fairbanks is characterized by shallow layers 

 with relatively intense gradients. 



These maximum duct widths may be used to estimate the maximum 

 radio wavelengths trapped. Kerr [34] gives the maximum wavelength, 

 X, trapped by given thickness, d: 



X„.ax = cy'" d''\ (4.24) 



