144 



N CLIMATOLOGY 



o 



300 



280 



260 



240 



220 



200 



180 



160 



140 



120 



100 



80 



60 



40 



20 







0! 0,2 0,5 I 2 5 10 20 30 40 50 60 70 80 90 95 98 99 995 99S999 



Percentage of Observations That Equal or Exceed ttie Ordinate Value 

 Figure 4.39. Observed ground-based duct thickness. 



The reader is cautioned that an atmospheric duct does not have the 

 sharp boundaries of a metallic waveguide. Thus the maximum wave- 

 lengths obtained by (4.24) do not represent cutoff frequencies but, as 

 Kerr is so careful to emphasize, merely suggest lower limits under the 

 assumptions of this rudimentary theory. 



The results given above were derived from a consideration of radio- 

 sonde data. Although the radiosonde is not an extremely sensitive in- 

 strument, it is readily available and is the only source of climatic informa- 

 tion involving the temperature and humidity structure of the atmosphere. 

 It is believed that the radiosonde data will at least yield the climatic trend 

 of radio ducts as well as their probable temperature and humidity distri- 

 butions. Further, it is evident that the choice of stations will definitely 

 affect the percentage of ducts observed. For example, it is almost certain 

 that a greater percentage of ducts would be observed over water in the 

 subtropics than over Swan Island. 



