96 



METEOROLOGY— THEORY 



based ducts or S-sliaped grouud-based ducts. Tbese 

 ground-based ducts are also a common occurrence on 

 the west coast of the United States. At San Diego the 

 average height of the inversion base is near 1,000 ft 

 during the summer.^* The height of tlie duct base has 

 a diurnal variation, being maximum in the morning 

 at about OSOO local time and minimum at about 1600. 

 The diurnal variations are governed by land and sea 

 breeze phenomena. 



6.9.3 Experimental Evidence 



During 1914, aircraft traffic to and from Ascension 

 Island at 8°S in the Atlantic was tracked by two 105- 

 mc radars sited 3,500 and 1,700 ft above mean sea 

 level. The following observed phenomena were re- 

 ported verbally. 



1. Eanges were greater during the dry season than 

 during the wet season. 



2. Ranges westward (370°) were greater than to 



the northeast (40°), and greater northeastward than 

 to the north (10°). 



The decrease in duct intensity equatorward de- 

 scribed herein is commensurate with observation (3), 

 in which ranges are reported to be less toward the 

 equator than along a parallel of latitude. 



*■''■ ^ Conclusions 



1. Over the trade wind areas of the oceans both 

 elevated and surface ducts often exist. 



2. The elevated duet is of maximum intensity and 

 frequency at 15° to 30° of latitude. It decreases in 

 intensity and frequency equatorward, disappearing 

 in the doldrums. 



3. The surface duct, dependent largely on wind 

 speed, is of maximum depth at about latitude 80° in 

 summer, 15° in spring and fall, and 10° in winter. 



4. Near the western coasts of continents the ele- 

 vated duct lowei's into an intense ground-based duct. 



