94 



METEOROLOGY— THEORY 



orological soundings of the Meteor expedition/^ taken 

 during 1925 to 1927 over the Atkintic Ocean were 

 utilized in analyzing elevated ducts. Climatological 

 data of the Atlantic and Pacific Oceans were employed 

 in study of low-level duets. A qualitative analysis of 

 95 soundings of the Meteor expedition had previously 

 been made in reference 23. 



*•'•* Elevated Ducts 



Tr.ADE WlXD AXD DoLDIiUilS ArEAS 



A semipermanent high-pressure system is located 

 over the oceans at about 30 degrees of latitude. The 

 northeast trade winds blow from 30°N to about 5°N. 

 Between the equator and 30°S the southeast trade 

 winds prevail. The doldrums, a region of light winds 

 and heavy rainfall, appears between the two wind 

 systems. 



Ducts in the Teade Wind Region 



In the trade winds a warm, dry subsiding air mass 

 exists over a cool, moist ground layer. The transition 

 zone between the two air masses is characterized by 

 a temperature inversion (increase with height) and 

 a sharp decrease of the water vapor content of the 

 air. It is this transition layer which coincides with 

 the duct, which in this paper is defined as a layer in 

 which the curvature of the path of high-frequency 

 electromagnetic waves exceeds the curvature of the 

 earth. Within these ducts these waves may be trapped, 

 and abnormally long ranges may occur. 



As the trade winds blow toward the equator over 

 warmer ocean areas the heating from below causes 

 the duct to rise and to become weaker until finally 

 near the equator the duct disappears and the two 

 air masses become thoroughly mixed. 



Height of the Duct Base 



The base of the inversion (or duct) increases in 

 elevation equatorward. According to the Meteor 

 soundings, taken during March and April, the aver- 

 age elevation of the base of the inversions rose from 

 700 m at latitudes 15°N to 20°ISr, to 1,020 m at 10°N 

 to 15°N, and to more than 2,000 m at latitudes 5°N 

 to 10°]Sr. Between the equator and 5°N, no ducts 

 existed to an elevation of 2,500 m. 



The elevation of the base of the inversion also in- 

 creases westward into the Atlantic from the African 

 Coast. At latitudes 15°]Sr to 20°N, its elevation in- 

 creases from less than 300 m off the African Coast 

 to 1,500 m in mid-Atlantic. 



Figure 13. Height of the temperature inversion base 

 over the Atlantic. (After von Ficker.) 



Figure 13-* depicts the height of the temperature 

 inversion base in the Atlantic, based largely on the 

 data from the Meteor expedition. South of the equa- 

 tor, soundings were made during the winter season 

 (June to August), while north of the equator the 

 soundings were made chiefly in the spring (March 

 to May). The height of the base of the inversion has 

 a seasonal variation, being greater in winter than in 

 summer. 



Figure li represents typical M curves computed 

 from the soundings of the Meteor expedition. Curves 

 A (sounding 182 of the Meteor expedition taken just 

 off the African Coast) show a ground-based duct of 

 elevation 140 m on the ascent curve and 90 m on the 

 descent curve. Oceanward, the duct becomes ground- 

 based, S-shaped, as is shown by curves B (sounding 

 183). 



uj ** 



? IP 



310 330 350 310 330 350 370 430 450 470 



FinunE 14. M curves, .l/r/co/- exjipdition, Marcli 1927. 



