ANALYSIS OF DUCTS IN THE TRADE WIND REGIONS 



95 



The descent curve' shows a decrease of 59 M units 

 ill 30 ni, corresponding to a ray curvature about 20 

 times greater than that of the earth's surface. West- 

 ward into the Atlantic the inversion base rises to about 

 1,000 m. (Curve C, sounding 184.) 



Frequency of Duct Occuekence 



Within the trades proper a duct is practically cer- 

 tain to exist. In Table 14 the percentage of duct 

 occurrence, by latitude according to Meteor sound- 

 ings, is tabulated. The extreme curve (ascent or de- 

 scent) was utilized in determining the existence of a 

 duct. 



Table 14. Frequency of duct occurrence by latitude 

 over the Atlantic Ocean based on the Meteor soundings. 



It is evident from Table 14 that in the vicinity of 

 the doldrums (5°N to 5°S) the existence of ducts is 

 rare. A maximum frequency occurs between latitudes 

 15° and 25°. Note that all soundings made between 

 latitudes 15° to 20°IS[ indicated the presence of a 

 duct. 



Thickness of Ddcts 



The average thickness of the duct in the trade 

 wind inversion, according to Meteor data, is about 

 130 m. According to the theory of the dissipation 

 of the ducts near the equator due to heating from 

 below, the thickness should decrease toward the equa- 

 tor. No evidence of such a decrease was found from the 

 Meteor soundings, probably because of the large height 

 interval between observations. For this reason too, the 

 figure for average thickness of 130 m is probably 

 too large. 



Intensity of Ducts 



The decrease of the modified index of refraction 

 within the duct averaged 28 M units between latitudes 



'The ascent and descent curves disagree largely because of 

 the lag of the humidity element in the sounding rig. The curve 

 showing sharper inversion is therefore probably the more 

 accurate. 



10° to 20°N, and only 14 M units between latitudes 

 5° and 10°N, indicating a decrease in the intensity 

 of the duct equatorward. The intensity of the duct 

 also decreases oceanward from the coast of Africa. 



*•'•'' Surface Ducts 



The thickness of surface ducts depends on the 

 wind speed and the magnitude of the vapor pressure 

 difference between the ocean surface and the air at 

 some representative level (say the ship's bridge). 

 It is probable that the wind factor is the more im- 

 portant. Near the west coasts of continents the low- 

 ering of the trade wind inversion becomes the most 

 important factor. Duct intensities over the ocean in 

 the Northern Hemisphere, based on climatic charts 

 of the ocean, have been computed by Montgomery and 

 Burgoyne.^^ 



Wind Speed 



According to observations taken in the Pacific 



north of New Guinea and northeast of Saipan, ducts 

 were less than 10 ft in depth at wind speeds of one 

 knot and were 40 to 60 ft at wind speeds of 10 to 20 

 knots.-" According to climatic charts of the ocean 

 (6), the average wind speed in the trade winds is 

 maximum in summer at 15° to 20°N and in winter 

 at 10° to 15°N. In the Southern Hemisphere maxi- 

 mums are at 10° to 15°S in summer (December 

 to February) and at 5° to 10°S in winter. These 

 latitudes in the respective seasons should also coincide 

 with the maximum thickness of surface ducts. 



Vapor Pressure Difference 



As the air flows toward the equator over continually 

 warmer water surfaces moisture is being supplied to 

 the air Ijy evaporation from the water surface. Nearer 

 the equator the increased rainfall decreases the water 

 vapor pressure difference between the ocean surface 

 and the air above. According to climatic charts^' the 

 maximum vapor pressure difference between ocean 

 surface and the air above in the trade exists at about 

 latitudes 20° in summer (in both hemispheres) and at 

 latitudes 10° to 15° in winter. This effect should also 

 contribute to the existence of a maximum duct height 

 in the trade winds at about 20° latitude in summer, 

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



Surface Ducts near the Western 

 Coasts of Continents 



All soundings of the Meteor expedition within 300 

 miles of the coast of Africa showed intense ground- 



