METEOROLOGY — THEORY 225 
tor. No evidence of such a decrease was found from the 
Meteor soundings, probably because of the large height 
interval hetween observations. For this reason too, the 
figure for average thic.ness 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 
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. 
Varor Pressuri Dirrerencr 
As the air flows toward the equator over continually 
warmer water surfaces moisture is being supplied to 
the air by 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. 
Surrace Ducts NEAR THE WESTERN 
Coasts oF CONTINENTS 
All soundings of the Meteor expedition within 300 
miles of the coast of Africa showed intense ground- 
based ducts or S-shaped ground-based ducts. These 
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 the duct base has 
a diurnal variation, being maximum in the morning 
at about 0800 local time and minimum at about 1600. 
The diurnal variations are governed by land and sea 
breeze phenomena. 
Experimental Evidence 
During 1944, aircraft traffic to and from Ascension 
Island at 8°S in the Atlantic was tracked by two 105- 
me radars sited 2,500 and 1,700 ft above mean sea 
level. The following observed phenomena were re- 
ported verbally. 
1. Ranges were greater during the dry season than 
during the wet season. 
2. Ranges westward (270°) 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 (2), 
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 duct is of maximum intensity and 
frequency at 15° to 20° 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 20° in 
summer, 15° in spring and fall, and 10° in winter. 
4. Near the western coasts of continents the ele- 
vated duct lowers into an intense ground-based duct. 
