248 RADIO WAVE PROPAGATION EXPERIMENTS 
fo" 
Figure 14. Substandard type of M curve. 
‘—_—— 
relatively cool land or sea surface, quite frequently in 
connection with the formation of surface fog. 
4, Simple surface trapping (Figure 15). The M 
curve has a negative slope in the inversion layer which 
comes down to the land or water surface. The duct is 
of the ground-based type, and its width is the height 
of the upper boundary of the inversion layer. Rays 
which are propagated at an angle of 1 degree or less 
with the horizontal may be trapped within the duct. 
As a consequence, radio and radar ranges may be 
exceedingly large. Simple surface trapping occurs 
quite frequently over the oceans—particularly where 
warm, dry air from over land flows out over a cooler 
sea surface—along coast lines with an afternoon sea 
breeze, and occasionally over land with radiational 
cooling at night. 
OUCT WIDTH 
-—-—_——_———- aw ————>] 1 — 
Ficure 15. Simple surface trapping. 
5. Elevated S-shaped type (Figure 16). The inver- 
sion layer has a width given by the difference in eleva- 
tion of the end points of the negative portion of the 
M curve, but the width of the duct extends downward 
to the level where the vertical projection of the upper 
minimum of the M curve intersects the latter. Trap- 
ping occurs when the transmitter is at an elevation 
which places it within (or close to) the duct and is 
most pronounced when the transmitter is at the eleva- 
tion of the base of the M inversion. This type of duct 
may be brought about by subsidence or as the result 
of a Fohn wind blowing off shore from mountains 
paralleling a coast. Examples of elevated S-shaped M 
curves are observed off the southern California coast 
and off the east coasts of Japan and New Guinea (see 
page 256-258). 
Ficure 16. Elevated S-shaped type. 
6. Ground-based S-shaped type (Figure 17). When 
the conditions which could produce type 5 (Figure 16) 
exist down to the surface of the earth or to the sea, 
this type of duct may occur. It usually has a width 
considerably greater than that found in the simple 
trapping case (type 4, Figure 15). 
That it is possible for ducts of two types to occur 
simultaneously has been shown from observations off 
Fiacure 17. Ground-based S-shaped type. 
the east coast of New Guinea when a Fohn wind flows 
out over a sea breeze, the latter producing simple sur- 
face trapping (Figure 18). (See pp. 256-258.) 
Factors Affecting the Extent of Trapping. The prin- 
cipal factors which determine the extent of trapping 
are: 
—_—__S 
Fiaurn 18. Combination of types 4 and 5. (See Figures 
15 and 16.) 
