122 



METEOROLOGY— FORECASTING 



legion may be reduced considerably l.)elow normal, even 

 to tlie point of producing radar and commiuiication 

 "blackout." The M deficit is negative with this type 

 of curve. Associated meteorological conditions are 

 usually those in which warm, moist air passes over a 



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 irapping (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. Eays 

 which are propagated at an angle of 1 degree or less 

 with the horizontal may l)e 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. 



Figure 16. Elevated S-shajied type. 



5. Elevated S-shaped type (Figure Hi). The inver- 

 sion layer has a width given by the diffi'erence 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 

 Section 8.3.5). 



Figure 15. Simple surface trapping. 



Figure 1". Ground-based S-shaped type. 



Ci. Ground-tjufied S-shaped tijpe (Figure 17). When 

 the conditions which could produce type 5 (Figure IG) 

 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 

 tra].)ping case (typo 4, Figure 15). 



That it is possible for ducts of two types to occur 

 simultaneously has been shown from observations off 



