ki:i'lk<:tio\ ikom i;i.i:\ \ti:i> laykks 



W 



SO 



SCO 



100 

 d IN FEET 



Figure 7. Maximum wavelength trapped in a simple 

 surface duct. Duct width d in feet. A M is total decrease 

 of M in duct. X max = 2.5 d V A M 10- 6 . 



67 REFLECTION FROM ELEVATED LAYERS 



Reflection from elevated layers has so far been 

 observed systematically only under the rather spe- 

 cial meteorological conditions at San Diego, but it 

 probably occurs elsewhere, though with a lesser 

 degree of regularity. It appears when there is a 

 strong elevated M inversion. Such an M curve is 

 very nearly equivalent to a true discontinuity of 

 refractive index, and the effect on a wave traversing 

 such a region is similar to that of a boundary between 

 two media, the more nearly so, the larger the M- 

 in version gradient. If there is a true discontinuity, 

 an incident wave is split up into a reflected and a 

 transmitted wave. If the discontinuity is replaced 

 by an M inversion layer, the reflected wave still 

 persists but becomes weaker the less steep the in- 

 version. The distinction between this phenomenon 

 and the apparent reflection in the duct where the 



rays become horizontal before turning downward is 

 usually fairly clear-cut. The true reflection described 

 here occurs primarily in waves which are so long as 

 to be below the cutoff. 



There exists a case of gradual transition between 

 two media with different refractive indices for which 

 the wave equation can be integrated. 44 * 1 "" 5 



This can be applied qualitatively to the case, 77 ' 91 

 in so far as earth's curvature can be neglected. 

 Figure 8 shows the calculated ratio in decibels of 



700 



D STRATUM THICKNESS 

 A "WAVELENGTH 



Figure 8. Calculated reflection ratio in decibels. 



reflected to incident wave for various angles of in- 

 cidence plotted against the ratio of thickness of the 

 transition layer to wavelength as abscissa. 



The verification of this theoretical concept in the 

 San Diego experiments will be discussed in the next 

 chapter. 



