154 



N CLIMATOLOGY 





Q 



60 80 100 120 140 



DUCT HEIGHT , h, IN m 



180 200 



Figure 4.46. Shadow zone half-lengths for minimum ducting conditions at 

 Washington, D.C. 



Table 4.9. Half-lengths at Washington, D.C. for various duct heights and surface 



gradient conditions 



Washington, D.C. Figures 4.47 and 4.48 show the distance measured 

 along the earth's surface that a ray with an initial elevation angle of zero 

 will travel in the A^^^ = 313.0 exponential atmosphere. The value of 

 zero for an initial elevation angle is used because the ray can be thought 

 of as leaving the position A, tangent to the duct in figure 4.41 and 

 "arriving" at the transmitter, even though the reverse process is actually 

 what is occurring. Thus, from figures 4.47 and 4.48, knowing the height 

 of the transmitter, ht, and the height of the ground-based duct, Ka, the 



