152 



A^ CLIMATOLOGY 



R^ 1 ST SHADOW ZONE 



ZONE BEYOND THE HORIZON 



DIRECT AND REFLECTED RAY 

 INTERFERENCE REGION 



^GROUND BASED DUCT 

 FiGUEE 4.43. Regions of fading from different effects of the radio rays. 



then (4.37) becomes 



dh = a 



2AN' X 10" 



+ (e. 



I 



dp + 02 



since 02 is no longer just equal to zero, but is such that 



< 02 < 0p, 



(4.35) 



(4.36) 



and AN' will no longer be the same as the AA'^ for the duct width. The 

 value of 02 may be determined from the formula: 



2 2h 



I 



a 



2AN' 



(4.37) 



In figures 4.44 to 4.46 the shadow zone half-lengths are given as a func- 

 tion of duct height for the various seasons of the year. As shown in 

 figure 4.39, the 50 percent level, or median of observed ground-based duct 

 thickness at Washington, D.C., is about 100 m. Figure 4.40 shows that 

 the percent level of observed maximum ground-based duct thickness is 

 about 200 for Washington, D.C. In table 4.9, the values oidht during the 

 four seasons are shown for the 100- and 200-m surface duct heights. 



As previously mentioned, the atmosphere above the assumed duct was 

 taken to coincide with the N ^ = 313.0 exponential atmosphere [9] because 

 this value most nearly corresponds to the average annual N s value at 



