pressure to the index of refraction i.s given by the well known expression: 



79/ 4,800e 



(n-l)10 6 =— (p-e+ '— 



The dependence of refractive index on pressure leads to a regular decrease 

 with height, but the change of barometric pressure with weather produces 

 only an insignificant effect on the gradient. The variations in this index 

 in the lower atmosphere owe their existence to stratifications in which 

 the temperature and moisture change rapidly with height. As tempera- 

 ture decreases with height n increases, and as humidity decreases n de- 

 creases. The effect of humidity variation is distinctly more pronounced 

 than the effect of temperature, and humidity variations constitute the 

 main cause of nonstandard conditions with temperature variations a 

 contributing factor. The major effects of nonstandard refraction on radar 

 propagation occur only for rays which emerge from the transmitter at 

 angles less than § degree. For angles between h and 1| degrees the re- 

 fractive effects consist merely in minor modifications of the expected 

 radar coverage, while -for angles above 1^ degrees the refractive effects 

 are negligible. These considerations are best clarified by figure 33 which 



SOUTHERLY WINDS 



NORTHERLY WINDS 



REFRACTIVE INDEX 



Figure 33. — Diagrammatic illustration of refractive index changes: A = moist warm 

 air blowing from the south over warm water; B = moist warm air from the 

 south blowing over cold water; C = cold dry air blowing over cold water; 

 and, D=cold air from the north blowing over warm water. 



75 



