REFRACTION OF RADIO RAYS 



169 



regardless of air mass type. The radio ray bending is also plotted on 

 these figures relative to the value expected in the Nih) = 313 exp 

 { — /i/6.95} atmosphere. Near the ground, bending departures are seen 

 to be mirrored by the A unit profiles. That is, a negative gradient of 

 A'^ or yl produces a positive increase of bending. Above a few kilometers, 

 however, the bending departures approach a fixed, usually nonzero, value. 

 It is apparent that the asymptotic value of T313 — r at large heights is 

 determined by the bending in the first few kilometers, where 60 to 75 

 percent of the low-angle bending normally occurs. The marked differ- 

 ences in air mass n structure and bending are confined, therefore, to the 

 first few kilometers above the earth, as is illustrated by figure 3.16. The 

 distribution of the meteorological elements within each air mass is re- 

 flected in the A{h, 313) profiles. For example, the steep humidity 

 gradient characteristic of maritime tropical air is reflected by the rapid 

 decrease of A{h, 313) with height. Comparatively, the high ground-level 

 temperatures and rapid decrease of temperature with height in continental 

 tropical air are reflected by the increase of A{h, 313) with height wdthin 

 that air mass. These, and .4-profile characteristics of other air masses, 

 are listed in table 4.11 and are evident from the form of the equation for 

 the refractive index at radio frecjuencies [see chapter 1, (1.20)]. 



Table 4.11. Refractive characteristics of air 7nasses. 



Ref- 

 erence 



Superior (S) S/mT, [48] 

 typical of Oulf coast 

 (Lake Charles, La.). 



Meteorological characteristics 



Formed from subsidence of high- 

 level air with resulting dry adiabatic 

 temperature lapse rate and low hu- 

 midity. Often found overlying other, 

 more humid air masses that show a re- 

 sultant characteristic drying with 

 height in the lower levels. 



Refractive characteristics 



Rapid decrease of N and A 

 with height in the lowermost 

 layer produces a superrefrac- 

 tive* layer. The overlying 

 superior air is nearly normal in 

 bending characteristics. 



Continental Polar (cP) 

 source region. 



Characterized by low temperature 

 and humidity with a pronounced tem- 

 perature inversion at the surface 

 created by progressive nocturnal cool- 

 ing during the arctic night. 



Ground-based superrefrac- 

 tive layer arising from the tem- 

 perature inversion. 



Maritome Polar (mP), [48] 

 typical low-level 

 ground modification 

 (Seattle, Wash.). 



Cool air, nearly saturated to a height 

 of several kilometers. Example shows 

 typical drying in lower levels when 

 this air mass moves over land. Over- 

 lying mP has increasing humidity and 

 decreasing temperature with height. 



Continental Tropical 

 (cT) source region 

 (El Paso, Tex.). 



[51] 



Characterized by superheated lower 

 layers, rapid decrease of temperature 

 with height, and very low humidity. 



Surface layer produces near- 

 normal bending. Increasing 

 N in the overlying mP air pro- 

 duces an elevated subrefrac- 

 tive layer. 



Strong temperature lapse 

 produces a subrefractive layer 

 reaching to several kilometers. 



Maritime Tropical 

 (mT) source region 

 (Pensacola, Fla.). 



[48] 



Relatively warm air, high water- 

 vapor content in lowest layer, which 

 decreases rapidly with height. Mod- 

 erate changes in temperature and hu- 

 midity structure produce large re- 

 fractive gradient changes. 



Strong superrefraction to 

 great heights arising chiefly 

 from rapid decrease of hu- 

 midity with height. 



'Normal refraction is taken to mean t(/i, 313), superrefraction to mean j-(ft)>7(h, 313), and subrefraction 

 t(/i)< r(ft, 313). 



