RADIO METEOROLOGY 



217 



is strong enough a temperature inversion will be 

 created. Since this process does not require the 

 presence of a ground surface, it may occur, and in 

 fact often does occur, aloft in the atmosphere. The 

 effects of subsidence frequently are the most pro- 

 nounced at an elevation of the order of a kilometer 

 or more. 



As a general rule, subsidence occurs in regions of 

 high barometric pressure. In fact, subsidence always 

 does occur in such regions, but it may not always be 

 intense enough to give rise to a strong temperature 

 inversion. The flow of air in a barometric High is 

 shown in Figure 32 as it appears on a weather map 



ILLUSTRATING SUBSIDENCE (SINKING) IN HIGH PRESSURE AREA 



AS IT APPEARS 

 ON THE WEATH- 

 ER MAP 



THE AIR AS IT SINKS GETS WARMER -MORE SO IN THE HIGHER LEV 

 ' ELS-AND A TEMPERATURE INVERSION IS CREATED 



/ 



\ ♦ l 





i \ 



INVERSION REGION 



VERTICAL 

 CROSS 

 SECTION 



^-— " i.mccc 



UNAFFECTED AIR 



Figtjre 32. Characteristics of subsidence. 



in horizontal projection, and also in a vertical cross 

 section. 



With subsidence, the air as a rule is very dry, and 

 there is nothing in the process which can change the 

 moisture content or produce moisture gradients. If, 

 however, the dry air finds itself over a surface capable 

 of evaporation, such as the sea surface, a steep 

 moisture gradient may be established and a duct 

 will be created. It is thus seen that subsidence in 

 itself does not produce a duct, except in extreme 

 cases, but it can act as an auxiliary factor and greatly 

 enhance the formation of a duct whenever other 

 conditions are favorable. Thus, forecasts of super- 

 refraction based on a purely advective mechanism, 

 or purely on radiative cooling or evaporation, may 

 have to be modified in the presence of subsidence; 

 an otherwise very weak duct may be converted into 

 a strong duct by the effect of subsidence upon the 

 lower strata. 



Strong subsidence effects are of frequent occur- 

 rence on the southern California coast where they 

 may continue with little change for days at a time. 



At times the duct is elevated, giving an elevated 

 S-shaped M curve like Ilia in Figure 20. Again the 

 duct may extend practically from the ground up 

 with M curves similar to curves II or Illb in Figure 

 20. The elevation of the top of the duct may vary 

 from 300 to 5,000 ft, and the thickness may lie 

 between a few feet and 1,000 ft. Coverage diagrams 

 and the corresponding M curves for several typical 

 situations are illustrated in Figure 24. 



For a number of reasons the meteorological condi- 

 tions in a barometric High are favorable for the 

 formation of ducts. Among the favorable factors 

 are: subsidence, creating very dry air into which 

 evaporation from the surface can take place; again 

 subsidence, creating temperature inversions; calm 

 conditions preventing mixing of the lowest layers by 

 frictional turbulence and maintaining the thermal 

 stratification caused by radiative cooling or local 

 breezes; clear skies producing nocturnal cooling 

 over land. 



The conditions in a barometric Low, on the other 

 hand, generally favor standard propagation. A lifting 

 of the air, the opposite of subsidence, usually occurs 

 in such regions and is accompanied by strong winds. 

 The combined effect is to destroy any local thermal 

 stratification and to create a deep layer of frictional 

 turbulence. The air is therefore well mixed, and 

 nonstandard vertical temperature and moisture 

 gradients are wiped out in the early stages of their 

 creation. Moreover, the sky is usually overcast in a 

 low-pressure area and nocturnal cooling, therefore, 

 is negligible. 



To summarize, high-pressure regions, clear skies, 

 and calm air are conducive to duct formation, while 

 low-pressure areas, cloudy skies, and winds favor 

 standard refraction. 



Fronts in the atmosphere are possible sources of 

 refractive effects. A front is a surface of discontinuity 

 which separates two air masses of different tempera- 

 tures. The surface slants at an angle of 1° to 2° with 

 the horizontal, with the colder air forming a wedge 

 under the warmer air. Fronts are a common occur- 

 rence in the atmosphere, and it might be thought 

 that they should have a considerable influence on 

 wave propagation. This is, however, not borne out 

 by English radar experience, which shows very little 

 superrefraction connected with fronts. The explana- 

 tion is probably that fronts are invariably accom- 

 panied by low-pressure areas, and turbulence along 

 a front is usually so strong that the transition from 

 the cold air to the overlying warm air takes place 



