134 A^ CLIMATOLOGY 



This type of inversion may cause the formation of an elevated super- 

 refractive layer where the air temperature usually decreases immediately 

 above the ground, rises through the inversion layer and decreases above 

 the layer. This is a common occurrence which may be observed at any 

 time. Subsidence has a tendency to destroy subrefractive layers and to 

 intensify superrefractive layers. Although the effects of subsidence are 

 generally observed at high levels, they are occasionally observed at lower 

 levels, especially in the subtropics. Since subsidence frequently occurs 

 in the lee of mountains and in the southeastern regions of northern 

 hemisphere highs, elevated ducts may also be observed. 



Conditions inimical to ducting are those which mduce mixing of the 

 lower atmosphere. Small scale atmospheric motions (turbulence) and 

 consequent mixing and mass exchange result from differential surface 

 heating and surface roughness effects. Both of these processes work to 

 destroy stratification. They ultimately result in uniform vertical distri- 

 bution of moisture through considerable depths of the lower atmosphere 

 and the establishment of neutral temperature lapse rates. Accordingly, 

 where the process of mechanical and convectively induced mixing are at 

 work, the probability of the occurrence of ducting is vanishingly small. 

 Thus, few, if any, ducts are observed over snow-free, low albedo land 

 areas from midmorning to late afternoon when the skies are clear, or in 

 areas of moderate to great surface roughness when the surface winds are 

 more than a few meters per second, irrespective of cloud conditions or the 

 time of the day. 



4.4.3. Refractive Conditions Due to Local 

 Meteorological Phenomena 



Land and sea breezes may produce ducts along the coastal regions since 

 the winds are of thermal origin, resulting from temperature differences 

 between land and sea surfaces. During the day, when the land gets 

 warmer than the sea, the air above the land rises and is replaced by air 

 from the sea, thus creating a circulation from the sea to the land, called a 

 sea breeze. During the night, the land becomes colder than the sea and a 

 circulation, called a land breeze, is set up in the opposite direction. This 

 type of circulation is generally shallow and does not extend higher than 

 a few hundred meters above the land or sea surface. 



A land or sea breeze may modify the refractive conditions in different 

 ways depending upon the distribution of moisture in the lower layers. 

 Since these breezes are of a local nature and generally extend only a few 

 miles, only coastal locations are usually affected. The very nature of sea 

 and land breezes results in a marked refractive index pattern. With a 



