214 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



(and distances) as the air moves out over the water. 

 The top of the duct is given by the elevation of the 

 minimum value of the M curve. It will be noticed 

 that the duct acquires a maximum depth some time 

 after the air has touched the cold water surface; 

 thereafter the depth decreases. The cause of this 

 behavior is found in the progressive decrease in 

 moisture and temperature differences which is the 

 final result of the diffusion process. Thus the final 

 stage of this transformation is an air mass whose 

 temperature and moisture distributions are in equi- 

 librium with the underlying water surface and no 

 longer show a rapid variation with height. 



Duct formation in such a case depends on two 

 quantities: (1) the excess of the unmodified air 

 temperature above that of the water and (2) the 

 humidity deficit, that is, the difference of the satura- 

 tion vapor pressure corresponding to the water tem- 

 perature minus the actual water vapor pressure in 

 the unmodified air. If these quantities are large, 

 especially the humidity deficit, a duct will develop. 

 A great variety of local conditions may, however, 

 be encountered in problems of this type, and empiri- 

 cal rules developed for one locality may not at all 

 apply to others. 



Advective processes may also occur over land, but 

 the conditions required for duct formation are likely 

 to be found much less frequently. Evaporation over 

 land need by no means be small unless the land 

 surface is very arid (desert) ; in fact, evaporation over 

 a moist soil or a ground covered with vegetation may 

 be comparable to, or even larger than, evaporation 

 from a sea surface. A duct may therefore be formed 

 when dry, warm air flows over a colder ground surface 

 capable of evaporation. The temperature excess and 

 humidity deficit may again be defined as above. 



Land and sea breezes often produce ducts near 

 coastal regions. These winds are of thermal origin 

 and are produced by temperature differences between 

 land and sea. The mechanism is illustrated in Figure 

 30. During the day, when the land gets warmer than 



WARM 



COLD 



COLD 



WARM 



LAND SEA LAND SEA 



SEA BREEZE LAND BREEZE 



Figure 30. Land and sea breezes. 



the sea, the air rises over the land and descends over 

 the sea and causes an air circulation in which the 

 wind blows from sea to land (sea breeze) in the lowest 



levels. Vice versa, if during the night the land becomes 

 colder than the sea, a circulation in the opposite 

 direction arises. This is the land breeze. As a rule, 

 this type of phenomenon is extremely shallow, and 

 the winds do not extend above a few hundred feet 

 at the most. Often there is a reverse wind in the 

 layer above the land or sea breeze layer. A sea breeze 

 may modify the advective conditions described above 

 in various ways, and extremely strong ducts have 

 been observed repeatedly under sea breeze condi- 

 tions. The land and sea breezes are of a strictly local 

 nature and in some cases will extend only a few 

 kilometers to both sides of the shore. Nevertheless 

 this region may be an important part of the trajec- 

 tory of radiation. These breezes develop only under 

 fairly calm conditions; under conditions of moder- 

 ately strong wind, the sea and land breeze will be 

 perceptible only as a slight modification of the 

 existing wind. Because of their limited extent, fore- 

 casting of these breezes requires a study of the local 

 wind and temperature conditions. 



Advective ducts caused in the manner described 

 here are often quite limited horizontally. This is 

 especially true if a sea breeze is involved. The 

 assumption made throughout this report, namely 

 that the stratification of the air is of infinite extent 

 horizontally, will no longer be valid, and superrefrac- 

 tion may be restricted to a stretch along the coast. 



17.3.4 



Ducts over the Open Ocean 



A type of duct that is somewhat similar to the 

 advective duct described above is found over the 

 open ocean where the air has had an extensive over- 

 water trajectory. It has been studied in experiments 

 carried out at the island of Antigua in the West 

 Indies. The subsequent description refers to this 

 particular location, but on the basis of experience 

 gained operationally and in other experiments it may 

 be presumed that similar conditions prevail in 

 numerous other regions of the world, particularly in 

 the trade wind regions. 



At Antigua, in winter and early spring when these 

 tests were made, the wind is usually from the north- 

 east since the island is situated at the southeastern 

 fringe of the so-called Bermuda High, a large semi- 

 permanent circulation system over the North 

 Atlantic, extending from about 10° to 30° North 

 latitude. The air at Antigua has thus had an ocean 

 trajectory of thousands of miles. The relative hum- 

 idity is of the order of 60 to 80 per cent, indicating 



