78 



GENERAL METEOROLOGY AND FORECASTING 



schematically in Figure 2. The successive M curves 

 correspond to a series of time intervals measured 

 from the passage of the air over the shore line. The 

 increase of the duct toward the maximum and the 

 subsequent flattening of the M curve as the air 

 approaches a new state of equilibrium is clearly seen 

 from the figure. 



Duct formation in such a case depends on two 

 quantities: (1) the excess of the unmodified air 

 temperature above the water temperature, and (2) 

 the humidity deficit, that is, the difference between 

 the saturation vapor pressure corresponding to the 

 water temperature and the actual water vapor pres- 

 sure in the unmodified air. The problem can be 

 treated by means of the mathematical theory of 

 diffusion in a turbulent medium, and a considerable 

 amount of effort has been spent in investigating this 

 type of advective duct. Extensive mathematical 

 work has been carried out in England 200 based 

 primarily on the large body of data on atmospheric 

 diffusion gathered in connection with chemical 

 warfare problems. 197 In the United States such 

 ducts have been studied very extensively in con- 

 nection with the propagation experiments in Massa- 

 chusetts Bay where conditions are favorable for 

 their formation. 201,204 



Another phenomenon often responsible for ducts 

 in coastal regions is the land and sea breeze. This 

 type of wind is of thermal origin and is produced 

 by temperature differences between land and sea. 

 During the day, when the land gets warmer than 

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

 sea descends, thus causing a circulation in which 

 the air in the lowest layers flows from sea to land. 

 This is the sea breeze. Vice versa, during the night 

 the land becomes colder than the sea, and circulation 

 is in the reverse direction, creating 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. A sea breeze modifies the 

 advective conditions described above in various 

 ways, and extremely strong ducts have occasionally 

 been observed under sea breeze conditions. The land 

 and sea breezes are of a strictly local nature and in 

 some cases will extend only a few miles to land or 

 sea from the shore. Nevertheless this region may be 

 an important part of the radiation trajectory of 

 coastal radars. These breezes develop only under 

 fairly calm conditions; they are wiped out by a 

 moderate or strong wind. 



The advective ducts of the types described here 



are by their very nature of only limited horizontal 

 extent. The horizontal variation of refractive index 

 presents a problem that till now has not been sys- 

 tematically studied from either the experimental or 

 the theoretical angle. 



A particular type of duct has been discovered in 

 purely maritime air, that is, air which has had an 

 extremely long sea trajectory and thus should have 

 reached an approximately steady state of diffusion 

 relative to the underlying sea surface. The Antigua 

 experiments described in the preceding chapter reveal 

 the existence of a type of low duct which seems to be 

 characteristic of maritime air. It appears probable 

 that similar ducts are permanent in the oceanic 

 regions of many parts of the earth. The relative 

 humidity of the air at Antigua was found to be 60 

 to 80 per cent, indicating that a continuous upward 

 diffusion of moisture must take place, since the air 

 immediately adjacent to the water surface is always 

 practically saturated. On the other hand, there is 

 little difference between the air and sea tempera- 

 tures in this case, the ocean being about 25 C while 

 the air temperature varies between 23 and 26 C. 

 The ducts are therefore caused solely by the varia- 

 tion of water vapor in the lowest layers and are 

 much lower than the advective ducts described 

 before, their height rarely exceeding 40 ft. Typical 

 M curves have been shown in Chapter 7, and, for 

 the particular effects caused by the low height of 

 these ducts, we refer to the discussion of the experi- 

 mental results. 



The diurnal change of ocean temperature is 

 insignificant, except in extremely shallow water, and 

 therefore, at some distance from the coast, propaga- 

 tion conditions do not show any appreciable diurnal 

 variation. 



95 DYNAMIC EFFECTS 



The physical processes in the lower strata of the 

 atmosphere which determine the formation of ducts 

 are to a considerable extent controlled by the large- 

 scale dynamics of the atmosphere. It is therefore 

 often possible to make at least a qualitative forecast 

 of propagation conditions on the basis of a knowledge 

 of the synoptic weather situation. An example in 

 point is the diurnal variation over land in clear 

 weather from standard conditions during the day 

 to duct conditions in the latter part of the night 

 and the early morning hours. 



Conditions in a barometric low pressure area 



