R-VIUO METEOROLOGY 



!13 



caused by convection is about — 1 (' per 100 in, and 

 the moisture lapse is steady and rather small. 

 Standard refraction will therefore prevail when winds 

 are moderate to strong over land, and over the ocean 

 also when the winds are sufficiently strong. 



Temperature inversions occur when the temperature 

 of the surface (sea or land) is appreciably lower than 

 the temperature of the air. The transition from the 

 ground temperature to the free air temperature takes 

 the form shown in Figure 28. The heat and moisture 



perhaps, a day or two. When the air in the inversion 

 is dry to begin with and flows over ground capable 

 of evaporation (the sea or moist land) there will be 

 established, in such an air mass, a steep moisture 

 lapse, since the water vapor that has been taken up 

 by the air near the ground will only gradually diffuse 

 into the dry air aloft. Conditions are then favorable 

 for the formation of an evaporation duct, in addition 

 to whatever tendency toward duct formation may 

 be caused by the temperature inversion itself. 



GROUND 



Figure 28. Air temperature versus height for an 

 inversion. 



transfer caused by turbulence in a temperature inver- 

 sion is less simple than that in a frictional layer. 

 The turbulent processes active in inversion regions 

 are highly complex and are not yet very well 

 explored. It is known, however, that the intensity of 

 the vertical transfer of heat and moisture is greatly 

 reduced as compared to the rate of transfer with 

 frictional turbulence. The reduction is the more 

 pronounced, the steeper the vertical increase of tem- 

 perature; in a steep inversion the rate of transfer 

 may be many times less than in a frictional layer. 

 This tends- to produce a vertical stabilization of the 

 air layers in the inversion region. As soon, therefore, 

 as a temperature inversion has begun to form, the 

 rapid mixing in the lowest layers, usually effected 

 by frictional turbulence, stops and is replaced by a 

 much more gradual diffusion. 



Assume now, for instance, that the rate of diffusion 

 has become so slow that the transfer of moisture over 

 a height of a few hundred feet takes many hours or, 



17.3.3 Advective Ducts — Coastal Conditions 



Advective formation of ducts may occur both over 

 land and over sea, but this process is most important 

 over the ocean near coasts. The most common illus- 

 tration is that of air above a warm land surface 

 flowing out over a cooler sea. Over the land the air 

 will usually have acquired a convective or nearly 

 convective temperature gradient of — 1 C per 100 m. 

 When this air flows out over the cool water surface, 

 a temperature inversion is rapidly formed which 

 grows in height as the process of turbulent transfer 

 progresses. The temperature inversion does not, in 

 itself, give rise to a pronounced duct because the 

 effect of a temperature gradient upon the M curve 

 is relatively small; but when the air is dry, evapora- 

 tion from the sea surface takes place simultaneously 

 with the heat transfer, and a moisture lapse rate is 

 established in the lowest layers. The combination of 

 temperature inversion and moisture lapse rate is 

 most favorable for the formation of a duct off shore. 



The gradual formation of this type of duct is 

 illustrated in Figure 29. This shows M curves, corres- 



700 



600 



uj 500 



UJ 



11. 

 400 



2 



fc300 



C9 



iTjZOO 



10 20 30. 40 50 60 70 

 M-Mo 



90 100 



Figure 29. Development of duct off coast. Initial state 

 corresponds to air at coast line. M hr, 14 hr, etc., refer 

 to time air has been over water. Initial conditions for 

 this set of curves: unmodified air To = 32 C, e = 12.3 

 mb; water T w = 22 C, e w = 26.5 mb saturation. 



ponding to the simple surface type of trapping (see 

 Figure 20, curve II) for a series of time intervals 



