SECT. 2] LARGE-SCALK INTERACTIONS 203 



the larger-scale instabilities in the flow, paving the way for a more limited 

 discussion of the fluctuations in middle latitudes and the longer period inter- 

 action anomalies which may persist over seasons and decades. 



A. Exchange Mechanisms in the Trade-Wind Region 

 a. The layer below cloud base 



We shall now take a closer look at the trade-wind sub -cloud layer, schemati- 

 cally indicated in Fig. 24. The physical nature of the motions occurring is 

 beautifully illustrated in Figs. 48 and 49, which are photographs made on the 



Fig. 48. Aerial photograph of smoke trail laid by stationary ship over Caribbean Sea, 

 August 29, 1944. Sea minus air temperature, 1.2°C. Wind speed, 7.7 m/sec. Note lateral 

 displacement of smoke to right and left of average wind direction. (After Woodcock 

 and Wyman, 1947, Plate 2. By courtesy of N.Y. Academy of Sciences.) 



first (1944) Woods Hole expedition to the trades (Woodcock and Wyman, 1947). 

 In Fig. 49a, a trail of smoke has been laid by an aircraft flying up-wind at 

 300 ft. Fig. 49b shows what has happened to the smoke trail 88 sec later. The 

 predominant scale of motion causing transport of the smoke has horizontal 

 dimensions about equal to the distance from A to B in the photograph, or 

 comparable to the 300-ft long destroyer. Smaller scales of motion are also 

 detectable: those of the nodules, about 50-100 ft across, and still smaller 

 scales giving rise to the thickening of the plume in the vertical and horizontal 

 (Fig. 48). Measurements, corrected for the smoke's own buoyancy, showed that 



