146 



[chap. 4 



layer extends to a height of about 2-3 km, topped by a much drier upper 

 troposphere. The transition zone, a few hundred meters in thickness, is the 

 celebrated "trade-wind inversion" which performs its function in moisture 

 accumulation by preventing upward leakage through cloud penetration. A 

 region of rapid drying and usually of stabilization in temperature lapse rate, it 

 dilutes away the buoyancy of the small cumuli by mixing or "entrainment" so 

 that few of them are able to poke their towers more than a few hundred meters 

 into the dry layer. The height of the inversion is determined by a balance 

 between the large-scale sinking motion and localized convection; the latter 



80 100 120 140 160 180 160 140 120 100 80 60 40 20 20 40 60 



^ 



-i^^^^^^^&^:^-^-/i#^ 



I I I I I I I L 



80 100° 120° 140° 160° 180° 160° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 



(a) 



40* 60" 



(b) 

 Fig. 23. Prevailing surface winds over the oceans in summer. (After U.S. Weather Bureau, 

 Atlas of Climatic Charts of the Oceans, 1938, Charts 9 and 29.) 



(a) Direction and constancy in July. Same notation as Fig. 22a. 



(b) Average wind velocity in knots — June, July and August. 



