SECT. 2] 



LAK(iK-K(!ALK INTERACTIONS 



145 



The trade-wind zone extends from about lO"^ to 25^ latitude in both hemi- 

 spheres (see Figs. 22 and 23). Easterly winds with an equatorward component 

 dominate the surface of the globe from the subtropical high pressure ridge lines 

 all the way to the equatorial trough. In the vertical, trade-wind air is charac- 

 terized by a layered structure : A lower moist, marginally stable convective 



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



60° 40° 20° 0° 20° 40° 60° 



jo^ ^ Sf!l_ 



(b) 

 Fig. 22. Prevailing surface winds over the oceans in winter. (After U.S. Weather Bureau. 

 Atlas of Climatic Charts of the Oceans, 1938. Charts 3 and 27.) 



(a) Direction and constancy in January. Direction lines based on dominant wind 

 arrows computed for each 5-degree square, with directional constancy as follows : 



=> 81% and over; -> 61-80% ; > 41-60% ; > 25-40% of all winds from 



the quarter within which the line is a median. Solid line : mean position of equatorial 

 trough. Dashed lines : mean positions of subtropical ridges. 



(b) Average wind velocity in knots — January, February and March, 

 fi— s. I 



