154 THE OCEAN RIVER 



The polar-front and trade-wind zones together function as 

 heat engines in which evaporation of water and heating of 

 air takes place at ground level and cause the air to rise, ex- 

 pand, and lose both its own heat and the latent heat of con- 

 densing vapor as it moves north. Then it sinks to the ground 

 and moves south to begin the heat cycle again. As we look 

 eastward both of these are seen to be engines with counter- 

 clockwise motion working against the brake of contrary 

 movement in the middle latitudes. 



This three-cell system is demonstrated on the surface of 

 the ocean by the northeast trades blowing toward the equator, 

 then by the westerlies blowing toward Europe, and finally as 

 the northeasterlies from the polar region. Thus the wheel 

 of air moves above the North Atlantic wheel of waters. 



Meteorology and oceanography have contributed much 

 to each other, and this not only because of action and reaction 

 between winds and the currents. The Ocean River and the 

 rivers of the air obey the same laws, are acted on by the same 

 mechanics of a spinning earth, and in their motions show 

 the same balanced adjustment of density against flow. In the 

 Gulf Stream there is a pihng up of water to the right of the 

 Stream in compensation for the Coriolis force, and again in 

 the easterlies and westerlies of the Atlantic Ocean we find 

 a pihng up of air, a high-pressure belt, to the right of the 

 wind's path. The arrangement of winds across the surface 

 of the ocean is balanced against a high-pressure belt in the 

 Horse Latitudes and another at the pole, with areas of low 

 pressure in the equatorial doldrums and in latitudes between 

 55° and 65° N. 



Convection, the vertical transfer of temperature by air 

 and water, does not work uniformly over the earth because 

 water, earth, and ice reflect heat and conduct, store, and radi- 

 ate it at different rates. This varying of heat transfer breaks 

 up the belts of high and low pressure into separate areas of 



