44 PRINCIPLES OF STRATIGRAPHY 



currents. The center of this zone shifts with the sun, but remains 

 near the equator. The prevailing icinds or planetary nnnds thus 

 produced belong strictly to the lower part of the atmosphere. This 

 is especially true of the trade winds, which appear to be confined 

 to the lower 10,000 feet, as shown by observations on the Peak of 

 Teneriffe in the Canary Islands (lat. 28°). The westerly winds of 

 the higher latitudes, however, coincide with the general movement 

 of the upper air, which, being poleward, is deflected to the east. 

 Thus, as shown by the movement of the upper clouds, the direction 

 of the upper winds is toward the east, and so a circumpolar whirl 

 is instituted. 



Influence of Continents on Winds. 



Sea-Breeses. The chief disturbing factor of the planetary winds 

 is the unequal heating of the atmosphere over the land and the 

 water, as explained on p. 31, which, in fact, is itself a cause of 

 local winds. As the land, after the rising of the sun, is more quickly 

 warmed in summer than the water, the air above the land expands 

 upward, crowding against the upper air and so producing a seaward 

 barometric gradient down which the air will flow. The resulting 

 pressure over the sea and the relief of pressure over the land will 

 cause a lower return current, the sea-breeze, to flow landward. The 

 velocity increases from the morning (8 or 9 A.M.) to the afternoon, 

 reaching its maximum at the time of greatest air temperature. At first 

 the sea-breeze blows, in general, at right angles to the coast, but 

 as the air is drawn from greater distances ofif-shore, it comes under 

 the influence of the deflective force of the earth's rotation and is 

 turned to the right in the northern hemisphere and to the left in the 

 southern hemisphere. Thus a sea-breeze on the eastern coast will 

 change to a southeast wind in the northern and a northeast wind in 

 the southern hemisphere, while a sea-breeze on the western coast 

 changes to a northwest and a southwest wind in the northern and 

 southern hemispheres respectively. Where the sea-breeze has the 

 direction of the prevailing winds it often increases in the afternoon 

 to the velocity of a gale. At Valparaiso "pebbles are torn up 

 from the walks and whirled about the street" and business and 

 shipping are interrupted. (]\Iaury-63.) At evening the calm 

 suddenly succeeds the uproar. Measurements on the Massachu- 

 setts coast have shown the velocity of the sea-breeze to be at first 

 from 5 to 13 km. per hour, reaching its greatest velocity of 16 to 

 30 km. late in the afternoon. At Boston the average velocity is 

 23 km. per hour at 3 P. AI. on sea-breeze days. The height of the 



