THE OCEANS, THEIR STRUCTURES AND FEATURES 



33 



water currents. Upwelling in this case occurs on the 

 inside of arcing segments of a current. On the inside 

 of such bends, there is created an area of low water 

 pressure, a place that is "refilled" by upwelling. Such 

 upwelling has very important effects upon surface 

 water temperature and coastal fog — perhaps just as 

 much as or even more than winds. However, the 

 ocean currents are not generally believed to have been 

 important in causing recent water temperature 

 changes. 



TIDES 



The cyclical rise and fall of the sea once or twice a 

 day varies in different parts of the world. Some 

 areas, primarily oceanic islands, have a tide dif- 

 ferential of only about a foot, whereas the Bay of 

 Fundy in Nova Scotia has tides approaching 50 feet. 

 The forces creating a high tide also cause the air 

 above the tide to be extended many miles into space; 

 even the land is affected, although insignificantly, by 

 the agents producing the tides. Therefore, any con- 

 sideration of tides has some bearing upon the air and 

 land as well as upon the ocean. 



Tides are produced by the gravitational attraction 

 of the sun and the moon upon the earth. Since gravi- 

 tational attraction is in direct proportion to mass, it 

 may be surprising to learn that the moon exerts more 

 influence than the sun. This is so because gravita- 

 tional attraction is affected by the distance between 

 objects as well as by their masses. For this reason, 

 the closer, although much smaller and lighter, moon 

 exerts more pull upon ocean waters than does the 

 sun. However, the position of sun and moon are im- 

 portant in the character of the tides. When the sun, 

 moon, and earth are in a direct line, waters of the 

 earth on the line have the highest high tides possible 

 and waters most remote from the line experience the 

 lowest low tides possible (Figure 3.5). {.\ote: It 

 makes no difference whether the straight-line order is 

 sun, moon, earth or sun, earth, moon.) This condi- 

 tion of highest rising of high tides and lowest receding 

 of low tides is called a spring tide. The opposite situa- 

 tion, called a neap tide, with the lowest high tides and 

 highest low tides, is produced when a line from sun to 

 earth and another from earth to moon form a right 

 angle (Figure 3.5). Because this relationship causes 

 the gravitational attraction of sun and moon to pull 



earth 



NEAP TIDE 



moon 



SPRING TIDE 



Figure 3.5 Relationships of sun, earth, and moon in producing maxi- 

 mum (spring) and minimum (neap) tides. Note the greater effect of the 

 moon as indicated for neap tides. 



against one another, with the moon "winning," the 

 result is the least pronounced high and low tides. 

 During neap tides, the highest waters are on the line 

 passing through the center of the moon and the cen- 

 ter of the earth, and the lowest waters on the line 

 perpendicular to the earth-moon line and passing 

 from the center of the earth to the center of the sun. 



It is easy to understand the high tide on the side 

 of the earth nearest the moon, because the water is 

 facing the moon. However, to understand the high 

 tide on the opposite side of the earth, one must ex- 

 amine differential attraction of gravity upon land and 

 water. In the case of the land, the rocks act as if they 

 were all at the earth's center. In other words, the 

 moon's gravitational effect is upon the center of the 

 globe. Therefore, the attraction of the moon upon 

 waters on the earth's opposite side is less than upon 

 the land; the waters are, in effect, 4000 miles farther 

 from the moon (and, of course, waters facing the 

 moon are 4000 miles closer than land, for the same 

 reason). Because there is less pull upon opposite 

 waters than upon the earth's center, a high tide is the 

 consequence of less attraction. One might think of 

 this as being the result of the earth's center being 

 pulled more than is the opposite water. This op- 

 posite-side high tide is almost as high as that on the 

 side nearest the moon. 



Although the pull of gravity explains a great deal, 

 tides are influenced by more than solar and lunar at- 

 traction. There is still the problem of why different 

 parts of the world have variations in tide height. To 

 understand these variations, one must first consider 



