38 WAVE DIMENSIONS 



(he latter blows in the original direction. But an alteration in the di- 

 rection of the wind does not cause waves that are already in motion to 

 diverge at all from their original lines of advance, though it may 

 widely alter their surface contours, as described below (p. 51). It 

 is only when a wave encounters some obstacle, or comes into shoaling 

 water at an angle with the coast line, that the line of advance of any 

 part of its crest or trough is diverted, as described below (p. 15:")). 



The tracks along which wave forms progress also differ in another 

 very important respect from those along which the winds blow, for 

 they are not deflected measurably from their original courses by the 

 effects of the rotation of the earth, whereas winds, unless at the Equator, 

 are thus deflected, to the right in the Northern Hemisphere, to the left 

 in the Southern, so widely that they actually blow at only a very small 

 angle with the isobars (or lines of equal pressure) in the atmospheric 

 disturbances that gave rise to them. 13 The reason for this difference 

 is that the net advance of the water particles of which a wave is com- 

 posed is so small as to be negligible in this connection, whereas the 

 masses of air that compose the winds do advance bodily. 



The end result of the contrast, outlined above, between the directions 

 of waves and of winds is that the former parallel the latter, wherever 

 and whenever the isobars follow straight lines, or curves of very long 

 radius as is characteristic of the winter gales of the West Wind Belts 

 (fig. 6) . But the isobars often follow curves of shorter radius. Under 

 such conditions, the directions of advance of the waves, that are gen- 

 erated at successive points along the isobars, diverge from the latter, 

 whereas the winds blow along the isobars so that they veer away more 

 or less sharply from the lines of advance of the waves. The difference 

 in this respect, that may exist in different parts of an atmospheric dis- 

 turbance, is illustrated diagrammatically in figure 7, where the wave 

 direction would parallel the wind direction within the area between 

 the lines AB and A'B', because the isobars there are nearly straight. 

 And the waves generated there would be the largest, because it is there 

 that the effective fetch would be the longest. But the winds in other 

 sectors of the disturbance would veer away from the waves because 

 of the curvature of the isobars, so that a very complex series of cross 

 seas, of different generations and running in different directions, 

 would result. It is not known precisely how much the wind must 

 deviate from its original direction to give rise to a distinct new train 

 of waves. 



As a rule, too, an atmospheric disturbance does not long remain 

 stationary over the ocean, but advances in one direction or in another, 



13 The direction of the wind averages about 10° to the left of the isobars in the Northern 

 Hemisphere, 10 to the right of the isobars in the Southern Hemisphere. For a readable 

 explanation of the deflective effects of earth rotation, see Pettersen, Sverre. 1941. Intro- 

 duction to Meteorology. New York. p. 100. 



