BAROMETRIC PRESSURES ON THE GREAT LAKES 125 



The magnitude of the wind effects on a given lake probably has little 

 relation to the natural periods of oscillation, or seiche periods, of that lake. 

 The fluctuation in wind impulses imparted to the lake by the surface drift 

 to leeward of the water is so rapid and so erratic, and the surface drift 

 normally remains nearly constant for so short a period, except during very 

 light winds, that the inertia effects, upon an average, probably tend about as 

 frequently to act counter to the wind impulse as to act in cooperation with 

 it. The one exception to the foregoing generalization is that the first half 

 wave of a new extremely large seiche tends frequently to be such as to 

 correspond to an exaggerated wind effect. 



GENERALIZATIONS AS TO SEICHES. 



The initial impulse or impulses which start a seiche in the Great Lakes 

 are probably much more frequently due to the wind than to the barometric 

 gradients. The wind impulses are in general much more sudden than the 

 barometric impulses. They are certainly much more frequent. The 

 barometric impulses are far from negligible, however, on the Great Lakes. 

 The impulse starting a seiche is clearly traceable to the barometric gra- 

 dients in some cases. The barometric gradients are probably especially 

 effective in starting the Strait of Mackinac seiches referred to on page 119. 



The development or non-development of seiches on a given lake is largely 

 dependent upon the depth of the lake, upon the uniformity of depth, and 

 upon the configuration of the shore. The greater the depth of the lake 

 the smaller will be the role played by friction and the smaller the true 

 damping of the seiche. The more uniform the depth over the greater part 

 of the lake the more will the lake tend to act as a single large seiche area and 

 the larger and more persistent will the seiches tend to be. Similarly, the 

 more regular the shore as seen in horizontal projection the more will the lake 

 tend to act primarily as one large seiche area, with correspondingly large 

 and persistent seiches. 



Normally, a lake surface has several natural periods of oscillation or 

 seiche periods. Each seiche period and each method of oscillation as a 

 seiche (lengthwise or crosswise) pertains to what may be called a seiche 

 area. Each seiche area is limited either (a) by portions of the shore of the 

 lake or (6) by a belt in which there is a steep slope in the bottom of the lake 

 separating areas of decidedly different depths. At boundaries of a seiche 

 area of class (a) there is no transmission of the oscillation beyond the 

 boundary. There is simply absorption and reflection. At boundaries of 

 a seiche area of class (6) there is much more or less effective reflection, de- 

 pending upon the steepness of the slope of the bottom and the amount of 

 the total change of depth, as well as upon the shape of such boundary as 

 seen in horizontal projection. To the extent that such reflection takes 

 place, the belt acts as a boundary. There is also at boundaries of class (6), 

 in addition to absorption of energy due to local movements, a decided 

 tendency for a part of the wave to be transmitted across the boundary into 



