124 EFFECTS OF WINDS AND OF 



wise as to maintain a moderately close approach to the condition of equilib- 

 rium, in which case the proportionality factors would tend to be unity. 



The magnitude of the barometric effects on a given lake is dependent 

 upon the position of that lake in regard to prevailing storm tracks. The 

 principal storm tracks for the United States pass near the Great Lakes. 

 Hence, there are frequent cases of large and rapidly changing barometric 

 gradients over these lakes. If lakes of the same shape and size in every 

 respect existed in some tropical region in which fluctuations in barometric 

 gradients were small, as a rule, the barometric effects would be correspond- 

 ingly small. The barometric effects on Gatun Lake, at the Panama Canal, 

 are probably relatively small, for the reason that the fluctuations in baro- 

 metric gradients are small. 



GENERALIZATIONS AS TO WIND EFFECTS. 



The wind effects upon the elevation of the water surface at a given point 

 are dependent to a small extent upon the distance of that point from the 

 nodal line. Note the function which L, the distance, plays in formula (59), 

 page 43. 



The wind effect at a given point is dependent largely upon the average 



value of the inverse cube of the depth, ~, between that point and the nodal 

 line. Consult formula (59). This average value is dependent mainly upon 

 the smaller depths involved. Dividing the depth by 10 multiplies ~ 



by 1,000. Hence, in making a first estimate of the probable magnitude of 

 the wind effects at a given point, attention should be riveted mainly upon 

 the shallow portions of the lakes which lie between the point and the nodal 

 line. 



The nodal line for each direction of wind tends to be located close to the 

 shallow parts of a lake which is unsymmetrical as to depths. Parts of the 

 nodal line tend to cross the mouths of bays of small depth. Note the 

 positions of the nodal lines on Lake Erie, and the several cases shown in 

 plates 2, 5, and 6, in which detached portions of a nodal line cross bays 

 tributary to Lake Michigan-Huron. There are several other such cases 

 on Lake Michigan-Huron, which could not be shown clearly on the small- 

 scale illustrations of this publication. 



The wind effects tend to be very large in bays of small depth, both be- 

 cause of the small depths involved and because of the fact that cross return 

 currents, with a component at right angles to the wind direction, such as 

 are referred to on page 43, which otherwise would hold down the wind 

 effects, are inhibited by land intervening between the bay and the open 

 lake. In making a first rough estimate of the wind effect at a point on a 

 bay it is important to note carefully the extent to which such cross currents 

 are inhibited for winds in each direction. 



