54 EFFECTS OF WINDS AND OF 



Note that this is due to the fact that the nodal line for a south wind is near 

 the extreme southern end of Lake Michigan, as shown on plate 5. 



The same apparently anomalous condition is also found at Harbor Beach 

 (see plate 6) . For a south wind the nodal line is near to Port Huron, at the 

 extreme southern end of Lake Huron. 2* for a south wind is +0.35 at 

 Harbor Beach, and a south wind raises the water surface at Harbor Beach. 



THE WIND EXPONENT. 



The greater the velocity of the wind blowing over a lake the more rapid 

 will be the drift of the surface water to leeward. The larger the surface 

 drift to leeward the greater will be the return current to windward ultimately 

 produced by gravity after the steady regime has been established. The 

 greater the return current the steeper will be the surface slopes of the water 

 and the greater the disturbance of elevation of the surface at any given point 

 for a wind from a given direction. 



As a first approximation, it might be assumed that the rate at which the 

 wind delivers water to leeward in the surface drift is proportional to the 

 velocity of the wind. If so, the velocity of the return gravity current to 

 windward would be proportional to the wind velocity. As shown by the 

 Chezy formula, (52) , page 40, the surface slope for a steady current produced 

 by gravity is proportional to the square of the velocity of the current. The 

 wind effects, as disturbances of elevation of the water surface at a gage, are 

 proportional to the slopes. Hence, on the first approximation suggested, 

 the wind effects would be proportional to the squares of the wind velocities, 

 and the exponent of h in the wind-effect formula (51), page 39, would be 2.0. 



But the validity of the approximate assumption suggested is decidedly 

 uncertain. So far as the writer knows, there is no proof, theoretical or 

 observed, that the drift of the water to leeward at the very surface is 

 proportional to the wind velocity, though that seems to be a plausible 

 assumption. 



The depth to which the surface drift extends is probably a function of the 

 wind velocity. With higher wind velocities, the drift at the very surface 

 will certainly be more rapid, and the depth to which the drift extends will 

 probably be greater than during light winds. If this is the case, then the 

 total drift to leeward in a given wind, expressed in volume per unit of time, 

 the Q of page 40, will be a different function of the wind velocity than is the 

 velocity of the drift at the very surface. 



The character of the water surface on which the wind acfs to drive the 

 water to leeward varies greatly for different wind velocities. It varies 

 greatly in roughness, and the roughnesses are themselves in motion at rates 

 which are in various relations to the wind velocities. During very light 

 winds the water surface is relatively very smooth, broken only in general by 

 wind ripples. With moderate winds, say 10 to 15 miles per hour, there is a 

 decided roughness in the form of wind waves, which are traveling to leeward 

 at a rate not differing greatly from the velocity of the wind. The wind has a 



