nean current direction should be 45° at the surface and that current 

 direction should rotate in a clockwise direction with increased depth. 

 Observations have shown that current direction does rotate in the direc- 

 tion indicated. This rotation to the right with increasing depth is 

 known as the Ehvan Spiral , and must be taken into account to understand 

 the relation between surface winds and wind-generated currents in the 

 sea. Shulman and Bryson (1961) have shown that this rotation of the 

 surface current to the right of the wind can be observed on Lake Mendota 

 in Madison, Wisconsin. Defant (1961) summarized many generalizations of 

 the Ekman theory aimed primarily at obtaining better descriptions of the 

 diffusion process. Defant also summarized many papers reporting empirical 

 evaluations of the theory. 



When the time -dependent response of the sea to a steady wind is 

 considered, it is found that water travels in a circular path making one 

 complete revolution in a time period equal to one-half a pendulum day. 

 The length of a half -pendulum day depends upon the angle of latitude, and 

 represents the period of rotation of the plane of rotation of a Foucault 

 pendulum. The radius of the circular path described by the water motion 

 is termed the radius of the inertia circle. It also depends on the angle 

 of latitude. These inertial motions in the sea were first explained by 

 Gustafson and Otterstedt (1932). Webster (1968) summarized most of the 

 observations of inertial period-motion in the deep sea from the time of 

 Gustafson to 1968. The importance of inertial motion has been demonstrated 

 in Lake Michigan where 54 current meter stations were established by the 

 Great Lakes -Illinois Rivers Basin Project of the Public Health Service in 

 1962. Holleyman (1966)' gives a map of station locations and an inventory 

 of the data collected. Verber (1965, 1966) gives several examples of 

 data collected on this network which shows the importance of inertial 

 circle motion. 



4. Wiad- Generated Surface Waves . 



In addition to currents, surface waves are also generated by wind. 

 If other things are equal, wave height and period will increase with 

 windspeed, with path length of the wind over the water, and with duration 

 of the wind. For low windspeeds, either path length or wind duration can 

 limit the growth of waves. When water depths exceed about one-half of 

 the wave length, the water motion due to waves consists of approximately 

 circular orbits in a vertical plane with the water moving in the direction 

 of wave propagation under a wave crest and in the opposite direction under 

 a wave trough. The speed of the water motion under a wave crest is 

 slightly greater than the speed under a trough. Because of this, a net 

 water motion in the direction of wave travel is present; this motion is 

 referred to as the mass transport. Its speed is proportional to the 

 square of the wave height. The existence of this wave -generated current 

 and a partial explanation was published by Stokes (1847) , and significant 

 extensions to the theory have been given by Longuet-Higgins (1953, 1958), 

 Longuet-Higgins and Stewart (1962, 1963, and 1964), Collins (1964), and 

 Piint (1962) . Mass transport is of little consequence in the open sea 

 where there is ample room for return flow. However, it can be significant 

 in shallow water near a beach. 



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