THE SLOPE OF LAKE SURFACES UNDER VARIABLE WIND STRESSES.

The inclination of a lake surface caused by a wind stress shows a distinct time lag in adjusting itself to changing winds. To analyze this phenomenon use is made of the hydrodynamic equations which are simplified by integrating over the whole depth of the lake. It is found that the time required by the lake to respond to changing wind stresses depends on the length of the seiche periods. This dependence of the rapidity with which the lake level adjusts itself to the variable wind can be expected a priori. Particular attention is given to the case of a wind which changes its direction. Such a wind shift took place during the passage of the hurricane of 26-27 August, 1949, over Lake Okeechobee, Florida, when the wind turned through about 180 degrees during a time of roughly three hours. This turning of the wind was accompanied by a turning of the height contours of the lake surface, but the latter rotated more slowly than the wind direction so that for some time the wind blew parallel rather than perpendicular to the isohypses. It is shown that the theory explains this behavior of the lake surface.