This paper demonstrates, for the first time, occasional occurrence of wind-induced inertial motion in a basin of width order 10 km. The classical model, applied to oceanic examples, predicts frictionless horizontal flow deflected by the Coriolis force into a circular track, an inertial circle completed in one inertial period (16.5 h at the latitude of Geneva). Records of wind, current, and water temperature, obtained by LHYDREP from various stations and depths in Lake Geneva, were scrutinized for episodes of inertial response to impulsive wind stress. Although the measurements were made only 3 km from a shoreline which certainly influenced the inertial response, approximations to the classical model were occasionally seen, in which short (< 8 h) wind impulses were followed by relatively regular rotations of current direction in upper layers (< 20 m) with periodicities indistinguishable by visual inspection from 16.5 h. Four episodes of inertial response are illustrated. They occurred during spring at a time of weakly developed stratification. During winter, regular (and as yet unexplained) rotations of weak currents were also seen in deep water only (below 100 m) in the presence of weak stratification there.
During the summer season of fully-developed, whole-basin stratification, responses to storms also occasionally appeared as rotating currents, but with periodicities much less than inertial. We interpret them as signals from one or more basin-tuned internal seiches, to be further studied.