Monthly mean air and water temperatures and winds and currents were calculated for Lake Ontario forJuly 1972 from data collected during the International Field Year for the Great Lakes. The mean air temperature pattern was similar to the lake surface temperature pattern except in the northwestern part of thelake due to warm air around Toronto. Surface water temperatures showed warm water (>19°C) along thesouth-central shore and a cold pocket (16°C) in the northwest. A subsurface cold pocket also occurred nearthe' middle of the lake. Maximum perturbations of the mean temperature field occurred near the surface andthermocline at the lowest frequencies (<0.02 cycle h-1). The diurnal temperature signal was significant nearthe surface, and the inertial signal was significant near the thermocline.

Winds were from the west at about 3 m s-1. In response, the thermocline tilted from 5 m along the north-western shore to 14 m along the southern shore of the lake.

Monthly resultant currents indicated cyclonic flow at all depths and a northbound flow off Rochester in theregion of a bottom ridge. Observed currents were consistent with geostrophic calculations. Maximum currentperturbations occurred near the surface at the lowest frequencies and at the inertial frequency.

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