Here’s an early afternoon picture from the Port Sheldon buoy in Lake Michigan west of Holland MI. The wind as I write this at the buoy is dead calm (0.0 mph). You don’t see that too often.
The graph is the water temperature at the buoy. The top line is the surface water temperature, which has remained fairly constant over the past week from 67-70 degrees. The bottom line is the water temperature at 75 feet. Here you can see the temperature took a big dive last Friday due to upwelling – falling from the upper 60s to the mid-upper 40s. That occurs when offshore winds push warmer surface water toward the middle of the lake and allowing colder water to move toward the surface.
In this case, the cold water made it up to 75 feet, but did not make it to the surface. It did reach the surface at the shore, where the water temperature at Grand Haven State Park was a chilly 53 this Sunday morning.
Inland lakes are much warmer for swimming. Reeds Lake in East Grand Rapids shows a water temperature of 76 at 1:45 pm.