Even in the dog days of summer, the water in Lake Superior can quickly become dangerously cold.

The phenomenon is called upwelling, this happens when southwest winds push the warmer surface water offshore which allows deeper, colder waters to the surface.

The Large Lakes Observatory, which is part of the University of Minnesota Duluth shared some data from their meteorological buoys located on the Western arm of Lake Superior that shows how quickly conditions can change making swimming dangerous.

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In the graph below, you can see the data from the two buoys, one offshore, and one nearshore. You can see that while temperatures remained in the 65° - 70° range, the nearshore buoy dropped to temps around 50° - 55°.

According to the National Weather Service, "Survival time is greatly diminished for someone immersed in water below 70 degrees."

If you'd like to monitor the buoy data in realtime, visit The Large Lakes Observatory website here. Looking at the data, you can really see how the temperature can vary so much from day to day and hour to hour.