The big chill: What causes dramatic changes in Lake Superior water temperatures

After a hot, sweaty hike, there’s nothing more refreshing than a dip in the lake at Julian Bay Beach on Stockton Island. If Lake Superior stays calm enough long enough, the shallowest water, warmed by the sun, can be pleasantly refreshing. Wade out farther and you’ll find the significantly colder water just a few feet below the surface. However, if you throw in some wind and waves, the water temperature can and does drop dramatically.

On a much larger scale, surface water temperatures in Lake Superior are always changing, as they are in Julian Bay. In late July surface water temperatures on the lake’s north shore dropped 20 degrees in just a few days, to near-record lows. The National Weather Service in Duluth reported the dramatic drop.

The Weather Service says the dramatically colder surface water near the North Shore was driven by off-shore wind and fluid dynamics. “Due to something known as Ekman transport, water in deep bodies of water like the ocean or Lake Superior move to the right of the wind direction,” the Weather Service said.

“With a southwesterly wind along the North Shore, for example, water moves easterly away from the coast, and deeper water rises to replace it. In Lake Superior, deeper water stays around 40°F year round, and when that mixes in, surface water temperatures can drop quickly.”

Deep water in Lake Superior stays around 40 degrees year round

As of July 31st, surface water temperatures are still much warmer around the Apostle Islands than they are on the North Shore, as you can see from this surface temperature map from the National Oceanic and Atmospheric Administration or NOAA.

The real-time Wisc-Watch wave height and temperature buoys, positioned among the Apostle Islands, confirm current surface water temperatures in the 60s.

Current conditions from https://infos.cee.wisc.edu/wisc-watch/.

60+ degree water may sound relatively warm, but water safety experts caution that hypothermia is a real risk for anyone in the water for too long. That’s why paddlers and others are advised to dress for the water temperatures and not the air. Showing Lake Superior the respect it demands often means wearing wetsuits, even on the hottest days of the year. It’s also really important to practice and know you can self-rescue if you capsize, or paddle with guides who can help you.

To learn more about safe paddling, watch our Sense of Adventure program, featuring Friends Executive Director Jeff Rennicke and certified kayak guide, photographer and author Maddy Marquardt. The National Center for Cold Water Safety also has some tips for paddling comfortably when the weather is warm but the water is cold.

To see real-time water temperature and wave conditions in the Apostle Islands National Lakeshore, visit the Water Information for a Safe Coast website, WISC-Watch. Enjoy the water – and be safe out there!


Jon Okerstrom
Jon Okerstrom

Jon Okerstrom is a Friends of the Apostle Islands National Lakeshore board member and digital and social media volunteer with a background in digital and television journalism, photography and graphic design. He knows what a treat it is to cool off in Julian Bay and to safely paddle the sea caves.