Why water temperatures on Lake Michigan dropped nearly 20 degrees this week

You're hot, then you're cold. You're yes, then you're no, Lake Michigan.

In less than two days, water temperatures on West Michigan beaches dropped nearly 20 degrees this week — a common oceanographic phenomenon referred to as upwelling — before rising again.

Shortly after midnight on Sunday morning, a National Weather Service-monitored buoy near Port Sheldon recorded a water temperature of 70 degrees on Lake Michigan. Then, temperatures began to fall. At around 6:30 a.m. on Monday, the buoy read 51 degrees, a 19-degree drop.

"It's a good process unless you want to swim, because then you're going to get a shock of cold water," said Dr. Al Steinman, the Allen and Helen Hunting Research professor at the GVSU Annis Water Resources Institute in Muskegon.

Upwelling, Steinman says, occurs when surface winds push warm surface water away from the shoreline, allowing colder, deeper water to rise up from the bottom of the lake, thus resulting in the temperature drop felt this week in Michigan.

"Good for vegetation. Good for the plankton, which then translates up the food web and to the fish," Steinman said. "Nutrient-rich water."

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Hours after temperatures dropped along Lake Michigan beaches, though, they began to rise.

"Really no different than the swimming pool," Steinman said. "Once the sun comes out, the pool will warm up. The only difference is you don't have displacement in the pool."

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In time for Independence Day, water temperatures on the Great Lake have returned to the 60s, with the Port Sheldon buoy reading 64 degrees on Wednesday afternoon.

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