Upwelling.

The water around the Apostle Islands was 70°F yesterday afternoon and 46°F this morning — a 24° drop, so it says on the Apostle Islands page at Facebook.

The cause is something called upwelling. Steady southwest winds pushed the warm surface water away from shore, and to replace it, frigid water from deep in the lake rose up to take its place. Lake Superior is cold and deep, so there's always an icy reserve waiting just below the surface. Give the wind the right angle and it comes straight up to the top....  Upwelling usually reverses once the winds shift, so the warmth will likely creep back in a few days. But this is a classic reminder of what makes Superior, Superior.

"In those two minutes, you ask yourself existential questions about what time even is, what a body even is, what a feeling even is."

"It’s just a sensation, right? But knowing that pain — and there is pain — is just a sensation does not help you right now because that took three seconds to figure out and you still have a wild wagon-train trip to California to go. Did you mention that there’s a man with a ukulele there? He appears to work for this cold-plunge outfit, and he is wearing that dumb hat and quietly strumming — is that? — Leonard Cohen’s 'Hallelujah'? You love that song and have never enjoyed it less. Your hatred for this man only buys seven or eight more seconds, and as you cast your mind about, looking for something else to get you through, a strange thing happens...."

Writes Taffy Brodesser-Akner, in "I Survived a Cold Plunge and All I Got Was Everything I Ever Wanted/I resisted the trend until I couldn’t any longer" (NYT).

By the way, do you need a fancy cold plunge machine or a session at a cold plunge commercial establishment? Can't you just fill up your bathtub with water from the cold tap and maybe toss in the ice that's accumulated in the bin inside the refrigerator? That was a good question for Grok.