A few days ago, the temperature reached 98F/37C in Saint Paul, with high humidity. Any trip outside was a swim through the air, desperately making as few waves as possible. But during all of this, I was always heartened to know that very cold air was only about 10 miles away.

Not across town, that is, but up. The temperature is as low as -50C directly over our heads in a mysterious place called the “tropopause” before you hit the stratosphere. It gets that was because of some unique features of living on a wet planet.

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