You know the feeling — you flip your pillow at 2 a.m. because one side is cool and perfect and the other is unbearably warm. Now imagine that pillow is the size of North America. That's basically how the jet stream works.
The atmosphere has a "cool side" and a "warm side" too. Cold, dense Arctic air sits over the top of the globe. Warm, lighter tropical air pools near the equator. And right where those two masses press against each other, about 6 to 9 miles above us, a ribbon of fast-moving wind rips across the sky from west to east.
That ribbon is the jet stream, and it acts a lot like the seam on your pillow. It's the boundary that keeps the cold side cold and the warm side warm. As long as that boundary holds, everyone stays comfortable — cool air stays north, warm air stays south, and weather moves along in an orderly fashion.
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