REPEAT AFTER ME: Divergence aloft means convergence at the surface. This fact is central to the dynamics within the areas of fastest winds aloft. These are found in the streaks and cores of 100-plus-knot winds embedded within the jet stream, which encircles the poles and undulates as it flows from west to east (in the northern hemisphere). Owing to the tilt of Earth's axis, in the colder months of the year its average track runs anywhere from 35 to 50 degrees north latitude; in the warmer months it migrates farther north. Look on a 300-millibar (about 30,000 feet) chart and you can see its flow.
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