San José State University

applet-magic.com
Thayer Watkins
Silicon Valley
& Tornado Alley
U.S.A.

Why do paths of hurricanes, typhoons
and other tropical cyclones recurve?

Generally tropical cyclones, known variously as hurricane, typhoons etc., start out moving west then turn toward the pole of their hemsphere and finally end up moving east. This pattern is called recurvature. Here is the worldwide pattern.

From the above display it is clear that recurvature has something to do with the rotation of the Earth. Local wind and temperature conditions and local topography can influence the path and produce erratic elements to the paths but the general pattern has to have a geophysical explanation.

That explanation is that a cyclone has two components of angular momenta. One is with respect to its own spin axis and the other is with respect to the spin axis of the Earth. When a body with angular momentum, such as a gyroscope, is subjected to a torque it precesses; i.e., it angular momentum vector rotates. If a body with angular momentum is forced to precess then it is subject to a torque. A cyclone rotates with the Earth and its angular momentum vector is kept pointing vertically because of the rising of warm air in its eye (center). The resulting torque forces the cyclone toward the pole in its hemisphere. But as the cyclone moves toward the pole it gets closer to the spin axis of the Earth. The preservation of angular momentum then results in the cyclone moving faster with respect to the pole and hence starts moving east. Its path has thus recurved.

The details of the physics of the process are given in hurricane paths.

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