WRAL WeatherCenter Blog

A low pressure cell in the Northern Hemisphere turns CCW. Does it turn CW in the Southern Hemisphere? If so, what would happen if a hurricane dropped below the equator? Would it fall apart or become a High pressure cell? Or is it impossible for it to cross the equator? Just curious.

Posted June 14, 2008

MIKE MOSS SAYS:      Robert,     As you noted, air tends to circulate arounnd low pressure in the northern hemisphere in a counter-clockwise fashion, while it circulates clockwise around low pressure south of the equator. Near the equator, the coriolis force that causes this organization to occur is very weak and is zero at the equator itself. The question of what would happen to a hurricane crossing the equator is a tricky one - as far as I know, no tropical cyclone has been documented to have done so (partly because there is a natural drift toward the nearest pole that a tropical cyclone undergoes in the absence of strong larger scale steering flow - since there is rarely a strong larger scale steering flow near the equator, storms that get within a few degrees of the equator tend to drift back away from it of their own accord). A storm that was somehow forced across the equator seems likely to just keep going (so long as other conditions remain favorable) by way of inertia and conservation of angular momentum. As it moved farther away from the equator on the opposite side from where it formed, there should be an increasing tendency for the coriolis force to oppose the initial sense of rotation - this would seem likely to result in enough disorganization to weaken or dissipate the system, but I can't say that with absolute certaintly.


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