Hurricane rotation

[swmochic]

Which in direction do hurricanes and tropical storms rotate? What causes the direction of rotation and do they ever rotate backward from their usual rotation?

[jfranklin]

Counter-clockwise in the northern hemisphere, and clockwise in the southern, and no, they don't ever rotate "backwards".

The explanation is complicated, but I'll try. The direction of rotation is basically a consequence of the earth's rotation, and the fact that a location on the equator is moving faster in its rotation than a point near the pole (after all, a point on the equator must cover a distance equal to the earth's circumference in 24 h, while a point just off the pole barely moves at all). So imagine an air parcel at the equator moving toward an area of low pressure located to its north. It starts its northward journey with the high eastward rotational velocity of the equator, which the parcel retains as it moves northward. But this air parcel soon finds itself surrounded by air parcels that do not have such a high eastward motion, and so the parcel ends up deflecting to the east of them. So what ends up happening is that motion initially from high to low pressure ends up being a counter-clockwise rotation around the low pressure.

This eastward deflection is called the Coriolis acceleration.

James