FactSentinel

Tropical cyclones, called hurricanes in the Atlantic and eastern Pacific, spin in opposite directions on either side of the equator. In the Northern Hemisphere they rotate counterclockwise, and in the Southern Hemisphere they rotate clockwise. This is a well-documented feature of atmospheric science, confirmed by decades of satellite imagery and observations from meteorological agencies such as NOAA and the World Meteorological Organization.

The cause is the Coriolis effect, which results from Earth's rotation. Air flowing toward a low-pressure center is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As air spirals inward toward the storm's low-pressure core, this deflection produces a counterclockwise circulation in the north and a clockwise circulation in the south. The effect is also why tropical cyclones rarely form within about five degrees of the equator, where the Coriolis force is too weak to organize a rotating storm.

A common misconception is that the Coriolis effect determines the direction water swirls in a bathtub or toilet. In reality, the force is far too weak to influence such small systems, whose drainage is governed by basin shape and residual motion in the water. The Coriolis effect matters only over large distances and long timescales, such as those of hurricanes and other large weather systems. Another point of confusion is terminology: the same type of storm is called a typhoon in the northwest Pacific and a cyclone in the South Pacific and Indian Ocean, but the rotation rule applies to all of them.