Anyone who has ever looked at an image of a hurricane knows it spins.
Part of this is due to the center of low pressure — the "eye" — at the center of the storm. But it also has to do with physics.
In fact, tropical cyclones — the general name for the storms called typhoons, hurricanes or cyclones in different parts of the world — always spin counterclockwise in the Northern Hemisphere, and spin in the opposite direction in the Southern Hemisphere.
The reason is something called the Coriolis effect, or Coriolis force, named for the French mathematician Gaspard-Gustave de Coriolis, who published work on the effect in the 19th century.
It works this way: Like a record on a turntable, the earth spins at a different speed at the equator than it does at the North Pole.
The same is true of anything that spins or rotates — the outside edge of something (in this case, the equator) always spins faster than the inside edge.
If you placed a marble in the center of a flat plate and then tried to push that marble to the edge of the plate, the marble would move in a straight line, as long as the plate was still. But if the plate was spinning, the marble would follow a curved pattern as it traveled from the center to the edge.
Winds passing to and from the North and South Poles and the equator are subject to this effect.
Imagine if a person were to stand at the North Pole and throw a ball far enough to reach the equator — say, to a person standing in Quito, Ecuador — the ball would not actually reach that person because it would not travel in a straight line. Since the equatorial region is moving faster than the North Pole, the ball would end up to the west of its target — somewhere in the Pacific Ocean, probably.
The same thing would happen in the Northern Hemisphere if someone were to throw a ball from south to north, only the ball would end up east of its intended target.
This is what happens to the winds that travel to and from the poles. In the Northern Hemisphere, winds moving north are diverted eastward, and winds moving south are diverted westward. In the Southern Hemisphere, winds traveling toward the equator will move eastward, and winds traveling toward the South Pole will curve west.
When these winds collide, they will swirl clockwise in the south, and counterclockwise in the north.
A variety of factors influence how hurricanes form. First they require warm water and warm, moist air — abundant in the mid and southern Atlantic Ocean regions.
Warm water evaporates and rises, forming clouds and releasing heat into the air. As the air rises, it leaves an area of low pressure near the surface of the water. If the clouds continue to build up a thunderstorm will form, and that low pressure area can become more intense, drawing more moist air toward it and continuing to intensify the storm.
This is where the Coriolis effect comes in. If there were no Coriolis effect, air would simply rush into the low pressure center, "since nature abhors a vacuum, and a low pressure area is a partial vacuum," said Chris Landsea, a Science and Operations Officer at the National Hurricane Center.
"But because the Coriolis is acting on it, as winds start moving toward the low-pressure area, they are diverted around, and they start spiraling into the center, so it takes much longer for the low pressure area to fill," Landsea said. "At the same time, that is drawing more moist air, and that causes more thunderstorms, and you get a big feedback process going on."
Interestingly, scientists have never seen a hurricane form or track exactly on the equator because the area does not feel the Coriolis effect.
"The farthest south we have seen a hurricane or tropical storm form in the Atlantic is about 7 or 8 degrees north of the equator," Landsea said. "Right on the equator, the winds and low pressure areas don't feel the force of the Coriolis itself," Landsea said. "So one of the safest places from hurricanes in the tropics is right on the equator, because hurricanes never form there or track there."