The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground as they travel long distances around Earth. The Coriolis effect is responsible for many large-scale weather patterns.
The key to the Coriolis effect lies in Earth’s rotation. Specifically, Earth rotates faster at the Equator than it does at the poles. Earth is wider at the Equator, so to make a rotation in one 24-hour period, equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour. Near the poles, Earth rotates at a sluggish 0.00008 kilometers (0.00005 miles) per hour.
Let’s pretend you’re standing at the Equator and you want to throw a ball to your friend in the middle of North America. If you throw the ball in a straight line, it will appear to land to the right of your friend because he’s moving slower and has not caught up.
Now let’s pretend you’re standing at the North Pole. When you throw the ball to your friend, it will again to appear to land to the right of him. But this time, it’s because he’s moving faster than you are and has moved ahead of the ball.
This apparent deflection is the Coriolis effect. Fluids traveling across large areas, such as air currents, are like the path of the ball. They appear to bend to the right in the Northern Hemisphere. The Coriolis effect behaves the opposite way in the Southern Hemisphere, where currents appear to bend to the left.
The impact of the Coriolis effect is dependent on velocity—the velocity of Earth and the velocity of the object or fluid being deflected by the Coriolis effect. The impact of the Coriolis effect is most significant with high speeds or long distances.
The development of weather patterns, such as cyclones and trade winds, are examples of the impact of the Coriolis effect.
Cyclones are low-pressure systems that suck air into their center, or “eye.” In the Northern Hemisphere, fluids from high-pressure systems pass low-pressure systems to their right. As air masses are pulled into cyclones from all directions, they are deflected, and the storm system—a hurricane—seems to rotate counter-clockwise.
In the Southern Hemisphere, currents are deflected to the left. As a result, storm systems seem to rotate clockwise.
Outside storm systems, the impact of the Coriolis effect helps define regular wind patterns around the globe.
As warm air rises near the Equator, for instance, it flows toward the poles. In the Northern Hemisphere, these warm air currents are deflected to the right (east) as they move northward. The currents descend back toward the ground at about 30° north latitude. As the current descends, it gradually moves from the northeast to the southwest, back toward the Equator. The consistently circulating patterns of these air masses are known as trade winds.
Impact on Human Activity
The weather impacting fast-moving objects, such as airplanes and rockets, is influenced by the Coriolis effect. The directions of prevailing winds are largely determined by the Coriolis effect, and pilots must take that into account when charting flight paths over long distances.
Military snipers sometimes have to consider the Coriolis effect. Although the trajectory of bullets is too short to be greatly impacted by Earth’s rotation, sniper targeting is so precise that a deflection of several centimeters could injure innocent people or damage civilian infrastructure.
The Coriolis Effect on Other Planets
The Earth rotates fairly slowly, compared to other known planets. The slow rotation of Earth means the Coriolis effect is not strong enough to be seen at slow speeds over short distances, such as the draining of water in a bathtub.
Jupiter, on the other hand, has the fastest rotation in the solar system. On Jupiter, the Coriolis effect actually transforms north-south winds into east-west winds, some traveling more than 610 kilometers (380 miles) per hour.
The divisions between winds that blow mostly to the east and those that blow mostly to the west create clear horizontal divisions, called belts, among the planet’s clouds. The boundaries between these fast-moving belts are incredibly active storm regions. The 180-year-old Great Red Spot is perhaps the most famous of these storms.
The Coriolis Effect Closer to Home
Despite the popular urban legend, you cannot observe the Coriolis effect by watching a toilet flush or a swimming pool drain. The movement of fluids in these basins is dependent on manufacturer’s design (toilet) or outside forces such as a strong breeze or movement of swimmers (pool).
You can observe the Coriolis effect without access to satellite imagery of hurricanes, however. You could observe the Coriolis effect if you and some friends sat on a rotating merry-go-round and threw or rolled a ball back and forth.
When the merry-go-round is not rotating, rolling the ball back-and-forth is simple and straightforward. While the merry-go-round is rotating, however, the ball won’t make to your friend sitting across from you without significant force. Rolled with regular effort, the ball appears to curve, or deflect, to the right.
Actually, the ball is traveling in a straight line. Another friend, standing on the ground near the merry-go-round, will be able to tell you this. You and your friends on the merry-go-round are moving out of the path of the ball while it is in the air.
The invisible force that appears to deflect the wind is the Coriolis force. The Coriolis force applies to movement on rotating objects. It is determined by the mass of the object and the object's rate of rotation. The Coriolis force is perpendicular to the object's axis. The Earth spins on its axis from west to east. The Coriolis force, therefore, acts in a north-south direction. The Coriolis force is zero at the Equator.
Though the Coriolis force is useful in mathematical equations, there is actually no physical force involved. Instead, it is just the ground moving at a different speed than an object in the air.
The Coriolis force is strongest near the poles, and absent at the Equator. Cyclones need the Coriolis force in order to circulate. For this reasons, hurricanes almost never occur in equatorial regions, and never cross the Equator itself.
flowing movement of air within a larger body of air.
a large volume of air that is mostly consistent, horizontally, in temperature and humidity.
a dip or depression in the surface of the land or ocean floor.
dark-colored band of clouds on Jupiter or Saturn.
line separating geographical areas.
light wind or air current.
type of map with information useful to ocean or air navigators.
person who is not in the military.
visible mass of tiny water droplets or ice crystals in Earth's atmosphere.
the result of Earth's rotation on weather patterns and ocean currents. The Coriolis effect makes storms swirl clockwise in the Southern hemisphere and counterclockwise in the Northern Hemisphere.
force that explains the paths of objects on rotating bodies.
circular motion to the left.
steady, predictable flow of fluid within a larger body of that fluid.
weather system that rotates around a center of low pressure and includes thunderstorms and rain. Usually, hurricanes refer to cyclones that form over the Atlantic Ocean.
to alter from a straight line.
imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.
material that is able to flow and change shape.
power or energy that activates movement.
Great Red Spot
enormous storm in Jupiter's Southern Hemisphere, which has been observed for more than 100 years.
left-right direction or parallel to the Earth and the horizon.
tropical storm with wind speeds of at least 119 kilometers (74 miles) per hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region.
structures and facilities necessary for the functioning of a society, such as roads.
largest planet in the solar system, the fifth planet from the Sun.
weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms.
to make or produce a good, usually for sale.
half of the Earth between the North Pole and the Equator.
fixed point that, along with the South Pole, forms the axis on which the Earth spins.
large, spherical celestial body that regularly rotates around a star.
extreme north or south point of the Earth's axis.
wind that blows from one direction.
object's complete turn around its own axis.
photographs of a planet taken by or from a satellite.
important or impressive.
gunman who fires from a concealed place.
the sun and the planets, asteroids, comets, and other bodies that orbit around it.
half of the Earth between the South Pole and the Equator.
severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.
winds that blow toward the Equator, from northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere.
path of an object moving in space under the influence of such forces as thrust, wind resistance, and gravity.
to change in appearance or purpose.
modern myth or piece of folklore.
measurement of the rate and direction of change in the position of an object.
repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures.
movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.