The Coriolis force is a fictitious or apparent force that acts on object that is moving relative to a rotating reference frame. In simple terms, it’s the force that you feel if you are moving along with a rotating object, like the Earth.
The Coriolis effect is the observable phenomenon that results from the Coriolis force. It’s why weather patterns, ocean currents, and even long-range artillery shots don’t go exactly where you’d expect them to if Earth were standing still.
Understanding the Coriolis force and Coriolis effect is crucial for various fields, such as meteorology, oceanography, and engineering. It helps explain complex phenomena, from hurricanes to the movement of airplanes and ocean currents.
- The Coriolis force is a virtual force that acts on a moving object relative to a rotating body. For example, a plane flying north over the rotating Earth is subject to the Coriolis force.
- The Coriolis effect is the observed consequence of the Coriolis force. In the case of a plane flying north, its movement is to the right.
- The Coriolis effect in the northern hemisphere is that the path of objects gets deflected to the right.
- In the southern hemisphere, the Coriolis effect is that that path of objects gets deflected to the left.
Historical Overview
The Coriolis force is named after Gaspard-Gustave de Coriolis, a French mathematician who first described this phenomenon in 1835. However, it’s worth noting that the general principles were hinted at long before by scientists like Giovanni Battista Riccioli, Francesco Maria Grimaldi, and Sir Isaac Newton. Coriolis extended these earlier ideas by explicitly showing how the laws of motion work in a rotating system, like a planet.
Simple Demonstrations
Understanding the Coriolis effect is easier if you see it for yourself.
- Merry-Go-Round: Sit on a spinning merry-go-round and try to throw a ball straight toward the center. You’ll notice that the ball seems to curve. From your perspective on the merry-go-round, this curving is due to the Coriolis force.
- Rotating Chair and Ball: Sit on a swivel chair and spin yourself while holding a ball. Throw the ball away from you, and you’ll notice a curve in its trajectory.
- Drawing on a Rotating Circle: Tack a paper circle onto a board. Place a ruler over the top of the circle through its center. Have someone rotate the circle around the pin while you draw a straight line along the ruler.
How the Coriolis Effect Works
The Earth rotates on its axis, causing different points on its surface to move at different speeds. At the equator, the speed is greatest, while it is zero at the poles. When an object moves freely over Earth’s surface, it tends to keep its initial speed and direction due to inertia.
Because of Earth’s rotation, an object moving in a straight line appears to follow a curved path, as viewed from Earth. The direction of this curve is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
The Coriolis force and effect apply to any rotating body, not just the Earth.
Where Is the Coriolis Effect Strongest? Weakest?
The Coriolis effect is strongest near the poles and weakest at the equator. This is because the rotational speed varies: the surface of the Earth moves faster at the equator and slower near the poles. As a result, the change in velocity experienced by a moving object is more pronounced near the poles, increasing the Coriolis force there.
Meteorology and Weather Patterns
The Coriolis effect plays a critical role in weather patterns. In the atmosphere, large masses of air are set into motion by differences in temperature and pressure. The Coriolis effect turns these air masses, creating cyclones and trade winds.
- Cyclones and Hurricanes: In the Northern Hemisphere, the Coriolis effect turns these large air masses to the right, creating a counter-clockwise spin. In the Southern Hemisphere, the effect is opposite, resulting in a clockwise spin.
- Trade Winds: These are the prevailing winds near the equator, formed due to the temperature difference between the equator and the poles. The Coriolis effect steers these winds to the west in both hemispheres.
What If the Earth Did not Rotate?
If the Earth did not rotate, the Coriolis force and effect would not come into play. The main driver for air movement would be the temperature difference between the equator and the poles. Air would flow directly from high-pressure to low-pressure areas, causing a much simpler, north-south air circulation pattern, with less variance in local weather systems.
Other Effects of the Coriolis Force
Aside from determining atmospheric circulation, the Coriolis force has other effect:
- Air Travel: Flight paths account for the Coriolis effect; otherwise, a plane flying a long distance would arrive at a different location than intended.
- Rockets and Artillery: Long-range missile systems also account for the Coriolis force to hit their targets accurately.
- Rifles: Even bullets are subject to the Coriolis effect over extremely long distances.
Coriolis Effect on Other Planets
Planets like Jupiter and Saturn, which have much faster rotations, exhibit more extreme forms of the Coriolis effect, creating more complex and powerful storm systems like Jupiter’s Great Red Spot.
Draining Water – A Common Misperception
Contrary to popular belief, the direction water spirals down a drain is not due to the Coriolis effect. This phenomenon is usually dictated by the shape of the drain and any existing water motion. The Coriolis effect is far too weak to affect such a small amount of quickly draining water. But, next time you take a bath, test this for yourself!
Coriolis Effect Formula
There are multiple formulas for the Coriolis effect, depending on the situation. One useful version calculates the acceleration to the right (northern hemisphere) or left (southern hemisphere) of an object gliding over the Earth’s surface (e.g., air, an aircraft, a train).
AC = -2ω x ν
The component that is orthogonal to the velocity over the Earth’s surface is:
ω ν 2 sin φ
Here:
- ω is the Earth’s rate of spin
- ν is the object’s velocity
- φ is the latitude, which is positive in the northern hemisphere and negative in the southern hemisphere
As viewed from above, the acceleration is to the right of the direction of motion in the northern hemisphere and to the left of the direction of motion in the southern hemisphere.
References
- Adams, Cecil (June 25, 2010). “Do Snipers Compensate for the Earth’s Rotation?“. Washington City Paper.
- Frautschi, Steven C.; Olenick, Richard P.; Apostol, Tom M.; Goodstein, David L. (2007). The Mechanical Universe: Mechanics and Heat (illustrated ed.). Cambridge University Press. ISBN 978-0-521-71590-4.
- Graney, Christopher M. (2011). “Coriolis effect, two centuries before Coriolis”. Physics Today. 64 (8): 8. doi:10.1063/PT.3.1195
- Persson, Anders (1998). “How Do We Understand the Coriolis Force?”. Bulletin of the American Meteorological Society. 79 (7): 1373–1386. doi:10.1175/1520-0477(1998)079<1373:HDWUTC>2.0.CO;2
- Trefethen, Lloyd M.; Bilger, R. W.; Fink, P. T.; Luxton, R. E.; Tanner, R. I. (September 1965). “The Bath-Tub Vortex in the Southern Hemisphere”. Nature. 207 (5001): 1084–1085. doi:10.1038/2071084a0