## Encyclopedic Entry

Storms in the north swing counter-clockwise: the Coriolis effect. Storms in the south swing with the clock, and winds tend to pass to the left!

Photograph courtesy NASA, Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC

Winds blow across the Earth from high-pressure systems to low-pressure systems. However, winds don’t travel in a straight line. The actual paths of winds—and of ocean currents, which are pushed by wind—are partly a result of the Coriolis effect. The Coriolis effect is named after Gustave Coriolis, the 19th-century French mathematician who first explained it.

The key to the Coriolis effect lies in the Earth’s rotation. The Earth rotates faster at the Equator than it does at the poles. This is because the Earth is wider at the Equator. A point on the Equator has farther to travel in a day.

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 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. The wind is like the ball. It appears to bend to the right in the Northern Hemisphere. In the Southern Hemisphere, winds appear to bend to the left.

In the Northern Hemisphere, wind from high-pressure systems pass low-pressure systems on the right. This causes the system to swirl counter-clockwise. Low-pressure systems usually bring storms. This means that hurricanes and other storms swirl counter-clockwise in the Northern Hemisphere. In the Southern Hemisphere, storms swirl clockwise.

Fast-moving objects such as airplanes and rockets are influenced by the Coriolis effect. Pilots must take the Earth’s rotation into account when charting flights over long distances. This means most planes are not flown in straight lines, even if the airports are directly across the continent from each other. The line between Portland, Maine, and Portland, Oregon, for instance, is very long, and fairly straight. However, a plane flying from Portland, Oregon, could not fly in a straight line and land in Portland, Maine. Flying east, the Coriolis effect seems to bend to the right, in a southerly direction. If the Oregon pilot flew in a straight line, the plane would end up near New York or Pennsylvania.

Military aircraft and missile-control technology must calculate the Coriolis effect for similar reasons. The target of an air raid could be missed entirely, and innocent people and civilian structures could be damaged.

The Earth rotates fairly slowly, compared with other planets. The slow rotation of the Earth means the Coriolis effect is not strong enough to be seen in small movements, such as the draining of water in a bathtub.

Jupiter, on the other hand, has the fastest rotation in the solar system. On Jupiter, north-south winds are actually transformed into east-west winds, some traveling more than 610 kilometers per hour (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 among the planet’s clouds. The boundary between these fast-moving winds can create strong, swirling storms, like the Great Red Spot.

Closer to home, you could observe the Coriolis effect if you and a friend stood on a rotating merry-go-round and threw a ball back and forth. To you and your friend, the ball’s path would appear to curve. Actually, the ball would be traveling in a straight line. You and your friend would be moving out of its path while it is in the air. A third person, standing on the ground near the merry-go-round, would be able to confirm that the ball travels in a straight line.

## Vocabulary

Term Part of Speech Definition Encyclopedic Entry

air raid

Noun

attack, usually bombing, by aircraft.

axial tilt

Noun

angle between an object's axis of rotation and its orbital axis, perpendicular to the orbital plane. Earth's axial tilt is about 23.5 degrees.

axis

Noun

an invisible line around which an object spins.

Encyclopedic Entry: axis

boundary

Noun

line separating geographical areas.

Encyclopedic Entry: boundary

calculate

Verb

to reach a conclusion by mathematical or logical methods.

chart

Noun

type of map with information useful to ocean or air navigators.

Encyclopedic Entry: chart

civilian

Noun

person who is not in the military.

cloud

Noun

visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

Encyclopedic Entry: cloud

confirm

Verb

to establish the truth or accuracy of a statement.

Coriolis effect

Noun

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.

Encyclopedic Entry: Coriolis effect

Coriolis force

Noun

force that explains the paths of objects on rotating bodies.

counter-clockwise

Adjective

circular motion to the left.

current

Noun

steady, predictable flow of fluid within a larger body of that fluid.

Encyclopedic Entry: current

deflect

Verb

to alter from a straight line.

Earth

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

Encyclopedic Entry: Earth

Equator

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

Encyclopedic Entry: equator

Great Red Spot

Noun

enormous storm in Jupiter's Southern Hemisphere, which has been observed for more than 100 years.

Gustave Coriolis

Noun

(1792-1843) French mathematician and engineer.

high-pressure system

Noun

weather pattern characterized by high air pressure, usually as a result of cooling. High-pressure systems are usually associated with clear weather.

horizontal

Adjective

left-right direction or parallel to the Earth and the horizon.

hurricane

Noun

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.

Jupiter

Noun

largest planet in the solar system, the fifth planet from the Sun.

low-pressure system

Noun

weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms.

mathematician

Noun

person who studies the theory and application of quantities, groupings, shapes, and their relationships.

military

Noun

armed forces.

missile

Noun

weapon that is guided toward a target.

Northern Hemisphere

Noun

half of the Earth between the North Pole and the Equator.

North Pole

Noun

fixed point that, along with the South Pole, forms the axis on which the Earth spins.

Encyclopedic Entry: North Pole

perpendicular

Noun

at a right angle to something.

planet

Noun

large, spherical celestial body that regularly rotates around a star.

Encyclopedic Entry: planet

pole

Noun

extreme north or south point of the Earth's axis.

rocket

Noun

device that moves through the atmosphere by release of expanding gas.

rotation

Noun

object's complete turn around its own axis.

Encyclopedic Entry: rotation

solar system

Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

Southern Hemisphere

Noun

half of the Earth between the South Pole and the Equator.

storm

Noun

severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

technology

Noun

the science of using tools and complex machines to make human life easier or more profitable.

transform

Verb

to change in appearance or purpose.

vertical

Noun

up-down direction, or at a right angle to Earth and the horizon.

wind

Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

### Credits

#### Media Credits

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Melissa McDaniel
Erin Sprout
Diane Boudreau
Andrew Turgeon

#### Illustrator

Tim Gunther, Illustrator
Mary Crooks, National Geographic Society

Jeannie Evers
Kara West

Nancy Wynne

#### Producer

Caryl-Sue, National Geographic Society

#### Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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