• Coriolis effect
    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

    Coriolis Force
    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.

    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.

  • 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.

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