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Coriolis effect

Once air has been set in motion, you’d imagine that this air would continue moving in a straight line, right?

But the Earth isn’t flat – it’s curved! Okay, then we’d imagine that the air would move in a line “as straight as possible”, following the curve of the Earth’s surface. That path is called a great circle: the shortest path, between two points, along the surface of a sphere.

Yet when we watch a mass of air move, it always gets deflected, as if by some invisible force.

This invisible (fictitious) force is called the Coriolis force, after the French scientist Gaspard-Gustave Coriolis (1835) who first figured out what was really going on.

Yes, the air masses do follow a great circle – but the land underneath them is rotating away – because the entire planet is rotating.

So even though the air is going “as straight as possible”, us folks stuck to the Earth (everyone who isn’t on an overhead space station) see the air as if it is deflected away.

In the inertial frame of reference (upper part of the picture), the black ball moves in a straight line. However, the observer (red dot) who is standing in the rotating/non-inertial frame of reference (lower part of the picture) sees the object as following a curved path due to the Coriolis and centrifugal forces present in this frame.

– Wikipedia

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A hockey puck is launched from the north pole. As it heads south, the earth turns to the east, causing the puck to appear to deflect to the west as viewed by an earthbound observer.

Blue: Inertial Great Circle
Red: Path on rotating earth
Gray: Path on stationary earth


A hockey puck is launched from London toward the west, on a stationary earth. The natural great circle motion of the puck takes it toward the equator, not along the original line of latitude, which we might normally call west.

The great circle path also coincides with the line of sight toward the west (projected radially down to the earth’ surface). Thus we must conclude that Costa Rica is due west of London.

Blue: Inertial Great Circle

A hockey puck is launched from Vancouver toward the east.

The inertial great circle path of the puck takes it south of the great circle path that the puck would follow on a stationary earth.

The earthbound observer attributes this deflection to the centrifugal and Coriolis forces. Note that even the stationary earth path takes the puck south of the original line of latitude.

  Blue: Inertial Great Circle
Red: Path on rotating earth
Gray: Path on stationary earth

Cyclone (over India)

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The direction of the Coriolis effect differs in the northern and southern hemispheres


Does water go down the drain in the opposite direction, on the other side of the equator?

Well, if the sink was wide enough…. yes.  But the sink would have to be miles wide for the effect to be strong enough to affect the motion of the water.

Toilets and sinks are so tiny, that the Coriolis effect is swamped out by random motions in the water.



Now let’s see how the Coriolis effect affects global wind systems.


PPT: Air Pressure and Wind. Tarbuck Lutgens Earth Science


* What is air pressure? Measuring air pressure with a barometer
* Pressure gradients and isobars
* The Coriolis effect describes how Earth’s rotation affects moving objects
* Factors affecting wind: Friction
* Jet streams
* Cyclones and anti-cyclonic winds
* Weather and air pressure. Weather forecasting
* Airflow patterns: divergence and convergence
* Pressure centers and winds
* Circulation on a non-rotating, and a rotating, earth model
* Global winds: circulation on a non-rotating earth, and on a rotating earth
* The influence of continents
* Monsoons. Regional wind systems, Local winds, land and sea breezes, valley and mountain breezes
* Wind direction and the prevailing wind
* Wind speed and the anemometer






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