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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.

* inert image of great circle

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

This invisible (really, 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?

No – the Coriolis effect is only large enough to move water over large scales (bodies of water miles across) Toilets and sinks are so tiny, that the Coriolis effect is swamped out by random motions in the water.





Chapter 19. 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

* El Niño and La Niña
* Where does wind come from?

Imagine Earth as a non-rotating sphere with uniform smooth surface characteristics.
Assume that the sun heats the equatorial regions much more than the polar regions.
In response to this, two huge convection cells develop.

On a hypothetical non-rotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form, as shown in Figure 9.
The heated air at the equator would rise until it reached the tropopause—the boundary between the troposphere and the stratosphere.
The tropopause, acting like a lid, would deflect this air toward the poles.
Eventually, the upper-level airflow would reach the poles, sink, spread out in all directions at the surface, and move back toward the equator.
Once at the equator, it would be reheated and begin its journey over again.
This hypothetical circulation system has upper-level air flowing toward the pole and surface air flowing toward the equator.

Figure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

Figure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

– – – – – – – – – –

If the effect of rotation were added to the global circulation model, the two-cell convection system would break down into smaller cells. Figure 10 illustrates the three pairs of cells that would carry on the task of redistributing heat on Earth. The polar and tropical cells retain the characteristics of the thermally generated convection described earlier. The nature of circulation at the middle latitudes, however, is more complex.

Near the equator, rising air produces a pressure zone known as the equatorial low—a region characterized by abundant precipitation. As shown in Figure 10, the upper-level flow from the equatorial low reaches 20 to 30 degrees, north or south latitude, and then sinks back toward the surface. This sinking of air and its associated heating due to compression produce hot, arid conditions. The center of this zone of sinking dry air is the subtropical high, which encircles the globe near 30 degrees north and south latitude. The great deserts of Australia, Arabia, and the Sahara in North Africa exist because of the stable dry conditions associated with the subtropical highs.

– – – –
We now allow the earth to rotate.
As expected, air traveling southward from the north pole will be deflected to the right.
Air traveling northward from the south pole will be deflected to the left.

However, by looking at the actual winds, even after averaging them over a long period of time, we find that we do not observe this type of motion.

In the 1920ís a new conceptual model was devised that had three cells instead of the single Hadley cell.
These three cells better represent the typical wind flow around the globe.

Idealized, three cell atmospheric convection in a rotating Earth.
“Three cell” being either three cells north or south of the equator.
The deflections of the winds within each cell is caused by the Coriolis Force.

Figure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001

Figure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001



Jet streams



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