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EFFECTS OF ROTATION (CORIOLIS EFFECT)

The Coriolis effect results from the rotation or the Earth. Motions of winds, ocean currents, airplanes, rockets, projectiles, etc. are deflected by this rotation. These demonstrations will use turntables as analogs to the rotation. The analogy is not perfect since the Earth is spherical, but the demonstrations do illustrate the effects of rotation. Note that when looking down on the North Pole the rotation of the Earth is in a counterclockwise sense while looking down on the South Pole the rotation is in a clockwise sense thus the effects will be opposite in the two hemispheres.

A segment from the video "Unchained Goddess" (Rhino Home Video, 1800-432-0020) illustrates the Coriolis effect with a carousel and explains its effects on large scale winds. When a ball is thrown from one person to another on a rotating carousel the path of the ball appears to curve due to the rotation of the turntable even though the ball travels in a straight line when seen from off of the carousel.
 

Effect of Rotation on Streams of Water

Turntables (lazy Susan) covered with dark paper
cylindrical containers
Thin dark tape to mark a large diameter of the cylindrical containers
1 liter plastic bottles with the top portion cut off and a small hole a few cm from the bottom

Procedures:

1) Place the large cylindrical container on a turntable and place one of the 1 liter bottles in the center.  Fill the bottle with water and line up the exiting stream of water with the diameter line. Viewed from above the water exits in a straight line. Rotate the turntable both counterclockwise (like the Northern Hemisphere) and clockwise (like the Southern Hemisphere) and observe how the stream is deflected to the right and to the left respectively relative to the direction it is moving.
 
2) Place the large container on the turntable and place two bottles against the wall of the container an opposite sides of the diameter line. Fill. both with water and orient the streams to line up with the diameter line when viewed from above and to meet each other near the center. Rotate the turntable both counterclockwise (CCW) and clockwise (CW). The streams should now diverge. They should both curve to the right (CW) or left (CW) when looking in the direction of the flow.

3) Place the large cylindrical container on a turntable and place two bottles against the wall of the container on opposite sides of the diameter line. Fill both with water and orient the streams to more or less parallel the side but in opposite directions relative to the direction of rotation. The streams should look straight when observed from above. Rotate the turntable both counterclockwise (CCW) and clockwise (CW). The stream that is flowing in the same direction as the direction of rotation should curve toward the wall of the large container. The stream that is flowing in the opposite direction as the direction of rotation should curve away from the wall of the cylindrical container. In both cases the curvature is to the right for CCW rotation and to the left for CW rotation when looking in the direction of the flow.

Explanation:

Even though each little bit of water travels in a straight line relative to the non-rotating surroundings after it has left the hole, the bottle moves and changes orientation as the water is moving. The parts at the stream farther from the bottle have traveled longer and farther from the point that they exited the bottle than the parts of the stream closer to the bottle. The bottle has also moved more relative to the exit point of the farther parts of the stream than relative to the closer parts of the stream.
This causes the stream to be curved. When the turntable is rotating counterclockwise the curvature is always to the right relative to the direction of the stream. This is analogous to the Northern Hemisphere of the Earth. When observed from above the North Pole the Earth's rotation is counterclockwise. Flows in the Northern Hemisphere are diverted to the right due to the Coriolis effect The opposite is true in the Southern Hemisphere: the rotation is clockwise when observed from above the South Pole and flows are diverted to the left.

It is important to remember that these demonstrations an rotating disks are not perfect analogies to the Coriolis effect on Earth. Since the Earth is spherical the Coriolis effect varies from zero at the equator to a maximum at the poles. Also these demonstrations include another effect called the centrifugal effect so they do not show the pure Coriolis effect.

The Motion of Wind around High and Low Pressure Systems

This enables an understanding of wind directions around high and low pressure systems. Air tries to flow directly outward from high pressure systems, but  the Coriolis effect diverts that motion and causes the wind to spiral outward instead with a
significant amount of motion circling around the high pressure. In the Northern Hemisphere (NH) the wind blows in a clockwise spiral around high pressure (air moving away from the high is diverted to the right causing a clockwise motion around
the high) while in the Southern Hemisphere (CI) the motion around high pressure is counterclockwise (outward motion diverted to the left). The demonstrations above are analogous to the high pressure case: the flow away from the high pressure in the bottle is diverted to the right for counterclockwise rotation (NH) and to the left for clockwise rotation (SH).

With a low pressure system air tries to flow directly in toward the low. In the Northern Hemisphere the wind blows in a counter-clockwise spiral around low pressure (air moving toward the low is diverted to the right causing a counter-clockwise motion around the low) while in the Southern Hemisphere the motion around low pressure is clockwise (inward motion diverted to the left). This is seen most dramatically in tropical cyclones (known to us as hurricanes) which intensely spiral counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
     Remember that whether air goes up or down (rises or sinks) affects whether or not clouds will form. When air rises clouds can form if there is enough water vapor and the air rises far enough clouds can form since rising air expands and cools. When air sinks the increasing pressure as it goes down causes the air to contract and heat up so cloud formation is suppressed. High pressure systems are generally associated with clear skies since in high pressure systems the air tends to sink (although some high pressure areas over oceans have a layer of low clouds) while low pressure systems tend to have cloudy skies due to rising air.
     Air tends to move away from the high pressure center (spiraling outward due to the Coriolis effect). That air has to be replaced by other air and the only place it can come from is from above so the sir sinks. In the bottles for the Coriolis demo the water moves outward from the hole near the bottom and the water level in the bottle goes down; the water sinks. In the atmosphere the sinking air is replaced from above.
     In the case of low pressure systems air tends to spiral toward the low pressure. All that air has to go somewhere and the only way is up so the air rises.
     In a hurricane the air is moving inward rapidly because of the very low pressure and rises rapidly in what is called the eye wall surrounding the eye. In the eye air is actually sinking so the eye is clear. This happens even though the pressure is low because the air is coming in so rapidly toward the center that it cannot all squeeze into the center. the air sinks in the eye to replace some air from the eye that is dragged upward in the rising air of the eye wall.

Prevailing Winds
      The  Coriolis effect is also important to the presence of the Easterly Trade Winds(winds blowing from the east) in the tropics(0^o-30^olatitude) and the prevailing westerlies (winds blowing from the west) in the mid-latitudes(30o-60 latitude).
    Heating from the sun is the greatest at the equator and  least at the poles on the average. Loss of heat out to space causes a cooling of the poles. Thus air rises at the equator and sinks at the poles.  Without rotation and on a featureless earth the air that rises at the equator would travel towards the poles in the upper atmosphere and the air that sinks at the poles would travel toward the equator near the Earth's surface thus making a huge loop.  Winds would blow north and south .   This is illustrated in the figure below.
                    Rotation (still on a featureless Earth) has a couple of effects.  The first is to break up the huge equator- to- pole loops into three loops for reasons that will not be explained here.  This and the other effects are illustrated in the right figure.
                    The other effect is to divert the north south motions into east west motions as well.  The air at the surface in the tropical loop tries to go from the sinking regions near 30^o N and S latitudes toward the equator where the air is rising.  The Coriolis effect deflects that motion to the right in the Northern Hemisphere and to the left in the Southern Hemisphere to produce easterly winds (blowing from the east) in both hemispheres in the tropics.  These winds are called the Trade Winds.
               Air at the surface in the mid-latitudes tries to move from the regions of sinking near 30^o latitude towards the regions of rising air near 60^o latitude in both hemispheres. The deflection by the Coriolis effect produces the prevailing westerlies in the mid- latitudes in both hemispheres. The Earth of course has features such as oceans, continents and mountain chains, so there are further complications to these patterns. Still the easterlies in the tropics and the westerlies in the mid latitudes exist on
average. The banding of clouds into certain latitudes is related to these motions and a time lapse picture shows clouds in the mid-latitudes moving west to east. That is why we can often look to the west to know what weather we might expect. Weather
systems in our latitudes often move from west to east. We live in mid latitudes, but we know that the wind does not always blow from the west. That is due to the effect of low and high pressure systems which determine the winds over distances of hundreds of miles. The wind resulting from such systems can blow from any direction depending on where you are relative to the center of low or high pressure. The prevailing westerlies represent the average tendency for the winds to blow from west to east in the mid-latitudes. Also the westerlies are the drivers that tend to make pressure systems move from west to east in mid-latitudes. Although storms (low pressure systems) usually move from west to east in mid-latitudes, their approach is generally signaled by winds from the east or northeast or southeast because air is moving toward the approaching low and diverted by the Coriolis effect. On the east coast of the US some severe storms move from southwest to northeast.
These are called Nor'easters due to the strong winds from the northeast. Benjamin Franklin was one of the first to realize that although the strong winds were coming from the northeast, the storm was moving in a direction opposite to the winds. He
discovered this by comparing storm arrival times Philadelphia with those observed by his brother in Boston.
 

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