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It all depends upon how the water was introduced and the geometric structure of the drain.
One can find both counterclockwise and clockwise flowing drains in both hemispheres. Some people would like you to believe that the Coriolis force affects the flow of water down the drain in sinks, bathtubs, or toilet bowls. Don’t believe them! The Coriolis force is simply too weak to affect such small bodies of water.
In his work “Sur les equations du movements relative des systems des corps” (1835) the French engineer Gaspard Gustav de Coriolis (1792-1843) first described this force. The Coriolis force is caused by the earth’s rotation. It is responsible for air being pulled to the right (counterclockwise) in the Northern Hemisphere and to the left (clockwise) in the Southern Hemisphere.
The Coriolis Effect is the observed curved path of moving objects relative to the surface of the Earth. Hurricanes are good visual examples. Hurricane air flow (winds) moves counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. This is due to the rotation of the Earth. The Coriolis force assists in setting the circulation of a hurricane into motion by producing a rightward (clockwise) deflection that sets up a cyclonic (counterclockwise) circulation around the hurricane low pressure.
What happens at the equator? The Coriolis force is too weak to operate on the moving air at the equator. This means that weather phenomena such as hurricanes are not observed at the equator, although they have been observed at 5 degrees above the equator. In fact, the Coriolis force pulls hurricanes away from the equator.
>Published: 11/19/2019. Author: Science Reference Section, Library of Congress
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This question would seem to be one of simple physics, and yet it continues to engender sharp disagreements. The main problem here is the division between theory and practice: whereas in principle the earth's rotation could affect the direction of draining water, in the real world that effect is probably swamped by other, less uniform influences.
Brad Hanson, a staff geologist with the Louisiana Geological Survey, presents the argument of why--in theory--water going down the drain would indeed spin in different directions depending on which hemisphere you're in:
"The direction of motion is caused by the Coriolis effect. This can be visualized if you imagine putting a pan of water on a turntable and then spinning the turntable in a counterclockwise direction, the direction in which the earth rotates as seen from above the north pole. The water on the bottom of the pan will be dragged counterclockwise direction slightly faster than the water at the surface, giving the water an apparent clockwise spin in the pan. But if you were to look at the water in the pan from below, corresponding to seeing it from the south pole, it would appear to be spinning in a counterclockwise direction. Likewise, the rotation of the earth gives rise to an effect that tends to accelerate draining water in a clockwise direction in the Northern hemisphere and counterclockwise in the Southern."
Fred W. Decker, professor emeritus of oceanic and atmospheric science at Oregon State University notes, however, that the Coriolis effect may actually have little to do with the behavior of real-world sinks and tubs:
"Really, I doubt that the direction of the draining water represents anything more than an accidental twist given by the starting flow. The local irregularities of motion are so dominant that the Coriolis effect is not likely to be revealed. An empirical test could help."
Robert Ehrlich, a physicist at George Mason University, expands on these ideas:
"Do bathtubs drain in different directions in the two hemispheres? If you had a specially prepared bathtub, the answer would be yes. For any normal bathtub you are likely to encounter in the home, however, the answer is no.
"The tendency of a circulation in a fluid to develop in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere can be traced to the earth's rotation. Imagine a cannon fired southward from any latitude above the equator. Its initial eastward motion is the same as that at a point on the spinning earth. This initial eastward velocity is less than that at a point later in its trajectory, because points closer to the equator travel in a bigger circle as the earth rotates. Therefore, the cannon shell is deflected westward (to the right), from the perspective of a person standing on the earth. A gunner firing a cannon northward would find that the shell is also deflected toward the right. These sideways deflections are attributed to the Coriolis force, although there really is no force involved--it is just an effect of being in a rotating reference frame.
"The Coriolis force accounts for why cyclones are counterclockwise-rotating storms in the Northern Hemisphere, but rotate clockwise in the Southern Hemisphere. The circulation directions result from interactions between moving masses of air and air masses moving with the rotating earth. The effects of the rotation of the earth are, of course, much more pronounced when the circulation covers a larger area than would occur inside your bathtub.
"In your tub, such factors as any small asymmetry of the shape of the drain will determine which direction the circulation occurs. Even in a tub having a perfectly symmetric drain, the circulation direction will be primarily influenced by any residual currents in the bathtub left over from the time when it was filled. It can take more than a day for such residual currents to subside completely. If all extraneous influences (including air currents) can be reduced below a certain level, one apparently can observe that drains do consistently drain in different directions in the two hemispheres."
Finally, Thomas Humphrey, a senior scientist at the San Francisco Exploratorium, discusses in more detail the reasons why we do not see the Coriolis effect at work in the bathroom:
"There is an African country near the equator where entrepreneurs have set up two toilets, one just north of the equator, the other just south of it. For a fee, they will allegedly demonstrate that the toilets flush in opposite directions. It is only for show, however; there is no real effect. Yes, there is such a thing as the Coriolis effect, but it is not enough to dominate the flushing of a toilet--and the effect is weakest at the equator.
"The telling comparison is between the magnitude of the Coriolis effect and the initial amount of angular momentum in the water--that is, how much is it spinning anyway, regardless of the earth's rotation. Coriolis acceleration at mid-latitudes is about one ten-millionth the acceleration of gravity. Because it is a very small acceleration, it needs a very long distance for it to produce an appreciable curvature--and hence directionality--to the motion. A toilet or sink is just not large enough. The Coriolis effect influences because wind velocities may be hundreds of times greater than the motions in a sink and because the distances involved are far larger than the tiny draining diameter in a sink or toilet.
"It is impossible to find a cup full of water that does not have some average net motion; it will always be going one way or the other, and that little amount of angular motion is enough to swamp the Coriolis effect. The net motion in the water becomes much more pronounced as the water is forced to move in toward the center of evacuation, causing the normally invisible flows in the water to become visible as the water nears the drain. The ultimate direction of that flow is random--it can go one way one time, the other way the next.
"If you run an experiment in your sink--fill the sink, then pull out the stopper--the water will almost always go down the same way, making you wonder if this is really a random effect. But you will find that the faucet is almost always off center or that there is some other asymmetry in the sink. As a result, filling the sink consistently gives it some net rotation in the same direction, which you see as the normal direction of evacuation. Toilets will always drain and fill the same way, for the same reason.
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Bowditch, N. 1995. American Practical Navigator. Bethesda, MD: Defense Mapping Agency Hydrographic/Topographic Center. pp. 873.
Duxbury, et al. 2002. Fundamentals of Oceanography, 4th edition. New York: McGraw Hill. pp. 344.
Lloyd, J.B. 1986. Eighteen Miles of History on Long Beach Island. Harvey Cedars, New Jersey. Down The Shore Publishing and the SandPaper, Inc. pp. 204.
National Oceanic and Atmospheric Administration (NOAA). 2005a. Ocean Explorer: Technology. Online at //oceanexplorer.noaa.gov/technology/technology.html.
National Oceanic and Atmospheric Administration (NOAA). 2005b. Rip Current Safety. Online at //www.ripcurrents.noaa.gov.
Pinet, P.R. 1998. Invitation to Oceanography. Sudbury, MA.: Jones and Bartlett Publishers. 596 pp. Sudbury, MA.
Ross, D. 1995 Introduction to Oceanography. New York: HarperCollins College Publishers. pp. 199-226, 339-343.
Rutgers University. Geography Department. Cartography homepage. Historical Maps of New Jersey Online at: //mapmaker.rutgers.edu/MAPS.html.
Thurman, H. 1994. Introductory Oceanography, 7th edition. New York: Macmillan Publishing Company. pp. 172-222.
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The speed at which waves approach the shore depends on sea floor and shoreline features and the depth of the water. As a wave moves toward the beach, different segments of the wave encounter the beach before others, which slows these segments down. As a result, the wave tends to bend and conform to the general shape of the coastline. Also, waves do not typically reach the beach perfectly parallel to the shoreline. Rather, they arrive at a slight angle, called the “angle of wave approach.”
When a wave reaches a beach or coastline, it releases a burst of energy that generates a current, which runs parallel to the shoreline. This type of current is called a “longshore current.”
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Longshore currents are affected by the velocity and angle of a wave. When a wave breaks at a more acute (steep) angle on a beach, encounters a steeper beach slope, or is very high, longshore currents increase in velocity. Conversely, a wider breaking angle, gentler beach slope, and lower wave height slows a longshore current’s velocity. In either case, the water in a longshore current flows up onto the beach, and back into the ocean, as it moves in a “sheet” formation.
As this sheet of water moves on and off the beach, it can “capture” and transport beach sediment back out to sea. This process, known as “longshore drift,” can cause significant beach erosion.
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As longshore currents move on and off the beach, “rip currents” may form around low spots or breaks in sandbars, and also near structures such as jetties and piers. A rip current, sometimes incorrectly called a rip tide, is a localized current that flows away from the shoreline toward the ocean, perpendicular or at an acute angle to the shoreline. It usually breaks up not far from shore and is generally not more than 25 meters (80 feet) wide.
Rip currents typically reach speeds of 1 to 2 feet per second. However, some rip currents have been measured at 8 feet per second—faster than any Olympic swimmer ever recorded (NOAA, 2005b). If wave activity is slight, several low rip currents can form, in various sizes and velocities. But in heavier wave action, fewer, more concentrated rip currents can form.
Because rip currents move perpendicular to shore and can be very strong, beach swimmers need to be careful. A person caught in a rip can be swept away from shore very quickly. The best way to escape a rip current is by swimming parallel to the shore instead of towards it, since most rip currents are less than 80 feet wide. A swimmer can also let the current carry him or her out to sea until the force weakens, because rip currents stay close to shore and usually dissipate just beyond the line of breaking waves. Occasionally, however, a rip current can push someone hundreds of yards offshore. The most important thing to remember if you are ever caught in a rip current is not to panic. Continue to breathe, try to keep your head above water, and don’t exhaust yourself fighting against the force of the current.
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Winds blowing across the ocean surface often push water away from an area. When this occurs, water rises up from beneath the surface to replace the diverging surface water. This process is known as upwelling.
This graphic shows how displaced surface waters are replaced by cold, nutrient-rich water that “wells up” from below. Conditions are optimal for upwelling along the coast when winds blow along the shore.
Upwelling occurs in the open ocean and along coastlines. The reverse process, called downwelling, also occurs when wind causes surface water to build up along a coastline. The surface water eventually sinks toward the bottom.
Subsurface water that rises to the surface as a result of upwelling is typically colder, rich in nutrients, and biologically productive. Therefore, good fishing grounds typically are found where upwelling is common. For example, the rich fishing grounds along the west coasts of Africa and South America are supported by year-round coastal upwelling.
Seasonal upwelling and downwelling also occur along the West Coast of the United States. In winter, winds blow from the south to the north, resulting in downwelling. During the summer, winds blow from the north to the south, and water moves offshore, resulting in upwelling along the coast. This summer upwelling produces cold coastal waters in the San Francisco area, contributing to the frequent summer fogs. (Duxbury, et al, 2002.)
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Since the age of exploration, mariners have needed to know the speed and direction (velocity) of ocean currents to steer their ships within harbors and along trade and exploration routes. A mariner needs to be able to measure the velocity of currents by observing distance, time, and direction.
The simplest method of determining the velocity of a current involves an observer, a floating object or drifter, and a timing device. The observer stands on an anchored ship with a timer. He or she then places the drifter (such as a piece of wood) into the water and measures the amount of time the drifter takes to move along the length of the ship. He or she then stops the timer after the object has traveled some distance, and measures that distance, noting the direction in which the object moved.
The observer then divides the distance the object traveled by the time it took the object to travel that distance, which equals the speed of the current. By combining the speed of the object with the direction in which it moved, the observer can then determine the current’s velocity. Ocean currents typically are measured in knots.
Although they still follow the same essential concept to measure ocean currents, mariners today use more accurate and sophisticated instruments. Today, drifters are often elaborate buoys equipped with multiple oceanographic instruments. Some are equipped with global positioning system technology and satellite communications to relay their position in the ocean back to observers on land. Other drifters submerge for long periods of time to measure the ocean currents at depth. The drifter occasionally rises to the surface to send a signal that relays its position.
All drifter measurements are termed “Lagrangian measurements,” named after mathematician Joseph Louis Lagrange (1736-1813), who first described the path followed by fluids. But current velocities can be measured another way as well—using “Eulerian measurements.” Named after Swiss mathematician Leonhard Euler (1707-1783), Eulerian measurements involve describing fluid flow by measuring the speed and direction of the fluid at one point only. In this method, an instrument is anchored in the ocean at a given location, and the water movement is measured as it flows past the instrument.
Measuring currents by Eulerian methods is becoming increasingly more common. One reason is that it is easier to retrieve these expensive but stationary instruments than it is to locate floating drifters.
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Thermohaline circulation drives a global-scale system of currents called the “global conveyor belt.” The conveyor belt begins on the surface of the ocean near the pole in the North Atlantic. Here, the water is chilled by arctic temperatures. It also gets saltier because when sea ice forms, the salt does not freeze and is left behind in the surrounding water. The cold water is now more dense, due to the added salts, and sinks toward the ocean bottom. Surface water moves in to replace the sinking water, thus creating a current.
This deep water moves south, between the continents, past the equator, and down to the ends of Africa and South America. The current travels around the edge of Antarctica, where the water cools and sinks again, as it does in the North Atlantic. Thus, the conveyor belt gets "recharged." As it moves around Antarctica, two sections split off the conveyor and turn northward. One section moves into the Indian Ocean, the other into the Pacific Ocean.
These two sections that split off warm up and become less dense as they travel northward toward the equator, so that they rise to the surface (upwelling). They then loop back southward and westward to the South Atlantic, eventually returning to the North Atlantic, where the cycle begins again.
The conveyor belt moves at much slower speeds (a few centimeters per second) than wind-driven or tidal currents (tens to hundreds of centimeters per second). It is estimated that any given cubic meter of water takes about 1,000 years to complete the journey along the global conveyor belt. In addition, the conveyor moves an immense volume of water—more than 100 times the flow of the Amazon River (Ross, 1995).
The conveyor belt is also a vital component of the global ocean nutrient and carbon dioxide cycles. Warm surface waters are depleted of nutrients and carbon dioxide, but they are enriched again as they travel through the conveyor belt as deep or bottom layers. The base of the world’s food chain depends on the cool, nutrient-rich waters that support the growth of algae and seaweed.
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