The Coriolis Effect: Making the Weather
Go Round
By: Jennifer Greer
Audience and Scope
The purpose of this technical definition and description is to give an
educational ride for anyone who is interested in learning more about this
physical phenomenon. Most people who would be reading this would not
know much about the effect itself or even some of the basic concepts that
describe it. This document will cover the basic meteorological and physics
based concepts that a person needs to understand the Coriolis Effect. By the
end of this description, a reader should be able to tell in the picture on the
left, where the hurricane is located by hemisphere.
Satellite Picture of a Hurricane
(Source: Hurricane Facts, hurricanefacts.com)
Introduction
The Coriolis Effect is a physical phenomenon that gives an apparent
deflection to objects moving in a straight path relative to a rotating surface.
It is commonly described in relation to weather and oceanic patterns. The
effect itself has little to do with causing weather, but its effects on earth
greatly matter to our weather behaviors and how the ocean currents move.
In order to properly understand this phenomenon, you must first understand
some basic physics and meteorology terminology.
In physics a frame of reference is used to determine the spatial location and
speed of an object. This means that for people in a car, the road appears to
be moving, but anyone standing on the sidewalk sees the car moving. The
second definition is that of a centrifugal force. This force is perpendicular to
the outward velocity of an object that is rotating which keeps the object
moving in a circular path.
Another important concept is Isaac Newton’s laws of physics. There are
four laws that Newton describes. The first is the law of inertia, an object in
motion tends to stay in motion. The second is the law of forces; an object’s
force equals its mass multiplied by its acceleration. Lastly, the third law is
that of reactions, all reactions have an equal and opposite reaction (See
picture on the right). All three of these laws will affect the way the Coriolis
Effect behaves.
In meteorology, a gyre is a system of rotating currents, this can be ocean
currents, vortexes in the air or even man-made. A geostrophic flow is a large
scale motion in the ocean or air that tends to occur perpendicular to the
pressure gradient.
Newton’s Third Law
(Source: Humor in Quotes,
humourinquotes.deviantart.com)
What is the Coriolis Effect?
The Coriolis Effect is caused by a rotating frame of reference. The force is
apparent because it is a consequence of inertia and does not have any direct
originating body. Return back to the idea of Newton’s Laws, the Coriolis
Effect falls perfectly into these. Without inertia, the rotation of the earth
wouldn’t affect motion above it. Without forces, there would be nothing to
“push” an object out of a straight line. Without equal reactions, the Coriolis
Effect would not have such a clean effect.
Look to the figure on the right, it shows the balance of forces on a flow
around a low-pressure area. The blue arrows are forces caused by a
difference in pressure, while the red arrows are forces due to the Coriolis
Effect. In this diagram the centrifugal force itself is negligible so we can
focus on the Coriolis force. As you can see by the figure, flow moving in
towards the center is deflected by the red Coriolis force into a circle. Flow
already in the circle is kept in that flow by the pressure-gradient force.
An example of how the Coriolis force behaves in the physical world is how
a pilot must plot his course. When flying over the earth, the Coriolis Effect
will move a straight path into a curved one. This means that a pilot must plot
a curved path to compensate for this. If the pilot flew in a straight line from
Florida to New York, his plane would end up over the Atlantic Ocean. So it
is important for him to correct his course along his journey to prevent this.
What does the Coriolis Effect do?
The Coriolis Effect plays a very important role in the movement of the
earth’s ocean and wind currents. Without it, currents would move in a
straight vertical line from the equator to the poles. This current would be
driven by the temperature and pressure differences across the globe. With
the Coriolis effect however, the currents take on the form seen below.
(Source: The North Coast Journal, northcoastjournal.com)
Diagram Depicting the Balance of Forces
(Source: The physics of rotation,
cleonis.nl)
Between the rotation of the earth and the geography of the earth, ocean
currents are formed in such a way as to allow passage across the oceans and
circulation of the oceans temperatures.
The Coriolis Effect also affects the way that a hurricane rotates. Because of
the deflection from the Coriolis Effect, hurricanes in the northern
hemisphere rotate clockwise while hurricanes in the southern hemisphere
rotate counter-clockwise. This comes from the earlier discussed effect of an
object moving along a straight path on a rotating surface will be deflected in
a certain direction from the rotation of the surface. The picture to the right
shows the directions of rotation for hurricanes.
What are the myths surrounding the Coriolis Effect?
Rotational Directions from Coriolis
Effect
(Source: cK-12, ck12.org)
Many people believe that the Coriolis Effect controls things that we see
every day at home, such as the direction the water will flow down a toilet
when flushed. This is not true. When the water rushes down a toilet’s drain,
its flow is affected on a much larger scale by things like friction and the
direction that the water is injected by the water tank.
While you can tell which hemisphere you are in by the direction of spin in a
hurricane, you cannot from a toilet. Another drain that people often think is
affected by the Coriolis Effect is a sink or bathtub drain. This is the same
case as your toilet, the angle at which the liquid enters the bowl and the
shape of the bowl is what really determines the spin in your draining liquid.
Conclusion
The purpose of the Coriolis Effect as said earlier is to provide the earth with
suitable currents both in the air and in the ocean. Without this effect the
currents would be forced by pressure and temperature gradients and there
would not be the mixing of currents as there is now. There would mostly
just be a strong equator to pole current and some mixing currents due to
geographical effects, but many of the benefits of the existing currents would
be lost.
Even if you believe yourself to be unaffected by this force, the global
impacts of the Coriolis Effect are impressive and useful. A major ocean gyre
is the North Atlantic Gyre, which is shown on the right. The major current
flow in this gyre moves up along the east coast of North America, across the
North Atlantic Ocean, down along the coast of France and northern Africa
and continues across the middle of the North Atlantic Ocean again to repeat
the cycle. The Coriolis Effect helps swirl the currents within this gyre to
keep it spiraling.
Vortex in a Toilet
(Source: Gephardt daily,
gephardtdaily.com)
North Atlantic Gyre
(Source: i-cool, i-cool.org)