Some physicists prefer to use the term “Coriolis Effect” over Coriolis Force claiming that it is only an apparent force.  I tend to agree with that.

The Coriolis force is something that just about everyone in school learns about at one time or another.  To be sure, it is a topic in secondary school earth science and physics courses.  A low percentage of students enroll in the latter but a very large number are exposed to the former partly because in many school systems earth and/or environmental science is required.  Non-science majors in college enroll in earth and/or environmental sciences partly because it is perceived to be far easier than some of the other options – e.g. physics or chemistry.

The crux of the Coriolis force with regard to earth is that because our planet is rotating – objects and fluids in motion tend to deflect to the right in the northern hemisphere and to the left in the southern hemisphere.  The larger the circulation system the more there is likely to be an obvious response to the force.  Physicists, by the way, tell us that it is but an “apparent” force and that it is more accurate to call it the Coriolis “effect” which I intend to do from here on. There is no need to debate the term here but if you want to learn more about the Coriolis effect I suggest you use both terms in your search.

Earth’s period of rotation is once per day.  The rotational direction is from west to east.  If you looked at earth from “above” the north pole you would discover that the rotation is counterclockwise, and if you looked at the earth from “below” the south pole you would find a clockwise rotation.  If you have difficulty envisioning that “reversal” I recommend that you pick up an item and rotate it watching the rotation from one end of the axis.  Then continue rotating it in the same direction – don’t stop – but view it from the other end of the axis.  You should observe the reversal; from one end it will be counterclockwise and from the other end it will be clockwise.

SPECIAL NOTE: One of the greatest myths or misconceptions in physics is that the Coriolis effect determines the direction of rotation of water down a toilet or other drain.  That is absolutely untrue.  If you live in the United States and observe the direction the water moves down a toilet in your dwelling, then, crate it up and ship it to New Zealand and have someone install it there, upon flushing the water would go down the same way.

Next, look at the demonstrations shown on Quick Time at the following site.  Before you go there take note of this.  The first boy, wearing the blue headgear is rotating clockwise when the playground device is viewed from atop the axis of rotation so his setup is analogous to the southern hemisphere.  The other two boys (one with red headgear and the other bare-headed) are rotating counterclockwise so their setup is analogous to the northern hemisphere.

Hopefully you saw that the first boy’s ball went to HIS left as would be expected in the southern hemisphere (clockwise) and the other two experienced the opposite (to THEIR right) as would be expected for the northern hemisphere (counterclockwise).  You might want to scroll down a little further on that page and you will find a Quick Time animation of a ball deflecting to the right on a rotating table.  The rotation will not be apparent because the camera was fixed above the table and rotating at exactly the same period.  Since the ball deflects to the right you should correctly deduce that the rotation of the table was counterclockwise like the rotation of earth from the northern hemisphere point of view.

There are many examples of the Coriolis effect here on earth.  Cold air masses in the northern hemisphere rotate clockwise because of the right turn of the air which, after sinking toward the surface flows outward from the domal system’s high pressure core; this is a great example of an anticyclone.  But my favorite example of the Coriolis phenomenon, surprisingly, is not an atmospheric example.  It is the manner in which most of the water being carried by the oceanic gyres turns right in the northern hemisphere, especially when it reaches a continental margin and left in the southern hemisphere especially when it reaches a continental margin.  Observe the image below where I have removed all but the gyre components of oceanic surface circulation.

SPECIAL NOTE:  Though not discussed here, it is the general circulation of the atmosphere at or near the surface that creates these gyres and general circulation is guided by the Coriolis effect.  If you wish to learn more about the “general circulation” of the atmosphere, other terms are global circulation, planetary circulation, and large macroscale circulation.

Left click image to enlarge.

Have you noticed – I have not explained the earth’s Coriolis effect!  I have described it, I have linked you to visual evidence, I have described a meteorological example and shown you an oceanic example via a very generalized map of the 5 oceanic gyres.  But I have not provided an explanation other than indicating that it is caused by rotation of the earth. If you have stuck with me to this point, I want to entice you with an “issue” that has often remained unaddressed/overlooked by some teachers and learners of meteorology.  That is this:  If the Coriolis effect is an important influence in large scale weather systems, and since hurricanes are synoptic scale (a type of macroscale) system, why do hurricane winds turn left in the northern hemisphere and right in the southern hemisphere?  THAT WILL BE THE TOPIC OF MY NEXT TUTORIAL POST AND IT WILL BE COMING SOON.  Now, let’s look at a hurricane.




Inflow consists of the harder-edged clouds with sharp contrast – Outflow consists of the more diffuse cirrus and cirrostratus of the upper layer.


With this post you can either simply enjoy the high resolution image of Ike and leave it at that point or explore deeper into the dynamics of storms such as this. I have provided a large image (above) of Ike completed earlier today followed by smaller images contrasting the circulation below with the circulation above.  Thirdly, you will find below an image of what might seem like a very odd looking hurricane compared to what you have been looking at this season.  To see it, you must ask for more detail when the invitation appears at the end of the next paragraph. Because of only a small amount of sheer and other factors, Ike is a well-formed system.  And – if you can get past its destructive character you might marvel at its beauty.  I speak of it as though it were a living thing.  In many respects, it is a separate entity with a life of its own.  We even talk about the life cycle of such a storm.  We personify it with a name, in this case a male name.  Its winds spiral because of the Coriolis effect and the whole storm’s path responds to the Coriolis effect – sometimes that is evident, sometimes it is not.

If you find yourself confused about the Coriolis effect, please be patient because I intend to post an item soon, with an explanation of certain aspects of hurricanes which might seem to be contradictions when they are not at all.  Believe me, misunderstandings about the Coriolis effect does cause considerable confusion. Wind is the movement of air in response to a pressure gradient, almost always moving closer to lower pressure environs.  It doesn’t move perfectly with the gradient because of the Coriolis effect.  The degree to which the wind moves contrary to the gradient direction depends upon many factors which may be addressed at another time. Suffice it to say, for now, that the Coriolis effect is what causes the rotation both below and above. Just as water flows downhill in response to a slope gradient, air flows from high to low in response to a pressure gradient.  The hotter the air and the greater the specific moisture content of the air – the less pressure it exerts.  The second half of that statement is paradoxical for many people because, sensibly, hot humid air “feels” stifling and heavy to us.  This paradox also will be discussed in a separate post soon.

Ike’s winds spiral inward toward a low pressure center cyclonically (counterclockwise in the Northern Hemisphere) and pick up velocity the closer to the eye wall because of the “conservation of angular momentum.”  That is illustrated when a figure skater spins faster as more mass approaches the axis of rotation.  This is achieved by him/her pulling in both arms and legs to spin faster.  Air rises within the storm, cools by expansion (forming cloud droplets once the dew point temperature is reached) and at some point very far up it becomes stable and then diverges outward anticyclonically (clockwise in the Northern Hemisphere). The condensation of water vapor into cloud droplets within the rising air provides the primary source of energy for the storm, the latent heat of condensation.  That extra burst of heat makes the rising air even more unstable so that it has a tremendous positive buoyancy force – just as the heated air in a hot air balloon only far more exaggerated.  As it continues rising more air rushes in behind it.  Interestingly, that extra heat is “taken” from the sea surface in the evaporation process and then travels as potential energy “hidden” until the cloud-forming condensation occurs thus revealing it.  “Latent” means “hidden.”

If the “L” for low central pressure near the bottom of the storm and the “H” for high central pressure aloft puzzle you – the explanation is straight forward.  Please understand that in the graphic above comparing the incoming air of the storm with the outgoing air, the pressure at the surface is NOT lower than the pressure at the top of the storm.  You see, because I have a red “L” below and a blue “H” at the storm’s top you might think that to be true.  But – the pressure always drops with an increase in altitude.  So, why do I have an H above and an L below?  The pressure in the core region of the storm is low compared to the pressure away from the core at that same elevation.  Likewise, the pressure aloft at the middle of the storm is high compared to the pressure at the same elevation away from the middle.

There is considerable confusion about the terms cyclone/cyclonic and anticyclone/anticyclonic.  The latter set is often used incorrectly. Hurricanes’ surface flow in the Northern Hemisphere is counterclockwise and they are cyclonic.  Hurricanes’ surface flow in the Southern Hemisphere is clockwise and they too are cyclonic.  Hurricanes are predominately rotating lows.  However, as you have seen, at the top they are anticyclonic – (rotating highs) but it’s the cyclonic component that is dominant and does the damage.  So, if you hear of a low pressure system rotating the “wrong” way (as do some tornadoes and waterspouts) do not make the mistake of calling them anticyclones.  They can’t be because they are rotating lows and, I repeat, anticyclones are rotating highs.  A great example of a rotating high is a cold air mass that moves into the U.S. from Canada.  They rotate clockwise around a central core of high pressure.  Instead of air converging and rising as it does in a hurricane, anticyclonic air sinks and diverges. Hurricanes and extra-tropical cyclones (rotating lows that typically have fronts) are too large to rotate the wrong way for their respective hemispheres.  No such thing has ever been observed.  But, as previously mentioned, tornadoes and waterspouts can sometimes develop with a wrong-way rotation.

The image below is a cyclone even though it is rotating clockwise; it is very unusual hurricane located off the coast of Brazil.  Of course it is rotating clockwise because it is in the Southern Hemisphere where the Coriolis effect is opposite what it is in the Northern hemisphere.

Part 2 has now been posted.  To find it easily please either scroll up to the following post or click on this link:


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