Archive for the ‘Macroscale weather’ Tag

The Coriolis Effect In the Real World – A Tutorial (Part 2) – Cyclones & Anticyclones

Left Click To Enlarge Image

I suggest that, though there may be some repetition, please read Part 1 first.  To go to it quickly, either scroll down or click on this link:

https://cloudman23.wordpress.com/2008/10/02/the-coriolis-effect-in-the-real-world-a-tutorial-part-1/

The set of weather maps, provided by NOAA, shows the remains of Ike after it began to head toward the northeast. At this latitude there is a tendency for weather systems in the middle latitudes to travel generally from west to east.  Notice the cold fronts which indicate that Ike had changed from tropical to extratropical.  The cold fronts represent the leading edge of cooler air being thrown out of the anticyclone (high with rotation) centered over the Eastern Dakotas. That air is coming “down” from some component of the north whereas the air on the “warm” side of the cold fronts is coming up from some component of the south and is being thrown out of the anticyclone centered off Florida.  So, we have a cyclone (low with rotation), Ike, between two anticyclones.

The only alterations I have made to the first map are 1) cropping of the original, 2) labeling of the fronts 3) placement of the red L and the two blue H’s, and 4) darkening of two of the isobar values making it easier for you to read.

Isobars are imaginary lines, of course, and plot equal pressure.  For example, every point on the 1020 isobar was believed to have had a pressure of 1020 millibars at the time of observation. In many ways isobars are analogous to contour lines on topographic maps.  In fact, the two “highs” on a topographic map would be hills and the “low” would be a trough-shaped valley between the hills.  Sticking with that analogy, surface runoff water would tend to flow down the hillsides along a stream gradient (or gravity gradient) toward the lower valleys.  In the simplest of topographic and geological settings the water would flow down the hills in a radial pattern, just as air would flow out of the highs were it not for the Coriolis effect.  Though water flowing down hillsides in stream channels does not respond to the Coriolis effect, air flowing on the scale depicted here does by deflecting to the right of the pressure gradient direction.  So, in the second version of the weather map I have drawn blue lines of which two are comparatively long, showing the direction that the air would flow if the earth did not rotate on it’s axis.  But, remember, rotation of the globe causes the Coriolis deflection to the right in the northern hemisphere and to the left in the southern hemisphere.

It’s important to note that the deflection is with reference to the object or fluid in motion.  For example, if someone driving directly toward you turns right, he/she will have turned to your left.  Though that person’s turn would be to your left, it is still a right turn.  So, in northern Texas the green line shows that the air is moving to the right of the pressure gradient direction (light blue) – even though that green arrow points toward the left side of the map.  Just put yourself in the position of the air in motion and you should not have difficulties with this.

This, then, shows why air in the northern hemisphere moves clockwise around anticyclones and counterclockwise around cyclones.

Near the end of my first tutorial on the Coriolis effect I revealed that the following question had come up often during my teaching career: “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?”

The “obvious” left turning of air within hurricanes causes confusion among many people who are trying to understand air circulation – particularly if they are starting from scratch without knowledge of the Coriolis effect or the pressure gradient force and how the two engage in a tug of war.  I understand the confusion because seeing the shape of hurricane rain bands on radar and arcuate cloud band alignment clearly shows how the air turns left as it gets closer and closer to the hurricane’s eye wall.

Not too many years ago I heard a person who should know better, during a television weather report, explain to the viewing audience that hurricanes were so powerful that they did not respond to the Coriolis effect – referring to the “left turns” that she was showing on the satellite loop that was being projected.  I don’t know whether or not in some previous weather report she had mentioned the Coriolis effect but it seemed to me that might have been the case.  In her honest attempt to educate some of her audience, she gave them information which was entirely incorrect – perhaps because of misinformation given her or maybe some general assumptions she had made.  You see, it is the Coriolis effect that forces the counterclockwise rotation in the first place!

In this last illustration (below) you are looking at a satellite image of hurricane Fran (1996).  I have drawn blue pressure gradient lines and red air flow lines which clearly show the rightward deflection (in spite of the fact that the air does turn left as it approaches the eye wall.  Notice, however, that no matter where pressure gradient lines are placed along the air flow lines, the deflection of the “real wind” is always to the right of the pressure gradient line.

Once again, as in Part 1, I have not truly explained the Coriolis effect; I have merely described it and illustrated it.  I have not explored the nitty-gritty.  I have implied that it is only an apparent force.  You might want to explore other attempts to describe the Coriolis effect – perhaps via an Internet search.

Finally, in the interest of accuracy, I must admit that I have simplified to the point of leaving out some important forces that play a roll in determining the actual direction that air moves (from high toward low) in its quest to reach pressure equilibrium.  Among those are friction, centripetal force, and centrifugal force.  The conservation of angular momentum is an important consideration and accounts for the increase in wind velocity as the air gets closer to the storm’s center.

Left Click To Enlarge Image
Left Click To Enlarge Image

THE CORIOLIS EFFECT IN THE REAL WORLD – A TUTORIAL – PART 1


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.

http://ffden-2.phys.uaf.edu/213.web.stuff/ET_AL_web_site/movies.html

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.

If you want a head start on understanding a hurricane’s circulation, read this repeat of a 9-9-2008 post using hurricane Ike as an example