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