In Which Direction does a Typhoon Spin?

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• A tray containing water is fixed at a location offset from the center of the tub. There is a hole at the center of the tray, where a pencil is inserted as a plug.
• First, the turntable is rotated counterclockwise and coffee creamer powder is sprinkled on the water to make the flow more visible. In this case, the direction of rotation is the same as that of the northern hemisphere of the earth.
• When you remove the pencil, the water starts to drain and water inside the tray flows slowly toward the hole (point P). Similar to the experiment “Inward Flow Increases the Rotational Speed,” the speed of water increases as the radius (distance) from the hole (point P) decreases. As a result, the water rotates faster when it is closer to point P and the rotational velocity is inversely proportional to the radius (OP). The direction of rotation is the same as that of the tub and is counterclockwise.

(First and second year of college-level physics are required to understand the following explanation.)

• The principle behind the water rotating in the tray is the same as that of typhoons and low-pressure atmospheric systems; it can be explained using the “Coriolis force.” The Coriolis force is an apparent force that occurs when an object placed on another rotating object (such as a carousel or the earth) is moving relative to it. The direction of this force is perpendicular to the direction of the relative speed.
  (Magnitude of Coriolis force) = 2 x (Mass) x (Rotational speed) x (Magnitude of relative speed)
  This Coriolis force could explain why the rotation increases as you get closer to point P. However, the Coriolis force is an apparent force and does not exist in reality (It is an apparent force acting when you think of a phenomenon on a rotating system. In that respect, it is similar to the centrifugal force).
• The rotation of water inside the tray can also be explained by the “law of conservation of angular momentum.” The angular momentum of the water inside the tray is the sum of the following (1) and (2). (1) The angular momentum of the center point of the tray (point P) rotating around point O. (The angular momentum when the mass is the entire mass of the water inside the tray, the radius is OP, and the velocity is the speed of point P.) (2) The angular momentum of rotation around point P. (The angular momentum when the mass is the mass at each radius point, the radius is the distance from point P to those points, and the velocity is the speed relative to point P.) When you make observations from a point on the tub (the rotating system), before extracting the pencil, it looks as though the water inside the tray is not rotating at all. However, when you observe it from a stationary point outside the tray (an inertial system or stationary system), you can see that it is rotating around point P at the same speed as the tub. In other words, the angular momentum of (2) is not zero. When the angular momentum of (2) is a constant, as the water approaches point P, the rotational speed will be inversely proportional to the distance to point P. Therefore, a vortex (free vortex) forms in the same direction as the rotation of the tub.  
• This explanation is in essence the same as that using the Coriolis force. In this experiment, the Coriolis force is induced from the conservation of angular momentum around point P (center of mass), and it is an apparent force as seen from the rotating system. This is similar to the fact that a centrifugal force is an apparent force on a rotating system, but it is fundamentally based on the “law of inertia” (a property to maintain straight-line motion).
• With typhoons, tornadoes, and low-pressure systems, an updraft is formed in the center area. To make up for the air extracted through this updraft, the surrounding air will converge to the center area, causing an inward flow, just like the one we saw in the tray in this experiment. In those events, similar to what was seen in the experiment, a strong vortex forms around the center.
• In the southern hemisphere, the direction of rotation of typhoons and tornadoes is opposite to that in the northern hemisphere. They rotate clockwise.
• How would a high-pressure system rotate? In a high-pressure system, a downdraft forms in the center section, and a radially diverging flow forms at the ground. Angular momentum around point P (2) is maintained, and further outboard, the rotational speed decreases. When you are observing from a viewpoint on the rotating earth, it looks as though the system is rotating in the opposite direction, because it is rotating more slowly than the earth. Therefore, it looks like a high-pressure system is rotating in the opposite direction to a low-pressure system. It appears that high-pressure systems rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

[Keywords]	Law of conservation of angular momentum, Free vortex, Coriolis’ force
[Related items]	The Shorter the String, the Faster it will Turn, Inward Flow Increases the Rotational Speed, Free Vortex and Forced Vortex
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 52–61.