## The Coriolis effect revisited

 E. Linacre and B. Geerts 4/'98

 A further explanation of the Coriolis effect (Section 11.4) is provided by Fig 1. The left part (a) shows a westerly wind blowing over Australia. Part (b) shows the wind blowing in the same direction (as seen in a fixed frame of reference) three hours later, when the Earth’s rotation has turned the continent around by 45 degrees. Now this identical wind appears to come from the southwest. In other words, the origin of the wind and its destination seem to have turned anticlockwise (i.e. ‘widdershins’), as though pushed to the left. This apparent leftwards torsion on the wind in the southern hemisphere is what we mean by the Coriolis effect. (It is rightwards in the northern hemisphere.) In reality the Coriolis force is largely balanced by the pressure gradient force, so that the wind direction typically changes little relative to the Earth over the course of 3 hours. This balanced wind is known as the geostrophic wind. Fig 1. Australia as seen from outer space at (a) time 0 and (b) time 0+3h.

Fig 2. Illustration of how an object on a straight path (from an outsider's perspective) is seen, by an observer on the clockwise-rotating platform, to rotate in a counterclockwise direction. In other words, from a 'southern hemisphere' perspective, the object is 'pulled' to the left.

Another handwaving attempt to explain the Coriolis force is given in Fig 2. The left diagram (a) shows an object flying along a straight path across a rotating platform, which can be thought of as the Earth with the South Pole in the center. Maintaining the analogy with the Earth, we assume that it takes about 12 hours to travel from left to right across the diagram. At various times (numbered 1 thru 20 on the diagram), an observer on the Earth sees this object in the directions shown in (a) and transposed to a common origin in (b). From the perspective of the observer on Earth, the freely moving object appears to be pulled to the left.

Yet another way of persuading people of the reality of the Coriolis effect was described by Durran and Domonkos (1). It involves a frictionless puck moving over a rotating parabolic dish, and two video cameras. The puck consisted of a 1.2-cm disk of dry ice, which automatically slides on a layer of carbon-dioxide gas.

Two Quicktime movie loops are included (2). The first one visualizes the Coriolis effect on a merry-go-round spinning in a clockwise direction (i.e., it applies to the southern hemisphere, Fig 1). The second one illustrates the development of a geostrophic wind balance.

References

(1) Durran, D.R. and S.K. Domonkos, 1996. An apparatus for demonstrating the inertial oscillation. Bull. Amer. Meteor. Soc. 77, 557-9.

(2) Source: WWW2010, both at the University of Illinois Urbana Champaign.