|MadSci Network: Other|
This is a really interesting question, and one that I, too, had trouble answering from the Web. The problem I had was that although several weather related websites explained it in terms of the Coriolis effect, they weren't very complete in explaining the effect. Most of them repeated the mantra: "in the northern hemisphere, the Coriolis force is to the right and in the south it is to the left" but never derived the the apparent force from first principles. Some of them used a merry-go-round explanation, but I still have not found an explanation that I feel is complete. The best I have found so far comes from Feynman's Lectures in Physics, Volume 1, chapter 19. I found this in my local Borders bookstore. The Great Teacher explained it in terms of conservation of angular momentum on a rotating sphere. From another explanation on one of the weather-oriented sites, I think it is helpful to remember that the strength of the Coriolis effect is related to the relative orientations of the vector of the earth's rotation and the vector of the object's (or air masses) motion. See note below. The Coriolis effect results in moving objects and air masses experiencing an apparent force that depends on latitude and velocity. In the Northern Hemisphere the apparent force will be to the right of the velocity vector and will increase with increasing velocity. It is to the left in the Southern Hemisphere. It will be present even if the velocity is in the same direction as the Earth's rotation. It is maximal at the Poles and zero at the Equator. As air masses move from high to low pressure zones there are two forces acting on them. The first is the pressure gradient force. The second is the Coriolis "force" which is really an "effect" or an "apparent force." The Coriolis effect increases with increasing speed so that the resultant force vector is no longer parallel to the pressure gradient, but almost perpendicular to it. So winds end up circling high and low pressure "hills" and "bowls". If they are descending into a low pressure "bowl" in the northern hemisphere, they are drawn toward the center but at the same time deflected to the right. So they end up circling the low pressure zone anti-clockwise. Conversely, the air sliding down a high pressure "hill" is deflected to the right as well and ends up going clockwise down the "hill." With those "facts" in hand, some of the website explanations begin to make some sense. Take a look at these site's diagrams. Once I understood that the real situation was more complex than the 2-dimensional merry-go-round analogy, some of these websites were more understandable:This wasn't a very convincing explanation but it does have some cute movies.
Excerpt from the last site above: "B. Coriolis Force: "Freely moving objects on the surface of the Earth appear to curve to the right in the northern hemisphere and to the left in the southern hemisphere due to the Earth's rotation. This effect is known as the Coriolis effect and it has a large influence on atmospheric and ocean circulation. The Coriolis effect arises from observing these motions from a moving reference frame: the spinning Earth. The object is actually moving in a straight line but the Earth where we are observing the motion is moving counterclockwise so the object appears to be veering away from us in a clockwise direction or to the right in the northern hemisphere and to the left in the southern hemisphere. The magnitude of the Coriolis effect increases from zero at the equator where the surface of the Earth is parallel to the spin axis to a maximum at the poles where the surface of the Earth is perpendicular to the spin axis. (Administrators note: As best as I can tell, the magnitude of the Coriolis force depends on the cross-product of the velocity vector with the angular velocity vector of the Earth. Cross-products of parallel vectors will be zero, so there will be no Coriolis effect at the Equator. They will be maximal at the Poles since the vectors will be perpendicular.) "The effect of the Coriolis Force is to produce the northeast and southeast trade winds in the latitudinal band between the equator and 30° north and south respectively. The surface circulation traveling from high pressure in high latitudes to low pressure at the equator is turned to the right (west) and left (west) by the Coriolis force in the north and south hemispheres respectively. The result is a wind that blows from the northeast to the southwest in the northern hemisphere (called the northeast trade wind) and from southeast to northwest in the southern hemisphere (called the southeast trade winds). This pattern is disturbed in the higher latitudes, primarily due to the increase in the Coriolis force which produces vortices or circular motions of the atmosphere called cyclones when rotation is towards a low pressure center or anticyclone when motion is away from a high pressure center. Also important is the distribution of land masses which heat up and cool down at different rates from the ocean and therefore complicated simple circulation patterns for a uniform surface." This question was a hard one for me. Since you didn't complete the form, I have no way of knowing if you are an adult whose background includes vector analysis or a high school student for whom this is all Greek. (Coriolis was Greek according to one of my sources, but he was actually French-born and published in French.) See what you can make of all this anyway. Respectfully submitted, David Winsemius, MD The Other Administrator.
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