From 1958: Coriolis Effect Explainer

April 16, 2018

Above, creative explanation of the Coriolis effect from the Bell Telephone Science hour, 1958.

Co·ri·o·lis ef·fect
ˌkôrēˈōləs iˌfekt
  1. an effect whereby a mass moving in a rotating system experiences a force (the Coriolis force ) acting perpendicular to the direction of motion and to the axis of rotation. On the earth, the effect tends to deflect moving objects to the right in the northern hemisphere and to the left in the southern and is important in the formation of cyclonic weather systems.

Below, more contemporary take from PBS. Which works best?


8 Responses to “From 1958: Coriolis Effect Explainer”

  1. PBS’s explanation is more accurate.

    • dumboldguy Says:

      I agree. The Bell programs were a boon to science education way back when, and were shown in most science classes. The Bell piece starts out well but gets “lost” at the end. It IS more fun, though (or is that my old 1950’s upbringing speaking?)

  2. Ken Lassman Says:

    They are about two different topics: the prevailing winds and hurricane wind directions around the low pressure core. They each do an excellent job of explaining the two distinct manifestations of the Coriolis affect, but don’t really address how they are related to each other.

  3. rhymeswithgoalie Says:

    I think it was a mistake to use what people think of as a windborne object in the PBS example (a paper airplane) rather than, say, a cannonball.

  4. valuethefuture Says:

    I’m a bit late to this, but here’s my vote… The video from PBS is better because it mentions the actual cause of the Coriolis effect (sort of). The Bell video’s use of a carousel, though, is utterly misleading. It shows an illusion that has nothing to do with Coriolis. In fact, if the boy could throw the ball slowly enough, it would be clear that the moment it leaves his hand its apparent path actually arcs off to the left, before it comes round, heads right, and passes over the center of the carousel. The real Coriolis effect Does Not Do That.

    The PBS video could have been better, though, because to really get what they’re saying, you have to understand conservation of angular momentum. And the best intuitive way to present that to the general public is with a spinning ice skater. Pretty much everyone knows what it looks like when spinning ice skaters pull their arms in—they dramatically speed up—and when they throw their arms out, they slow down.

    So. Start with a mass of air in the northern hemisphere. When it’s not moving relative to the ground, it’s rotating (like everything else) in a circle of latitude that’s centered on the earth’s axis of rotation. The farther north it happens to be, the smaller the circle in which moves, and for a given angular momentum, the faster it moves. If something (like a low pressure system) starts to draw it north to a higher latitude, following the curvature of the earth makes it spiral in towards its center of rotation, and like a skater conserving angular momentum it speeds up relative to the earth and curves off to the east. A mass of air moving south does the opposite. Following the curvature of the earth as it moves south toward the equator, it spirals away from its center of rotation, slows down relative to the earth, and curves off to the west. (These are real effects and would be seen as such even from a non-rotating frame, off earth. They’re not a visual illusion caused by using a rotating frame to view something that’s actually seen from outside that frame to be going straight the whole time, as in the Bell video).

    And now it’s clear why the Coriolis effect is strongest near the poles and almost nothing near the equator. Travel 100 miles north or south near the equator and your distance from the earth’s axis and your angular velocity change very little. Do the same near the poles and they change by a lot.

    And now we can also see why hurricanes and typhoons don’t form exactly on the equator. They form in those sweet spots that are still close enough to the equator for the ocean to heat up and produce strong evaporative updrafts, pulling in air from hundreds of miles around, and yet far enough from the equator that a change in latitude will change its distance from the earth’s axis enough to appreciably affect its speed and deflect it’s path east or west. Whether east or west, the deflections are to the right in the northern hemisphere and to the left in the south.

    The result: As air all around a center of low pressure tries to move toward it, it’s deflected to the right in the northern hemisphere, merging into a counterclockwise flow around the center, and deflected to the left in the southern hemisphere becoming a clockwise flow around the center.

    So, apologies for the length, but yeah, Bell is just wrong (carousel fun notwithstanding!). PBS is basically right, but they could have done better.

    • wilddouglascounty Says:

      Thanks for injecting the effect of angular momentum into the conversation. In a cursory glance of other videos explaining the Coriolis Effect, the BBC video here incorporates both angular momentum and also incorporates the fact that the relative speed of the surface of a sphere slows down as you go from the equator to the poles. I think it does a better job than either of the choices initially presented because it covers all of the factors better.

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