Why a hurricane and bathwater always spin counter-clockwise … er, one of those, anyway: As pilots, we are students of the atmosphere — our well-being can often depend on our ability to “read” what we see around us (as well as what’s printed in weather reports). If flying becomes, to whatever extent, a part of your life, it soon becomes a part of the lives of your friends and family; sometimes we’re approached with “why” questions about what we do, and one topic of mystery is Coriolis force. So, here’s ‘why’…
WHAT AND WHO: Coriolis force “explains” why air circulates clockwise around an area of high pressure, and counter-clockwise about a low (in the Northern Hemisphere — opposite down under). You heard about it in ninth grade science class, and you probably saw it again in ground school when the instructor covered the unit on weather. OK Mr. Wizard. Who was Gaspard Coriolis? (answer: a 19th century French engineer and mathematician who lived from 1792 to 1843) And does a bathtub REALLY drain counter-clockwise in the land of the billabong?
THE FORCE THAT DOESN’T EXIST: That’s right, it’s make-believe … sorta. Coriolis force is an “apparent” force; it cannot cause a motion. Rather, it is caused by a motion, and a special kind of motion: rotation. We should actually be calling it something like the Coriolis “deflection.” Here’s the classic explanation…
WHAT IT DOES
Example (Part 1): Remember record players? There was this spinning plastic disc, usually horizontal, with a very fine spiral groove. In that groove rode a diamond needle that was wired to a transducer, generating audible sound. Let’s say we took a round piece of paper and speared it onto the spindle of a record player. If you turned the thing on, then drew a straight line from the center of the spinning paper towards its edge, the line would trace a spiral on the paper. The pencil in “inertial space” is traveling in a straight line, but relative to the spinning disc (and since record players rotate clockwise, when viewed from above) the line curves to the left.
Example (Part 2): Now imagine an ant on the turntable The ant would see the pencil point (and any other objects: clouds, etc.) veering off mysteriously!
Example (Part 3, the Finale): Now, the earth turns counter-clockwise as viewed from the North Pole, so that curve to the “left” becomes a curve “right” for those of us standing on the northern half of our turntable (the earth’s northern hemisphere.)
Example 2: Try a merry-go round, they usually rotate counter-clockwise (I know, fun facts to know and tell…) If you stand at its center and a friend stands on the edge and throws a ball to you, because the ball already has a much greater tangential velocity or “edge speed”, it will ‘curve’ off to your friend’s right. If you try to throw the ball to your friend, the ball will ‘curve’ off to your right. Neither one of you will be able to catch it — and you’ll both get sick long before you learn how to compensate for the tangential velocity, so bring some bags.
WHAT’S GOING ON IN THE AIR…
We all know that air moves from areas of high pressure to areas of low pressure and Coriolis force “makes the winds blow clockwise” as it moves down and out around a high, and ‘counter-clockwise’ as it moves up and in around a low — and if you understood that, try figuring out why it’s the other way ‘round, Down Under.
…AND THE MAELSTROM IN YOUR TUB
I want to debunk once and for all the silly myth about clockwise or counter-clockwise draining bathtubs! On this scale, frictional and viscous forces greatly eclipse Coriolis forces (by more than a million times at mid-latitudes), and a draining bathtub is really just cyclostrophic flow. The water will spiral down mostly because of the shape of the drain, its location, the way the water was moving in the first place, how you pulled the plug, etc. However: If you had a tank about 50 feet wide, a few feet deep, in a thermally stable room, with a hole in the center, waited a week, then pulled the plug—very slowly, from the outside if possible—then, yes, it would always drain the same way. Now, which way was that again.