HOW COME? Location won't sway bathwater swirl

Is it true that water goes down a bathtub drain in opposite directions, depending on the hemisphere you're in? asks a reader.Swirling bathwater, spinning faster and faster as the tub empties, resembles a mini-hurricane. In the Northern Hemisphere -- say, off the coast of Florida -- real hurricanes turn counterclockwise. Meanwhile, in the Southern hemisphere, tropical cyclones spin clockwise.

So if, in a Florida hotel room, you fill the tub and pull the plug, will the water depart in a counterclockwise swirl? Maybe. Or maybe not. Whether your bathwater is north or south of the equator turns out to be the least important factor in how it circles the drain. For cyclonic storms, however, it's location, location, location.

How come? Imagine a ball lying on a patch of ground at the North Pole, and another sitting on the ground at the equator. Meanwhile, the Earth is speedily spinning, though we don't sense its carousel movement.

At its bulging waistline, our planet measures about 25,000 miles around. In the 24 hours it takes Earth to turn once, the North Pole ball, at the top of Earth's axis, has effectively stood still. But the ball at the equator has been carried 25,000 miles, at a speed of more than 1,000 mph.

Now imagine that the North Pole ball is picked up and thrown by a pitcher with superhuman strength -- all the way to Florida. Because the Earth is turning faster nearer the equator, the ball's path would gradually appear to curve, the ball landing to the right of its target. Starting from the South Pole, a ball's path would be deflected to the left. This "Coriolis effect" is named after 19th-century French scientist Gustave Coriolis. Coriolis studied forces that arise when a reference frame (like the Earth) is rotating.

Because of the Coriolis effect, a liquid or gas flowing across the spinning Earth feels an accelerating force, at a 90-degree angle to its forward motion. North of the equator, as rushing air drops into a low-pressure area, it's deflected in a counterclockwise direction. Presto: Counterclockwise hurricanes. South of the equator, low-pressure storms are nudged into a clockwise rotation.

But Earth's rotation has virtually no effect on water in a bathtub. Scientists say the rotation rate of a tiny parcel of water around a drain (one rotation in a few seconds) is tens of thousands of times higher than the rotation rate of the Earth (one rotation in 24 hours). The Coriolis effect, at 3 ten-millionths of the strength of gravity, is too weak to make swiftly draining bathwater turn one way. Instead, the directional die is cast as water swirls against the sides of the tub, developing a slight rotating motion that increases when you open the drain.