Windswept_ The Story of Wind and Weather - Marq de Villiers [36]
On a very small scale, say that of the average bathtub or kitchen sink, the angle of deflection is so tiny as to be unnoticeable, estimated at i/3600th of a degree, so the way water flows out of a bathtub depends on the angle it was poured in, the configuration of the tub itself, abrasions on the surface, the temperature differentials in the water, and dozens of other minute effects. It is theoretically possible, in a carefully controlled laboratory experiment, to show the effect at work; most of us, to see it at all, would need a tub the size of one of the Great Lakes.8
I once saw a "demonstration" of the Coriolis force by a cheerful Maasai at the Equator near the town of Eldoret, in Kenya. I had come to Eldoret after a rather harrowing all-night, breakneck drive in an aging Renault, piloted by a former game warden; we shared the car with a massive python, which the game warden called Brenda, that creaked and rustled around in the backseat and kept me awake. I was therefore feeling somewhat fragile when the car stopped at the "Line," where we joined a busload of tourists who were having their pictures taken with one leg in each hemisphere, and who were being subjected to the aforementioned Coriolis demo. Two buckets had been filled with water and placed a yard on either side of the line. Then the demonstrator, with a flourish, pulled the plug on each in turn, and lo and behold, the water exited by spinning counter clockwise in the northern hemisphere bucket, and clockwise in the southern. This produced the requisite "ooohs" and "aaahs" from the busload, as well as a satisfactory yield in tips, but it was all a fraud, set up by the direction in which the con artist had poured the water into the buckets, and by a minute difference in the way he pulled the plugs. The game warden, whose name was Ibrahim, gave him a disgusted snort for his pains, and so did I, but he wasn't offended; he was making a good living from human gullibility and the occasional skeptic wasn't going to deter him.
The Coriolis effect influences massive movements of air just as much as it does artillery shells or aircraft, and is therefore important for an understanding of global winds. Just like an artillery shell, freely moving air (winds) will deflect to the right in the northern hemisphere, and to the left in the southern. So air moving toward a low-pressure system will deflect to its right; but because the forces which got the air moving toward the north in the first place are still in play, the result will be a vortex of air, spinning counterclockwise— air will try to turn to the right, the low pressure pocket will try to draw the air into itself, and the result is that the air is held into a circle that actually turns to the left. Without the Coriolis effect, air rushing into a point could still form a vortex, but the direction of rotation would be random.
With the Coriolis effect in play, the randomness disappears, and northern hemisphere cyclones, including hurricanes, always revolve in the same direction.
Bear in mind that the Coriolis force is not the only factor determining large scale winds. One force is generated by the pressure gradient, of course, but there are three others: the curvature of the wind (wind that turns speeds up or slows down depending on whether it is turning clockwise or counterclockwise [the opposite in the southern hemisphere]); changing pressure differences (called the isallobaric effect), which can dramatically boost or inhibit the wind, a major consideration in east coast "weather bombs"; and the stability of the air in the lowest part of the atmosphere (stable air, either warm or cold, is reluctant to overturn, whereas unstable air is highly turbulent, and its overturning and mixing brings down the established winds from higher elevations to where we live).
III
The general circulation model—the Hadley and Ferrel cells and the generally