Windswept_ The Story of Wind and Weather - Marq de Villiers [35]
This intricately three-dimensional pattern of winds is the planet's general circulation. It explains why most of the air movement in the lower atmosphere is vertical rather than horizontal. It is also directly responsible for the latitudinal movement of air masses, and therefore of weather and the long drifts of weather called climate.
II
This business of the Coriolis force, named after French physicist and mathematician Gustave Gaspard Coriolis, who first described it in 1835,7 merits a small digression, because it is not quite as simple as it sounds. The physics that govern what scientists call a rotating frame of reference are quite complex, but the effects of the earth's rotation on natural phenomena are simple enough to see. East-west motions create a Coriolis force, or Coriolis effect, which is directed radially inward (for easterly motion) or outward (for westerly motion) from the axis of rotation. Motion relative to the earth's, to the west or the east, will produce an acceleration (because of the force) to the north or the south.
Hadley and Ferrel cells, showing a simplified version of the major vertical air movements that help balance planetary heat distribution.
North or south motions also give rise to a Coriolis force because the motions are toward (or away) from the axis of rotation. Vertical motions also give rise to a horizontal Coriolis force, but it is negligible and usually ignored.
For our purposes, the result is what is important: Whether from east-west or north-south motions, there is always a deflection to the right in the northern hemisphere, and to the left in the southern hemisphere.
The easiest commonsensical way to see the effect is to imagine firing a rocket or an artillery shell while you are standing at the equator. Aim the rocket to hit a target a thousand miles away. If you fire it toward the north pole, something odd seems to happen. Even if your calculations were spot on, and there was no wind, the rocket wouldn't land due north of where you are. Instead, it will appear to have drifted off course, to the east. Surely something must have diverted it? Was Newton wrong?
Gustave Coriolis
The answer lies in the rotation of the earth. All points on Earth rotate 360 degrees in twenty-four hours, a planetary day, but obviously some points must be rotating much faster than others. The fastest speeds are at the equator, perhaps a thousand miles an hour; close to the poles the rotational velocity is negligible, a mere dozen or so miles an hour.
So what has happened to your rocket is this: At the moment you fire it in a northward direction, you and the launcher that fired it, and therefore the rocket itself, are traveling eastward at a high rate of speed—the speed the equator is traveling. Your rocket travels in a straight line, but throughout its flight it keeps moving eastward at a constant rate, the speed of the equator. When it nears its destination, however, the ground below it is not moving eastward nearly as fast as the rocket itself. And so the ground "seems" to have moved westward, and the missile "seems" to have drifted eastward. That is, an object traveling away from the equator will eventually be heading east faster than the ground below it, and may seem to have been driven east by some unknown force. Objects traveling toward the equator, similarly, will seem to be have been driven west. Which means, if you can turn your head to squint in the right direction, that in the northern hemisphere objects will turn to the right, and in the southern hemisphere to the left.
South Pole
The Coriolis force, showing how movement is deflected to the right in the northern hemisphere and to the left in the southern hemisphere.
Newton's law is conserved, after all. It is just that we are in a rotating frame of reference.
Some of the effects are critical ones, and not just for long-range ballistic missiles. Over the course of a four-hour flight,