Windswept_ The Story of Wind and Weather - Marq de Villiers [34]
The differentials are set in motion by one simple governing principle: entropy. Entropy, disorder—or "mixed-upness," as Richard Dawkins called it—is the substance of the second law of thermodynamics, which is that entropy, or disorder, always increases in a closed system. In this definition, order means that different parts of a system have different characteristics (heat, pressure, odor); disorder means no part is different from any other. In nature, if something is hot and something is cool, reordering will occur. That is, if a zone of high pressure is near a zone of low pressure, nature will try to equalize the two zones through movement of air from the high to the low, and winds result. Nature is striving for balance; climate—derived from the Greek word klima, meaning degree of latitude—is the earth's way of seeking to balance its energy intake.
This is just as well. If this didn't happen—if entropy wasn't at work, if there was no balancing—there would be no wind, no weather, and no life on earth as we know it. The poles would go into a much deeper freeze, the equatorial zones would overheat, and what little organic life remained would be huddled at the interstices.
And so, in some ways winds, the movement of the air relative to ' the surface of the earth, are simplicity itself: Hot air one place, cooler air another, and there you have it—wind. Pressure differentials— wind. Adjacent climate zones—wind. Altitude differentials— wind. Planetary rotation—deflected wind. The physics is not complicated: Wind is air moving from high pressure to low pressure, in a straight line, deflected by the rotation of the earth (the Coriolis force).
Because winds begin with the sun, the key to understanding global wind patterns is to start where solar radiation is most intense, at the equator. The air warmed by the radiation rises quickly, causing a quasi-vacuum of low pressure that draws air toward the equator from semi tropical latitudes. The winds so created head directly for the equator but because of the planet's rotation are turned by the same Coriolis force that twists the ocean's currents. This "turn" pushes the winds "right" in the northern hemisphere and "left" in the southern hemisphere, until they parallel the equator. These are the most reliable winds on earth, the so-called trade winds; they are the winds that made transoceanic travel possible in the days of sail. Eventually these steady trade winds, because they are paralleling the equator, are also warmed, and they then follow the same pattern— they rise, are cooled, drift toward the poles, and sink again, at about 30 degrees of latitude, more or less the southern Mediterranean and northern California.
Some of this newly cooled air moves at high altitudes back toward the equator, completing what is known as the Hadley cell, named after George Hadley, an English lawyer in the eighteenth century. But some of it moves toward the midlatitudes, and on its way the Coriolis force "turns" the wind right in the northern hemisphere and left in the southern, causing the prevailing midlatitude westerlies, the winds that the canny New Englanders learned to exploit when their trade with Europe began to expand. But the westerlies only account for a fraction of the air mass in motion. The rest, which forms a second overturning cycle of air, carries surface air poleward and upper-level air back toward the Hadley cells. These are Ferrel cells, which generally have a motion opposite to planetary rotation;