Windswept_ The Story of Wind and Weather - Marq de Villiers [20]
Above the stratosphere is—big surprise, this—the stratopause, and above that is the mesosphere, reaching up to about 53 miles. The air in the mesosphere is too thin to circulate much at all, and winds hardly exist there. Not too thin, though, to burn up most of the meteors that enter our atmosphere. Temperatures in the mesosphere deline, from about o° Celsius to somewhere around — 100° Celsius.
Then, of course, there's the mesopause.
Beyond that is the thermosphere, which stretches out as far as around 370 miles and represents the outer limits of earth's thermal reach. Temperatures in the thermosphere range from around —8o° Celsius to as low as — i,ooo° Celsius, though the number of air molecules at that altitude is so small, and the consequent heat transfer so meager, that it would not feel at all cold to the human skin (provided that skin's owner could somehow deal with the absence of oxygen and the vacuumlike pressure).
Just to complete the set, however, atmospheric scientists generally include one more layer, which they call the exosphere. As its name implies, the exosphere is simply that part of space beyond any influence from Earth; the exosphere, therefore, is a near vacuum, containing not much at all, and yet includes pretty much all the remaining universe—the human species is a parochial one, measuring influence by its own small corner of its own small galaxy. The satellites that beam TV signals into your living room circulate on the lower fringes of the exosphere.
The air gets thinner the higher you go. Unsurprisingly, then, atmospheric pressure—and the density of air's life-sustaining gases—diminishes rapidly with altitude. At 9,000 feet the air pressure is already only three quarters of that at sea level, and almost everyone—except hardy Andes dwellers, Sherpas of the Himalayas, and a few Ethiopian and Kenyan marathoners—feels the effects. The reduced pressure causes the brain to swell slightly, resulting in headaches and nausea. At double that height, 18,000 feet, the pressure has dropped to half earth-normal; no permanent settlements exist at these altitudes because the human lungs just can't cope, and expel too much CO, fatally disrupting the body's balance. At Everest's peak, just over 29,000 feet, pressure is only 30 percent, about 300 millibars instead of the earth-normal of 1,000; at those altitudes, the human body begins an inevitable, and final, breakdown.7 It's possible—at least for a few extrahardy mountaineers—to survive for brief periods at Everest's summit without an oxygen tank, but survival at that altitude is measured in minutes, not hours.
It's interesting to contemplate what a perilously thin layer our sustaining atmosphere really is. Earth's diameter is only a little more than 7,000 miles. The troposphere, in which our weather (and the greenhouse effect) happens, is at most 10 miles deep. Put another way, if the earth was a ball 4 feet or so in diameter, the troposphere would be a fraction of an inch thick, about the thickness of the lead in a common pencil.
But the theater of the wind is more complicated than just stacking decreasingly dense layers one on the next.
In the popular imagination, and in the scientific consensus before the space age, "outside" seemed empty, dark, and frigid, a void through which to view distant planets and stars; the very word space seemed imprecise, lacking in definition, an afterthought, coined only in opposition to nonspace, or Earth. In fact, our planet's hinterland is not empty at all, but filled with magnetic fields, electric fields, matter, energy, ionized gases, and radiation, generally invisible to the eye or the telescope but easily apparent to instrumentation. Some of these are worth noting because they affect our climate and our wind patterns and therefore the weather.
The most curious