THANK GOODNESS FOR the atmosphere. It keeps us warm. Without it, Earth would be alifeless ball of ice with an average temperature of minus 60 degrees Fahre. In additiomosphere absorbs or deflects ining swarms of ic rays, charged particles,ultraviolet rays, and the like. Altogether, the gaseous padding of the atmosphere is equivalentto a fifteen-foot thiess of protective crete, and without it these invisible visitors fromspace would slice through us like tiny daggers. Even raindrops would pound us senseless if itweren』t for the atmosphere』s slowing drag.
The most striking thing about our atmosphere is that there isn』t very much of it. It extendsupward for about 120 miles, which might seem reasonably bounteous when viewed fromground level, but if you shrank the Earth to the size of a standard desktop globe it would onlybe about the thiess of a couple of coats of varnish.
For stifiveniehe atmosphere is divided into four unequal layers: troposphere,stratosphere, mesosphere, and ionosphere (now often called the thermosphere). Thetroposphere is the part that』s dear to us. It alone tains enough warmth and oxygen to allowus to fun, though even it swiftly bees ungenial to life as you climb up through it.
From ground level to its highest point, the troposphere (or 「turning sphere」) is about ten milesthick at the equator and no more than six or seven miles high iemperate latitudes wheremost of us live. Eighty pert of the atmosphere』s mass, virtually all the water, and thusvirtually all the weather are tained within this thin and wispy layer. There really isn』tmuch between you and oblivion.
Beyond the troposphere is the stratosphere. When you see the top of a storm cloudflattening out into the classivil shape, you are looking at the boundary betweeroposphere and stratosphere. This invisible ceiling is known as the tropopause and wasdiscovered in 1902 by a Fren in a balloon, Léon-Philippe Teisserenc de Bort. Pause inthis sense doesn』t mean to stop momentarily but to cease altogether; it』s from the same Greekroot as menopause. Even at its greatest extent, the tropopause is not very distant. A fastelevator of the sort used in modern skyscrapers could get you there in about twenty mihough you would be well advised not to make the trip. Such a rapid ast withoutpressurization would, at the very least, result in severe cerebral and pulmonary edemas, adangerous excess of fluids in the body』s tissues. When the doors ope the viewingplatform, anyone inside would almost certainly be dead or dying. Even a more measuredast would be apanied by a great deal of disfort. The temperature six miles up be -70 degrees Fahre, and you would need, or at least very much appreciate,supplementary oxygen.
After you have left the troposphere the temperature soon warms up again, to about 40degrees Fahre, thanks to the absorptive effects of ozone (something else de Bortdiscovered on his daring 1902 ast). It then pluo as low as -130 degrees Fahre inthe mesosphere before skyrocketing to 2,700 degrees Fahre or more ily very erratic thermosphere, where temperatures vary by a thousand degrees from dayto night—though it must be said that 「temperature」 at such a height bees a somewhatnotional cept. Temperature is really just a measure of the activity of molecules. At sealevel, air molecules are so thick that one molecule move only the ti distance—aboutthree-millionths of an inch, to be precise—before banging into another. Because trillions ofmolecule