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ting to 2；700 degrees fahrenheit or more in the aptly namedbut very erratic thermosphere； where temperatures can vary by a thousand degrees from day to night鈥攖hough it must be said that 鈥渢emperature鈥潯t such a height bees a somewhatnotional concept。 temperature is really just a measure of the activity of molecules。 at sealevel； air molecules are so thick that one molecule can move only the tiniest distance鈥攁boutthree…millionths of an inch； to be precise鈥攂efore banging into another。 because trillions ofmolecules are constantly colliding； a lot of heat gets exchanged。 but at the height of thethermosphere； at fifty miles or more； the air is so thin that any two molecules will be milesapart and hardly ever e in contact。 so although each molecule is very warm； there are fewinteractions between them and thus little heat transference。 this is good news for satellitesand spaceships because if the exchange of heat were more efficient any man…made objectorbiting at that level would burst into flame。
even so； spaceships have to take care in the outer atmosphere； particularly on return trips toearth； as the space shuttle columbia demonstrated all too tragically in february 2003。
although the atmosphere is very thin； if a craft es in at too steep an angle鈥攎ore thanabout 6 degrees鈥攐r too swiftly it can strike enough molecules to generate drag of anexceedingly bustible nature。 conversely； if an ining vehicle hit the thermosphere attoo shallow an angle； it could well bounce back into space； like a pebble skipped across water。
but you needn鈥檛 venture to the edge of the atmosphere to be reminded of what hopelesslyground…hugging beings we are。 as anyone who has spent time in a lofty city will know； youdon鈥檛 have to rise too many thousands of feet from sea level before your body begins toprotest。 even experienced mountaineers； with the benefits of fitness； training； and bottledoxygen； quickly bee vulnerable at height to confusion； nausea； exhaustion； frostbite；hypothermia； migraine； loss of appetite； and a great many other stumbling dysfunctions。 in ahundred emphatic ways the human body reminds its owner that it wasn鈥檛 designed to operateso far above sea level。
鈥渆ven under the most favorable circumstances；鈥潯he climber peter habeler has written ofconditions atop everest； 鈥渆very step at that altitude demands a colossal effort of will。 youmust force yourself to make every movement； reach for every handhold。 you are perpetuallythreatened by a leaden； deadly fatigue。鈥潯n the other side of everest； the british mountaineerand filmmaker matt dickinson records how howard somervell； on a 1924 british expeditionup everest； 鈥渇ound himself choking to death after a piece of infected flesh came loose andblocked his windpipe。鈥潯ith a supreme effort somervell managed to cough up theobstruction。 it turned out to be 鈥渢he entire mucus lining of his larynx。鈥�
bodily distress is notorious above 25；000 feet鈥攖he area known to climbers as the deathzone鈥攂ut many people bee severely debilitated； even dangerously ill； at heights of nomore than 15；000 feet or so。 susceptibility has little to do with fitness。 grannies sometimescaper about in lofty situations while their fitter offspring are reduced to helpless； groaningheaps until conveyed to lower altitudes。
the absolute limit of human tolerance for continuous living appears to be about 5；500meters； or 18；000 feet； but even people conditioned to living at altitude could not tolerate suchheights for long。 frances ashcroft； in life at the extremes； notes that there are andean sulfurmines at 5；800 meters； but that the miners prefer to descend 460 meters each evening andclimb back up the following day； rather than live continuously at that elevation。 people whohabitually live at altitude have often spent thousands of years developing disproportionatelylarge chests and lungs； increasing their density of oxygen…bearing red blood cells by almost athird； though there are limits to how much thickening with red cells the blood supply can stand。 moreover； above 5；500 meters even the most well…adapted women cannot provide agrowing fetus with enough oxygen to bring it to its full term。
in the 1780s when people began to make experimental balloon ascents in europe；something that surprised them was how chilly it got as they rose。 the temperature drops about3 degrees fahrenheit with every thousand feet you climb。 logic would seem to indicate thatthe closer you get to a source of heat； the warmer you would feel。 part of the explanation isthat you are not really getting nearer the sun in any meaningful sense。 the sun is ninety…threemillion miles away。 to move a couple of thousand feet closer to it is like taking one stepcloser to a bushfire in australia when you are standing in ohio； and expecting to smell smoke。
the answer again takes us back to the question of the density of molecules in the atmosphere。
sunlight energizes atoms。 it increases the rate at which they jiggle and jounce； and in theirenlivened state they crash into one another； releasing heat。 when you feel the sun warm onyour back on a summer鈥檚 day； it鈥檚 really excited atoms you feel。 the higher you climb； thefewer molecules there are； and so the fewer collisions between them。
air is deceptive stuff。 even at sea level； we tend to think of the air as being ethereal and allbut weightless。 in fact； it has plenty of bulk； and that bulk often exerts itself。 as a marinescientist named wyville thomson wrote more than a century ago： 鈥渨e sometimes find whenwe get up in the morning； by a rise of an inch in the barometer； that nearly half a ton has beenquietly piled upon us during the night； but we experience no inconvenience； rather a feeling ofexhilaration and buoyancy； since it requires a little less exertion to move our bodies in thedenser medium。鈥潯he reason you don鈥檛 feel crushed under that extra half ton of pressure is thesame reason your body would not be crushed deep beneath the sea： it is made mostly ofinpressible fluids； which push back； equalizing the pressures within and without。
but get air in motion； as with a hurricane or even a stiff breeze； and you will quickly bereminded that it has very considerable mass。 altogether there are about 5；200 million milliontons of air around us鈥�25 million tons for every square mile of the planet鈥攁 notinconsequential volume。 when you get millions of tons of atmosphere rushing past at thirty orforty miles an hour； it鈥檚 hardly a surprise that limbs snap and roof tiles go flying。 as anthonysmith notes； a typical weather front may consist of 750 million tons of cold air pinnedbeneath a billion tons of warmer air。 hardly a wonder that the result is at timesmeteorologically exciting。
certainly there is no shortage of energy in the world above our heads。 one thunderstorm； ithas been calculated； can contain an amount of energy equivalent to four days鈥櫋se ofelectricity for the whole united states。 in the right conditions； storm clouds can rise to heightsof six to ten miles and contain updrafts and downdrafts of one hundred miles an hour。 theseare often side by side； which is why pilots don鈥檛 want to fly through them。 in all； the internalturmoil particles within the cloud pick up electrical charges。 for reasons not entirelyunderstood the lighter particles tend to bee positively charged and to be wafted by aircurrents to the top of the cloud。 the heavier particles linger at the base； accumulating negativecharges。 these negatively charged particles have a powerful urge to rush to the positivelycharged earth； and good luck to anything that gets in their way。 a bolt of lightning travels at270；000 miles an hour and can heat the air around it to a decidedly crisp 50；000 degreesfahrenheit； several times hotter than the surface of the sun。 at any one moment 1；800thunderstorms are in progress around the globe鈥攕ome 40；000 a day。 day and night across theplanet every second about a hundred lightning bolts hit the ground。 the sky is a lively place。
much of our knowledge of what goes on up there is surprisingly recent。 jet streams； usuallylocated about 30；000 to 35；000 feet up； can bowl along at up to 180 miles an hour and vastlyinfluence weather systems over whole continen