万物简史英文版_比尔·布莱森-第48章

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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　fahrenheit。　in　addition；the　atmosphere　absorbs　or　deflects　ining　swarms　of　cosmic　rays；　charged　particles；ultraviolet　rays；　and　the　like。　altogether；　the　gaseous　padding　of　the　atmosphere　is　equivalentto　a　fifteen…foot　thickness　of　protective　concrete；　and　without　it　these　invisible　visitors　fromspace　would　slice　through　us　like　tiny　daggers。　even　raindrops　would　pound　us　senseless　if　itweren鈥檛　for　the　atmosphere鈥檚　slowing　drag。

the　most　striking　thing　about　our　atmosphere　is　that　there　isn鈥檛　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　thickness　of　a　couple　of　coats　of　varnish。

for　scientific　convenience；　the　atmosphere　is　divided　into　four　unequal　layers：　troposphere；stratosphere；　mesosphere；　and　ionosphere　（now　often　called　the　thermosphere）。　thetroposphere　is　the　part　that鈥檚　dear　to　us。　it　alone　contains　enough　warmth　and　oxygen　to　allowus　to　function；　though　even　it　swiftly　bees　uncongenial　to　life　as　you　climb　up　through　it。

from　ground　level　to　its　highest　point；　the　troposphere　（or　鈥渢urning　sphere鈥潱�　is　about　ten　milesthick　at　the　equator　and　no　more　than　six　or　seven　miles　high　in　the　temperate　latitudes　wheremost　of　us　live。　eighty　percent　of　the　atmosphere鈥檚　mass；　virtually　all　the　water；　and　thusvirtually　all　the　weather　are　contained　within　this　thin　and　wispy　layer。　there　really　isn鈥檛much　between　you　and　oblivion。

beyond　the　troposphere　is　the　stratosphere。　when　you　see　the　top　of　a　storm　cloudflattening　out　into　the　classic　anvil　shape；　you　are　looking　at　the　boundary　between　thetroposphere　and　stratosphere。　this　invisible　ceiling　is　known　as　the　tropopause　and　wasdiscovered　in　1902　by　a　frenchman　in　a　balloon；　l茅on…philippe　teisserenc　de　bort。　pause　inthis　sense　doesn鈥檛　mean　to　stop　momentarily　but　to　cease　altogether；　it鈥檚　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　minutes；though　you　would　be　well　advised　not　to　make　the　trip。　such　a　rapid　ascent　withoutpressurization　would；　at　the　very　least；　result　in　severe　cerebral　and　pulmonary　edemas；　adangerous　excess　of　fluids　in　the　body鈥檚　tissues。　when　the　doors　opened　at　the　viewingplatform；　anyone　inside　would　almost　certainly　be　dead　or　dying。　even　a　more　measuredascent　would　be　acpanied　by　a　great　deal　of　disfort。　the　temperature　six　miles　up　canbe　…70　degrees　fahrenheit；　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　fahrenheit；　thanks　to　the　absorptive　effects　of　ozone　（something　else　de　bortdiscovered　on　his　daring　1902　ascent）。　it　then　plunges　to　as　low　as　…130　degrees　fahrenheit　inthe　mesosphere　before　skyrocketing　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�at　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�the　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�in　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�with　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�the　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�use　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　a