A Short History of Nearly Everything-第54章

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imself　choking　to　death　after　a　piece　of　infected　flesh　came　loose　andblocked　his　windpipe。”　with　a　supreme　effort　somervell　managed　to　cough　up　theobstruction。　it　turned　out　to　be　“the　entire　mucus　lining　of　his　larynx。”

bodily　distress　is　notorious　above　25；000　feet—the　area　known　to　climbers　as　the　deathzone—but　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’s　day；　it’s　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：　“we　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。”　the　reason　you　don’t　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—25　million　tons　for　every　square　mile　of　the　planet—a　notinconsequential　volume。　when　you　get　millions　of　tons　of　atmosphere　rushing　past　at　thirty　orforty　miles　an　hour；　it’s　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’　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’t　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—some　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　continents；　yet　their　existence　wasn’t　suspected　untilpilots　began　to　fly　into　them　during　the　second　world　war。　even　now　a　great　deal　ofatmospheric　phenomena　is　barely　understood。　a　form　of　wave　motion　popularly　known　asclear…air　turbulence　occasionally　enlivens　airplane　flights。　about　twenty　such　incidents　a　yearare　serious　enough　to　need　reporting。　they　are　not　associated　with　cloud　structures　oranything　else　that　can　be　detected　visually　or　by　radar。　they　are　just　pockets　of　startlingturbulence　in　the　middle　of　tranquil　skies。　in　a　typical　incident；　a　plane　en　route　fromsingapore　to　sydney　was　flying　over　central　australia　in　calm　conditions　when　it　suddenlyfell　three　hundred　feet—enough　to　fling　unsecured　people　against　the　ceiling。　twelve　peoplewere　injured；　one　seriously。　no　one　knows　what　causes　such　disruptive　cells　of　air。

the　process　that　moves　air　around　in　the　atmosphere　is　the　same　process　that　drives　theinternal　engine　of　the　planet；　namely　convection。　moist；　warm　air　from　the　equatorial　regionsrises　until　it　hits　the　barrier　of　the　tropopause　and　spreads　out。　as　it　travels　away　from　theequator　and　cools；　it　sinks。　when　it　hits　bottom；　some　of　the　sinking　air　looks　for　an　area　oflow　pressure　to　fill　and　heads　back　for　the　equator；　pleting　the　circuit。

at　the　equator　the　convection　process　is　generally　stable　and　the　weather　predictably　fair；but　in　temperate　zones　the　patterns　are　far　more　seasonal；　localized；　and　random；　whichresults　in　an　endless　battle　between　systems　of　high…pressure　air　and　low。　low…pressuresystems　are　created　by　rising　air；　which　conveys　water　molecules　into　the　sky；　forming　cloudsand　eventually　rain。　warm　air　can　hold　more　moisture　than　cool　air；　which　is　why　tropical　andsummer　storms　tend　to　be　the　heaviest。　thus　low　areas　tend　to　be　associated　with　clouds　andrain；　and　highs　generally　spell　sunshine　and　fair　weather。　when　two　such　systems　meet；　itoften　bees　manifest　in　the　clouds。　for　instance；　stratus　clouds—those　unlovable；featureless　sprawls　that　give　us　our　overcast　skies—happen　when　moisture…bearing　updraftslack　the　oomph　to　break　through　a　level　of　more　stable　air　above；　and　instead　spread　out；　likesmoke　hitting　a　ceiling。　indeed；　if　you　watch　a　smoker　sometime；　you　can　get　a　very　goodidea　of　how　things　work　by　watching　how　smoke　rises　from　a　cigarette　in　a　still　room。　atfirst；　it　goes　straight　up　（this　is　called　a　laminar　flow；　if　you　need　to　impress　anyone）；　and　thenit　spreads　out　in　a　diffused；　wavy　layer。　the　greatest　superputer　in　the　world；　takingmeasurements　in　the　most　carefully　controlled　environment；　cannot　tell　you　what　forms　theseripplings　will　take；　so　you　can　imagine　the　difficulties　that　confront　meteorologists　when　theytry　to　predict　such　motions　in　a　spinning；　windy；　large…scale　world。

what　we　do　know　is　that　because　heat　from　the　sun　is　unevenly　distributed；　differences　inair　pressure　arise　on　the　planet。　air　can’t　abide　this；　so　it　rushes　around　trying　to　equalizethings　everywhere。　wind　is　simply　the　air’s　way　of　trying　to　keep　things　in　balance。　airalways　flows　from　areas　of　high　pressure　to　areas　of　low　pressure　（as　you　would　expect；　thinkof　anything　with　air　under　pressure—a　balloon　or　an　air　tank—and　think　how　insistently　thatpressured　air　wants　to　get　someplace　else）；　and　the　greater　the　discrepancy　in　pressures　thefaster　the　wind　blows。

incidentally；　wind　speeds；　like　most　things　that　accumulate；　grow　exponentially；　so　a　windblowing　at　two　hundred　miles　an　hour　is　not　simply　ten　times　stronger　than　a　wind　blowing　attwenty　miles　an　hour；　but　a　hundred　times　stronger—and　hence　that　much　more　destructive。

introduce　several　million　tons　of　air　to　this　accelerator　effect　and　the　resu