万物简史英文版_比尔·布莱森-第49章

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p；　can　bowl　along　at　up　to　180　miles　an　hour　and　vastlyinfluence　weather　systems　over　whole　continents；　yet　their　existence　wasn鈥檛　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鈥攅nough　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鈥攖hose　unlovable；featureless　sprawls　that　give　us　our　overcast　skies鈥攈appen　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鈥檛　abide　this；　so　it　rushes　around　trying　to　equalizethings　everywhere。　wind　is　simply　the　air鈥檚　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鈥攁　balloon　or　an　air　tank鈥攁nd　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鈥攁nd　hence　that　much　more　destructive。

introduce　several　million　tons　of　air　to　this　accelerator　effect　and　the　result　can　be　exceedinglyenergetic。　a　tropical　hurricane　can　release　in　twenty…four　hours　as　much　energy　as　a　rich；medium…sized　nation　like　britain　or　france　uses　in　a　year。

the　impulse　of　the　atmosphere　to　seek　equilibrium　was　first　suspected　by　edmondhalley鈥攖he　man　who　was　everywhere鈥攁nd　elaborated　upon　in　the　eighteenth　century　by　hisfellow　briton　george　hadley；　who　saw　that　rising　and　falling　columns　of　air　tended　toproduce　鈥渃ells鈥潯。╧nown　ever　since　as　鈥渉adley　cells鈥潱�。　though　a　lawyer　by　profession；　hadleyhad　a　keen　interest　in　the　weather　（he　was；　after　all；　english）　and　also　suggested　a　linkbetween　his　cells；　the　earth鈥檚　spin；　and　the　apparent　deflections　of　air　that　give　us　our　tradewinds。　however；　it　was　an　engineering　professor　at　the　茅cole　polytechnique　in　paris；gustave…gaspard　de　coriolis；　who　worked　out　the　details　of　these　interactions　in　1835；　andthus　we　call　it　the　coriolis　effect。　（coriolis鈥檚　other　distinction　at　the　school　was　to　introducewatercoolers；　which　are　still　known　there　as　corios；　apparently。）　the　earth　revolves　at　a　brisk1；041　miles　an　hour　at　the　equator；　though　as　you　move　toward　the　poles　the　rate　slopes　offconsiderably；　to　about　600　miles　an　hour　in　london　or　paris；　for　instance。　the　reason　for　thisis　self…evident　when　you　think　about　it。　if　you　are　on　the　equator　the　spinning　earth　has　tocarry　you　quite　a　distance鈥攁bout　40；000　kilometers鈥攖o　get　you　back　to　the　same　spot。　if　youstand　beside　the　north　pole；　however；　you　may　need　travel　only　a　few　feet　to　plete　arevolution；　yet　in　both　cases　it　takes　twenty…four　hours　to　get　you　back　to　where　you　began。

therefore；　it　follows　that　the　closer　you　get　to　the　equator　the　faster　you　must　be　spinning。

the　coriolis　effect　explains　why　anything　moving　through　the　air　in　a　straight　line　laterallyto　the　earth鈥檚　spin　will；　given　enough　distance；　seem　to　curve　to　the　right　in　the　northernhemisphere　and　to　the　left　in　the　southern　as　the　earth　revolves　beneath　it。　the　standard　wayto　envision　this　is　to　imagine　yourself　at　the　center　of　a　large　carousel　and　tossing　a　ball　tosomeone　positioned　on　the　edge。　by　the　time　the　ball　gets　to　the　perimeter；　the　target　personhas　moved　on　and　the　ball　passes　behind　him。　from　his　perspective；　it　looks　as　if　it　has　curvedaway　from　him。　that　is　the　coriolis　effect；　and　it　is　what　gives　weather　systems　their　curl　andsends　hurricanes　spinning　off　like　tops。　the　coriolis　effect　is　also　why　naval　guns　firingartillery　shells　have　to　adjust　to　left　or　right；　a　shell　fired　fifteen　miles　would　otherwisedeviate　by　about　a　hundred　yards　and　plop　harmlessly　into　the　sea。

considering　the　practical　and　psychological　importance　of　the　weather　to　nearly　everyone；it鈥檚　surprising　that　meteorology　didn鈥檛　really　get　going　as　a　science　until　shortly　before　theturn　of　the　nineteenth　century　（though　the　term　meteorology　itself　had　been　around　since1626；　when　it　was　coined　by　a　t。　granger　in　a　book　of　logic）。

part　of　the　problem　was　that　successful　meteorology　requires　the　precise　measurement　oftemperatures；　and　thermometers　for　a　long　time　proved　more　difficult　to　make　than　you　mightexpect。　an　accurate　reading　was　dependent　on　getting　a　very　even　bore　in　a　glass　tube；　andthat　wasn鈥檛　easy　to　do。　the　first　person　to　crack　the　problem　was　daniel　gabriel　fahrenheit；　adutch　maker　of　instruments；　who　produced　an　accurate　thermometer　in　1717。　however；　forreasons　unknown　he　calibrated　the　instrument　in　a　way　that　put　freezing　at　32　degrees　andboiling　at　212　degrees。　from　the　outset　this　numeric　eccentricity　bothered　some　people；　and　in1742　anders　celsius；　a　swedish　astronomer；　came　up　with　a　peting　scale。　in　proof　of　theproposition　that　inventors　seldom　get　matters　entirely　right；　celsius　made　boiling　point　zeroand　freezing　point　100　on　his　scale；　but　that　was　soon　reversed。

the　person　most　frequently　identified　as　the　father　of　modern　meteorology　was　an　englishpharmacist　named　luke　howard；　who　came　to　prominence　at　the　beginning　of　the　nineteenthcentury。　howard　is　chiefly　remembered　now　for　giving　cloud　types　their　names　in　1803。

although　he　was　an　active　and　respected　member　of　the　linnaean　society　and　employedlinnaean　principles　in　his　new　scheme；　howard　chose　the　rather　more　obscure　askesiansociety　as　the　forum　to　announce　his　new　system　of　classification。　（the　askesian　society；you　may　just　recall　from　an　earlier　chapter；　was　the　body　whose　members　were　unusuallydevoted　to　the　pleasures　of　nitrous　oxide；　so　we　can　only　hope　they　treated　howard鈥檚presentation　with　the　sober　attention　it　deserved。　it　is　a　point　on　which　howard　scholars　arecuriously　silent。）howard　divided　clouds　into　three　groups：　stratus　for　the　layered　clouds；　cumulus　for　thefluffy　ones　（the　word　means　鈥渉eaped鈥潯�in　latin）；　and　cirrus　（meaning　鈥渃urled鈥潱�　for　the　high；thin　feathery　formations　that　generally　presage　colder　weather。　to　these　he　subsequentlyadded　a　fourth　term；　nimbus　（from　the　latin　for　鈥渃loud鈥潱�；　for　a　rain　cloud。　the　beauty　ofhoward鈥檚　system　was　that　the　basic　ponents　could　be　freely　rebined　to　describe　everyshape　and　size　of　passing　cloud鈥攕tratocumulus；　cirrostratus；　cumulocongestus；　and　so　on。　itwas　an　immediate　hit；　and　not　just　in　england。　the　poet　johann　von　goethe　in　germany　wasso　taken　with　the　system　that　he　dedicated　four　poems　to　howard。

howard鈥檚　system　has　been　much　added　to　over　the　years；　so　much　so　that　the　encyclopedicif　little　read　international　cloud　atlas　runs　to　two　volumes；　but　interestingly　virtually　all　thepost…howard　cloud　types鈥攎ammatus；　pileus；　nebulosis；　spissatus；　floccus；　and　mediocris　area　sampling鈥攈ave　never　caught　on　with　anyone　outside　meteorology　and　not　terribly　muchthere；　i鈥檓　told。　incidentally；　the　first；　much　thinner　edition　of　that　atlas；　produced　in　1896；divided　clouds　into　ten　basic　types；　of　which　the　plumpest　and　most　cushiony…looking　wasnumber　nine；　cumulonimbus。

1that　seems　to　have　been　the　source　of　the　expression　鈥渢o　be　oncloud　nine。鈥�

for　all　the　heft　and　fury　of　the　occasional　anvil…headed　storm　cloud；　the　average　cloud　isactually　a　benign　and　surprisingly　insubstantial　thing。　a　fluffy　summer　cumulus　severalhundred　yards　to　a　side　may　contain　no　more　than　twenty…five　or　thirty　gallons　of　water鈥斺�渁bout　enough　to　fill　a　bathtub；鈥潯�as　james　trefil　has　noted。　you　can　get　some　sense　of　theimmaterial　quality　of　clouds　by　strolling　through　fog鈥攚hich　is；　after　all；　nothing　more　than　acloud　that　lacks　the　will　to　fly。　to　quote　trefil　again：　鈥渋f　you　walk　100　yards　through　a　typicalfog；　you　will　e　into　contact　with　only　about　half　a　cubic　inch　of　water鈥攏ot　enough　togive　you　a　decent　drink。鈥潯�in　consequence；　clouds　are　not　great　reservoirs　of　water。　only　about0。035　percent　of　the　earth鈥檚　fresh　water　is　floating　around　above　us　at　any　moment。

depending　on　where　it　falls；　the　prognosis　for　a　water　molecule　varies　widely。　if　it　lands　infertile　soil　it　will　be　soaked　up　by　plants　or　reevaporated　directly　within　hours　or　days。　if　itfinds　its　way　down　to　the　groundwater；　however；　it　may　not　see　sunlight　again　for　manyyears鈥攖housands　if　it　gets　really　deep。　when　you　look　at　a　lake；　you　are　looking　at　acollection　of　molecules　that　have　been　there　on　average　for　about　a　decade。　in　the　ocean　theresidence　time　is　thought　to　be　more　like　a　hundred　years。　altogether　about　60　percent　of1if　you　have　ever　been　struck　by　how　beautifully　crisp　and　well　defined　the　edges　of　cumulus　clouds　tend　to　be；while　other