万物简史英文版_比尔·布莱森-第68章

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cation。　（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　clouds　are　more　blurry；　the　explanation　is　that　in　a　cumulus　cloud　there　is　a　pronounced　boundarybetween　the　moist　interior　of　the　cloud　and　the　dry　air　beyond　it。　any　water　molecule　that　strays　beyond　the　edgeof　the　cloud　is　immediately　zapped　by　the　dry　air　beyond；　allowing　the　cloud　to　keep　its　fine　edge。　much　highercirrus　clouds　are　posed　of　ice；　and　the　zone　between　the　edge　of　the　cloud　and　the　air　beyond　is　not　soclearly　delineated；　which　is　why　they　tend　to　be　blurry　at　the　edges。

water　molecules　in　a　rainfall　are　returned　to　the　atmosphere　within　a　day　or　two。　onceevaporated；　they　spend　no　more　than　a　week　or　so鈥攄rury　says　twelve　days鈥攊n　the　skybefore　falling　again　as　rain。

evaporation　is　a　swift　process；　as　you　can　easily　gauge　by　the　fate　of　a　puddle　on　asummer鈥檚　day。　even　something　as　large　as　the　mediterranean　would　dry　out　in　a　thousandyears　if　it　were　not　continually　replenished。　such　an　event　occurred　a　little　under　six　millionyears　ago　and　provoked　what　is　known　to　science　as　the　messinian　salinity　crisis。　whathappened　was　that　continental　movement　closed　the　strait　of　gibraltar。　as　the　mediterraneandried；　its　evaporated　contents　fell　as　freshwater　rain　into　other　seas；　mildly　diluting　theirsaltiness鈥攊ndeed；　making　them　just　dilute　enough　to　freeze　over　larger　areas　than　normal。

the　enlarged　area　of　ice　bounced　back　more　of　the　sun鈥檚　heat　and　pushed　earth　into　an　iceage。　so　at　least　the　theory　goes。

what　is　certainly　true；　as　far　as　we　can　tell；　is　that　a　little　change　in　the　earth鈥檚　dynamicscan　have　repercussions　beyond　our　imagining。　such　an　event；　as　we　shall　see　a　little　furtheron；　may　even　have　created　us。

oceans　are　the　real　powerhouse　of　the　planet鈥檚　surface　behavior。　indeed；　meteorologistsincreasingly　treat　oceans　and　atmosphere　as　a　single　system；　which　is　why　we　must　give　thema　little　of　our　attention　here。　water　is　marvelous　at　holding　and　transporting　heat。　every　day；the　gulf　stream　carries　an　amount　of　heat　to　europe　equivalent　to　the　world鈥檚　output　of　coalfor　ten　years；　which　is　why　britain　and　ireland　have　such　mild　winters　pared　with　canadaand　russia。

but　water　also　warms　slowly；　which　is　why　lakes　and　swimming　pools　are　cold　even　on　thehottest　days。　for　that　reason　there　tends　to　be　a　lag　in　the　official；　astronomical　start　of　aseason　and　the　actual　feeling　that　that　season　has　started。　so　spring　may　officially　start　in　thenorthern　hemisphere　in　march；　but　it　doesn鈥檛　feel　like　it　in　most　places　until　april　at　the　veryearliest。

the　oceans　are　not　one　uniform　mass　of　water。　their　differences　in　temperature；　salinity；depth；　density；　and　so　on　have　huge　effects　on　how　they　move　heat　around；　which　in　turnaffects　climate。　the　atlantic；　for　instance；　is　saltier　than　the　pacific；　and　a　good　thing　too。　thesaltier　water　is　the　denser　it　is；　and　dense　water　sinks。　without　its　extra　burden　of　salt；　theatlantic　currents　would　proceed　up　to　the　arctic；　warming　the　north　pole　but　deprivingeurope　of　all　that　kindly　warmth。　the　main　agent　of　heat　transfer　on　earth　is　what　is　knownas　thermohaline　circulation；　which　originates　in　slow；　deep　currents　far　below　the　surface鈥攁process　first　detected　by　the　scientist…adventurer　count　von　rumford　in　1797。

2what　happensis　that　surface　waters；　as　they　get　to　the　vicinity　of　europe；　grow　dense　and　sink　to　greatdepths　and　begin　a　slow　trip　back　to　the　southern　hemisphere。　when　they　reach　antarctica；they　are　caught　up　in　the　antarctic　circumpolar　current；　where　they　are　driven　onward　intothe　pacific。　the　process　is　very　slow鈥攊t　can　take　1；500　years　for　water　to　travel　from　the2the　term　means　a　number　of　things　to　different　people；　it　appears。　in　november　2002；　carl　wunsch　of　mitpublished　a　report　in　science；　〃what　is　the　thermohaline　circulation？；〃　in　which　he　noted　that　the　expressionhas　been　used　in　leading　journals　to　signify　at　least　seven　different　phenomena　（circulation　at　the　abyssal　level；circulation　driven　by　differences　in　density　or　buoyancy；　〃meridional　overturning　circulation　of　mass；〃　and　soon）…though　all　have　to　do　with　ocean　circulations　and　the　transfer　of　heat；　the　cautiously　vague　and　embracingsense　in　which　i　have　employed　it　here。

north　atlantic　to　the　mid…pacific鈥攂ut　the　volumes　of　heat　and　water　they　move　are　veryconsiderable；　and　the　influence　on　the　climate　is　enormous。

（as　for　the　question　of　how　anyone　could　possibly　figure　out　how　long　it　takes　a　drop　ofwater　to　get　from　one　ocean　to　another；　the　answer　is　that　scientists　can　measure　poundsin　the　water　like　chlorofluorocarbons　and　work　out　how　long　it　has　been　since　they　were　lastin　the　air。　by　paring　a　lot　of　measurements　from　different　depths　and　locations　they　canreasonably　chart　the　water鈥檚　movement。）thermohaline　circulation　not　only　moves　heat　around；　but　also　helps　to　stir　up　nutrients　asthe　currents　rise　and　fall；　making　greater　volumes　of　the　ocean　habitable　for　fish　and　othermarine　creatures。　unfortunately；　it　appears　the　circulation　may　also　be　very　sensitive　tochange。　according　to　puter　simulations；　even　a　modest　dilution　of　th