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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’spresentation 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 “heaped” in latin）； and cirrus （meaning “curled”） for the high；thin feathery formations that generally presage colder weather。 to these he subsequentlyadded a fourth term； nimbus （from the latin for “cloud”）； for a rain cloud。 the beauty ofhoward’s system was that the basic ponents could be freely rebined to describe everyshape and size of passing cloud—stratocumulus； 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’s 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—mammatus； pileus； nebulosis； spissatus； floccus； and mediocris area sampling—have never caught on with anyone outside meteorology and not terribly muchthere； i’m 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 “to be oncloud nine。”
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—“about enough to fill a bathtub；” as james trefil has noted。 you can get some sense of theimmaterial quality of clouds by strolling through fog—which is； after all； nothing more than acloud that lacks the will to fly。 to quote trefil again： “if you walk 100 yards through a typicalfog； you will e into contact with only about half a cubic inch of water—not enough togive you a decent drink。” in consequence； clouds are not great reservoirs of water。 only about0。035 percent of the earth’s 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—thousands 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—drury says twelve days—in the skybefore falling again as rain。
evaporation is a swift process； as you can easily gauge by the fate of a puddle on asummer’s 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—indeed； making them just dilute enough to freeze over larger areas than normal。
the enlarged area of ice bounced back more of the sun’s 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’s 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’s 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’s 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’t 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—aprocess 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—it 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