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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 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—but 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’s m