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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！hough 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n latin）； and cirrus （meaning 鈥渃urled鈥潱or 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鈥潱弧or 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s 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n 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