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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！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