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un warm onyour back on a summer鈥檚 day； it鈥檚 really excited atoms you feel。 the higher you climb； thefewer molecules there are； and so the fewer collisions between them。
air is deceptive stuff。 even at sea level； we tend to think of the air as being ethereal and allbut weightless。 in fact； it has plenty of bulk； and that bulk often exerts itself。 as a marinescientist named wyville thomson wrote more than a century ago： 鈥渨e sometimes find whenwe get up in the morning； by a rise of an inch in the barometer； that nearly half a ton has beenquietly piled upon us during the night； but we experience no inconvenience； rather a feeling ofexhilaration and buoyancy； since it requires a little less exertion to move our bodies in thedenser medium。鈥潯he reason you don鈥檛 feel crushed under that extra half ton of pressure is thesame reason your body would not be crushed deep beneath the sea： it is made mostly ofinpressible fluids； which push back； equalizing the pressures within and without。
but get air in motion； as with a hurricane or even a stiff breeze； and you will quickly bereminded that it has very considerable mass。 altogether there are about 5；200 million milliontons of air around us鈥�25 million tons for every square mile of the planet鈥攁 notinconsequential volume。 when you get millions of tons of atmosphere rushing past at thirty orforty miles an hour； it鈥檚 hardly a surprise that limbs snap and roof tiles go flying。 as anthonysmith notes； a typical weather front may consist of 750 million tons of cold air pinnedbeneath a billion tons of warmer air。 hardly a wonder that the result is at timesmeteorologically exciting。
certainly there is no shortage of energy in the world above our heads。 one thunderstorm； ithas been calculated； can contain an amount of energy equivalent to four days鈥櫋se ofelectricity for the whole united states。 in the right conditions； storm clouds can rise to heightsof six to ten miles and contain updrafts and downdrafts of one hundred miles an hour。 theseare often side by side； which is why pilots don鈥檛 want to fly through them。 in all； the internalturmoil particles within the cloud pick up electrical charges。 for reasons not entirelyunderstood the lighter particles tend to bee positively charged and to be wafted by aircurrents to the top of the cloud。 the heavier particles linger at the base； accumulating negativecharges。 these negatively charged particles have a powerful urge to rush to the positivelycharged earth； and good luck to anything that gets in their way。 a bolt of lightning travels at270；000 miles an hour and can heat the air around it to a decidedly crisp 50；000 degreesfahrenheit； several times hotter than the surface of the sun。 at any one moment 1；800thunderstorms are in progress around the globe鈥攕ome 40；000 a day。 day and night across theplanet every second about a hundred lightning bolts hit the ground。 the sky is a lively place。
much of our knowledge of what goes on up there is surprisingly recent。 jet streams； usuallylocated about 30；000 to 35；000 feet up； 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