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Lesson 14
TheButterflyEffect
蝴蝶效应
Firstlistenandthenanswerthefollowingquestion.
听录音，然后回答以下问题。
Whydosmallerrorsmakeitimpossibletopredicttheweathersystemwithahighdegreeofaccuracy?
Beyondtwoorthreedays, theworld'sbestweatherforecastsarespeculative, andbeyondsixorseventheyareworthless.
TheButterflyEffect is the reason. For small pieces of weather -- and to a global forecaster, small can mean thunderstorms and blizzards -- any prediction deteriorates rapidly. Errors and uncertainties multiply, cascading upward through a chain of turbulent features, from dust devils and squalls up to continent-size eddies that only satellites can see.
Themodernweathermodelsworkwithagridofpointsoftheorderofsixtymilesapart, andevenso, somestartingdatahastoguessed, sincegroundstationsandsatellitescannotseeeverywhere. Butsupposetheearthcouldbecoveredwithsensorsspacedonefootapart, risingatone-footintervalsallthewaytothetopoftheatmosphere. Supposeeverysensorgivesperfectlyaccuratereadingsoftemperature, pressure, humidity, andanyotherquantityameteorologistwouldwant. Preciselyatnoonaninfinitelypowerfulcomputertakesallthedataandcalculateswhatwillhappenateachpointat 12.01, then 1202, then 12.03...
ThecomputerwillstillbeunabletopredictwhetherPrinceton, NewJersey, willhavesunorrainonadayonemonthaway. Atnoonthespacesbetweenthesensorswillhidefluctuationsthatthecomputerwillnotknowabout, tinydeviationsfromtheaverage. By 12.01, thosefluctuationswillalreadyhavecreatedsmallerrorsonefootaway. Soontheerrorswillhavemultipliedtotheten-footscale, andsoonuptothesizeoftheglobe.