新概念英语4册第14课

The Butterfly Effect

Beyond two or three days, the world`s best weather forecasts are speculative, and beyond six or seven they are worthless.

The Butterfly Effect 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.

The modern weather models work with a grid of points of the order of sixty miles apart, and even so, some starting data has to be guessed, since ground stations and satellites cannot see everywhere. But suppose the earth could be covered with sensors spaced on foot apart, rising at one-foot intervals all the way to the top of the atmosphere. Suppose every sensor gives perfectly accurate readings of temperature, pressure, humidity, and any other quantity a meteorologist would want. Precisely at noon an infinitely powerful computer takes all the data and calculates what will happen at each point at 12:01, then 12:02, then 12:03 ...

The computer will still be unable to predict whether Princeton, New Jersey, will have sun or rain on a day one month away. At noon the spaces between the sensors will hide fluctuations that the computer will not know about, tiny deviations from the average. By 12:01, those fluctuations will already have created small errors one foot away. Soon the errors will have multiplied to the ten-foot scale, and so on up to the size of the globe.

两到三天的天气预报是一种猜测，而六到七天的天气预报就毫无价值了。这是由蝴蝶效应引起的。从一次小的雷暴或暴风雪到全球的天气预报，预报不准确的程度大大增加。从小的由尘暴和暴风引起的空气流直到只有从卫星上才能观察到的大陆范围内的旋涡，错误和不确定性成倍增加。

现代的天气模型是由一个六英里间隙的网格点确定的。即使这样，因为地面站和卫星不可能观察到地面的每个点，一些初始数据仍然要依靠猜测。但假如地球被间隔一英尺的传感器覆盖，直到大气层的顶端，并且每个探测器都可以获得完全准确的温度、压力、湿度、以及其他任何气象学家所需要的值，在正午时一台运算功能无限强大的计算机获得了所的数据并计算出12:01， 12:02， 12:03...会发生什么。

这台计算机仍然不能预测出新泽西州普林斯顿在一个月后是晴天还是雨天。在正午时，传感器的间隔会隐藏一些波动，使计算机无法得到，则平均数值会有一点偏移。当12:01时，这种波动会造成一英寸的误差，而很快误差就会以大概10英寸成倍增长。直到影响全部的数据。