超过两三天，世界上最好的天气预报也只是推测性的；而超过六七天，它们便毫无价值。
Beyond two or three days, the world's best weather forecasts are speculative, and beyond six or seven they are worthless.
蝴蝶效应就是原因所在。
The Butterfly Effect is the reason.
对于小规模的天气系统——而对于一名全球尺度的预报员来说，"小"可以意味着雷暴和暴风雪——任何预测都会迅速恶化。
For small pieces of weather — and to a global forecaster, small can mean thunderstorms and blizzards — any prediction deteriorates rapidly.
误差和不确定性不断倍增，如瀑布般向上级联，穿越一系列湍流特征，从尘卷风和飑，一路延伸至只有卫星才能观测到的大陆尺度涡旋。
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.
现代天气模型使用间距约为六十英里的网格点阵进行运算，即便如此，一些初始数据仍不得不靠猜测，因为地面站和卫星无法覆盖每一个角落。
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.
但是，假设地球表面能够被以一英尺为间距排列的传感器所覆盖，这些传感器以一英尺为间隔向上延伸，一直延伸到大气层的顶端。
But suppose the earth could be covered with sensors spaced one foot apart, rising at one-foot intervals all the way to the top of the atmosphere.
再假设每一个传感器都能给出温度、气压、湿度以及气象学家所需的其他任何量的完全精确读数。
Suppose every sensor gives perfectly accurate readings of temperature, pressure, humidity, and any other quantity a meteorologist would want.
恰好在正午十二点，一台无限强大的计算机获取所有数据，并计算出12点01分、接着12点02分、再接着12点03分……各个点将会发生什么。
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...
这台计算机仍然无法预测普林斯顿（新泽西州）在一个月后的某天会是晴天还是雨天。
The computer will still be unable to predict whether Princeton, New Jersey, will have sun or rain on a day one month away.
在正午时分，传感器之间的空隙将会隐藏一些计算机无从得知的波动——那是一些微小的偏离平均值的偏差。
At noon the spaces between the sensors will hide fluctuations that the computer will not know about, tiny deviations from the average.
到12点01分时，那些波动将已经在相距一英尺处制造出了小误差。
By 12.01, those fluctuations will already have created small errors one foot away.
很快，误差将已经扩大到十英尺的尺度，如此类推，直至扩展到全球的规模。
Soon the errors will have multiplied to the ten-foot scale, and so on up to the size of the globe.