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  3. "Weather and Chaotic Systems" Weather and climate are closely related, but they are not quite the same thing. In any particul

"Weather and Chaotic Systems" Weather and climate are closely related, but they are not quite the same thing. In any particul

admin2018-07-24  61
问题 "Weather and Chaotic Systems"
   Weather and climate are closely related, but they are not quite the same thing. In any particular location, some days may be hotter or cooler, clearer or cloudier, calmer or stormier than others. The ever-varying combination of winds, clouds, temperature, and pressure is what we call weather. Climate is the long-term average of weather, which means it can change only on much longer time scales. The complexity of weather makes it difficult to predict, and at best, the local weather can be predicted only a week or so in advance.
   Scientists today have a very good understanding of the physical laws and mathematical equations that govern the behavior and motion of atoms in the air, oceans, and land. Why, then, do we have so much trouble predicting the weather? To understand why the weather is so unpredictable we must look at the nature of scientific prediction.
   Suppose you want to predict the location of a car on a road 1 minute from now. You need two basic pieces of information: where the car is now, and how fast it is moving. If the car is now passing Smith Road and heading north at 1 mile per minute, it will be 1 mile north of Smith Road in 1 minute.
   Now, suppose you want to predict the weather. Again, you need two basic types of information: (1) the current weather and (2) how weather changes from one moment to the next. You could attempt to predict the weather by creating a "model world." For example, you could overlay a globe of the Earth with graph paper and then specify the current temperature, pressure, cloud cover, and wind within each square. These are your starting points, or initial conditions. Next, you could input all the initial conditions into a computer, along with a set of equations (physical laws) that describe the processes that can change weather from one moment to the next.
   Suppose the initial conditions represent the weather around the Earth at this very moment and you run your computer model to predict the weather for the next month in New York City. The model might tell you that tomorrow will be warm and sunny, with cooling during the next week and a major storm passing through a month from now. But suppose you run the model again, making one minor change in the initial conditions—say, a small change in the wind speed somewhere over Brazil. A   This slightly different initial condition will not change the weather prediction for tomorrow in New York City.    B But for next month’s weather, the two predictions may not agree at all!   C
   The disagreement between the two predictions arises because the laws governing weather can cause very tiny changes in initial conditions to be greatly magnified over time. D   This extreme sensitivity to initial conditions is sometimes called the butterfly effect: If initial conditions change by as much as the flap of a butterfly’s wings, the resulting prediction may be very different.
   The butterfly effect is a hallmark of chaotic systems. Simple systems are described by linear equations in which, for example, increasing a cause produces a proportional increase in an effect. In contrast, chaotic systems are described by nonlinear equations, which allow for subtler and more intricate interactions. For example, the economy is nonlinear because a rise in interest rates does not automatically produce a corresponding change in consumer spending. Weather is nonlinear because a change in the wind speed in one location does not automatically produce a corresponding change in another location.
   Despite their name, chaotic systems are not necessarily random. In fact, many chaotic systems have a kind of underlying order that explains the general features of their behavior even while details at any particular moment remain unpredictable. In a sense, many chaotic systems—like the weather—are "predictably unpredictable." Our understanding of chaotic systems is increasing at a tremendous rate, but much remains to be learned about them.
Why do the predictions disagree for the computer model described in paragraph 5?
选项 A、The conditions at the beginning were very different.
B、The model was not accurately programmed.
C、Computer models cannot predict weather.
D、Over time models are less reliable.
答案D
解析 "For tomorrow’s weather, this slightly different initial condition will not change the weather prediction ... But for next month’s weather, the two predictions may not agree at all!" Choice A is not correct because the change in the initial conditions was minor. Choice B is not correct because it is not mentioned in the passage. Choice C is not correct because computer models are used to deal with all the data for weather prediction on time scales shorter than a few weeks.
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