Snowflakes
You’ve probably heard that no two snowflakes are alike. Of course, nobody has ever confirmed that statement by examining every one of the estimated one septillion snowflakes that drift to Earth each year. still, Kenneth Libbrecht, a professor at the California Institute of Technology, is confident that the statement is true.
Snowflakes aren’t flaky, says Libbrecht. At their basic level, they’re crystalline. The lattice of every snowflake is six-sided in shape. The simplest snow crystals are six-sided flat plates and six-sided columns. Such crystals are common in places where the air is extremely cold and dry. Snow crystals acquire their special beauty when their simple six-sided symmetry blossoms. Under the right conditions, each of the six corners of a crystal sprouts what is called an arm. In a matter of minutes, the arms can become highly ornate and give the crystal a star like appearance.
Several factors in the environment affect the shape and growth rate of a snow crystal. One factor is humidity. Crystals grow faster and in more intricate shape as humidity increases. A second factor is air temperature. A snowflake is born when several molecules of water vapor in a could land on a speck of dust and freeze to form a simple crystal. As the young crystal bops around in the cloud, it passes through air pockets of varying temperatures. If the crystal passes through a pocket of air that is, says,—15 degrees Celsius, it will grow quickly and sprout six arms, says Libbrecht. If the crystal is then tossed into a warmer pocket, one about-10℃, the arms’ tips will stop growing quickly and form six-side plates. If the crystal then drifts into an even warmer pocket of about -℃, its top and bottom will grow more quickly than its sides and become more column like in shape.
In the course of its life span, a snow-crystal might flutter through many warmer and colder pockets, acquiring a complicated and unique growth history. Such a history will give rise to a snowflake that is unlike any other. Each arm on the snowflake will look exactly like every other one, but the crystal itself will be one of a kind.
Using his cooling tanks, Libbrecht has learned how to create snow crystals of different shapes—plates, colhuns, needles etc. Libbrecht has even refined his techniques so that he can make crystals that look highly similar to one another. Still, he lacks the control to manufacture identical twin snowflakes. A slight difference in humidity and temperature can upset the growth profile of a crystal.
1. What does Professor Libbrecht believe to be true?
A. No two snowflakes are exactly the same in shape.
B. Somebody has examined all the snowflakes that on Earth.
C. The statement that no two snowflakes are alike is confirmed.
D. None of the above.
2. What do the simplest snow crystals look like?
A. They have six columns.
B. They are flaky.
C. They are cubic in shape.
D. They are six-sided.
3. What are the factors that affect the shape and growth rate of a snow crystal?
A. Humidity and temperature.
B. Water and falling speed.
C. Air and altitude.
D. Both B and C.
4. It can be felt from the description in the 2nd paragraph that the author
A. admires the beauty of the snowflakes.
B. dislikes the changing growth history of the snowflakes.
C. has a particular feeling for those flower-like crystals.
D. likes to compare snowflakes to the stars in the sky.
5. Libbrecht is not able to
A. create snow crystals of different shapes.
B. make crystals that look similar to one another.
C. create snowflakes that are exactly alike.
D. refine his techniques.
Powering a City? It’s a Breeze
The graceful wooden windmills that have broken up the flat Dutch landscape for centuries—a national symbol like wooden shoes and tulips—yielded long ago to ungainly metal-pole turbines.
Now, windmills are breaking into a new frontier. Though still in its teething stages, the “urban turbine” is a high-tech windmill designed to generate energy from the rooftops of busy citles. Lighter, quieter, and often more efficient than rural counterparts, they take advantage of the extreme turbulence and rapid shifts in direction that characterize urban wind patterns.
Prototypes have been successfully tested in several Dutch cities, and the city government in the Hague has recently agreed to begin a large-scale deployment in 2003. Current models cost US$8,000 to US$12,000 and can generate between 3,000 and 7,000 kilowatt hours of electricity per year. a typical Dutch household uses 3,500 kilowatt hours per year, while in the United States, this figure jumps to around 10,000 kilowatt hours.
But so far, they are being designed more for public or commercial buildings than for private homes. The smallest of the current models weigh roughly 200 kilograms and can be installed on a roof in a few hours without using a crane.
Germany, Finland and Denmark have also been experimenting with the technology, but the ever-practical Dutch are natural pioneers in urban wind power mainly because of the lack of space. The Netherlands, with 16 million people crowded into a country twice the size of Slovenia, is the most densely populated in Europe.
Problems remain, however, for example, public safety concerns, and so strict standards should be applied to any potential manufacturers. Vibrations are the main problem in skyscraper-high turbine. People don’t know what it would be like to work there, in an office next to one of the big turbines. It might be too hectic.
Meanwhile, projects are under way to use minimills to generate power for lifeboats, streetlights, and portable generators. “I think the thing about wind power is that you can use it in a whole range of situations,” said Corin Millais, of the European Wind Energy Association. “It’s a very local technology, and you can use it right in you backyard. I don’t think anybody wants a nuclear power plant in their backyard.”
1. What are the symbols of Netherlands according to the first paragraph? 考资网(www.kaozi.com)
A. The flat landscape.
B. Wooden shoes and wooden windmills.
C. Metal-pole turbines.
D. Both A and B.
2. Which statement is best describes the urban turbine mentioned in the second paragraph?
A. It is a windmill put on rooftops of buildings for energy generation.
B. It is a high-tech machine designed to generate energy for urban people.
C. It is light and quiet and therefore more efficient.
D. It is driven by urban wind.
3. The smallest models of an urban turbine
A. is designed for private homes.
B. weighs 2,000 kilograms.
C. can be carried up to the rooftop without a crane.
D. can be installed with a crane.
4. The Netherlands leads in the urban turbine technology because 考资网(www.kaozi.com)
A. the Dutch are natural pioneers.
B. the Dutch have a tradition with windmills.
C. Netherlands is windier than Germany, Finland and Slovenia.
D. Netherlands is a small country with a large population.
5. According to the last paragraph, what are the advantages of wind power technology?
A. It can be used for different purposes.
B. It can replace nuclear power plant.
C. It can be in stalled in one’s backyard.
D. It can be installed in one’s backyard.
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