Snowflakes have six angles and crystal clear.

W: Sure, but they usually melt too fast for me to get a close look. Why do you ask？

M: I'm just curious. I was reading an article about the formation of snowflakes, and I real- ized that I had never paid much attention to them before.

W: WeI1, there is a big variety, isn't there？

M: Yes, but they all have one of the three basic forms: hexagonal columns, thin hexagonal plates and the blanching starshaped form.

W: I wonder why are forms different, maybe because ice starts to form. on dust particles with different shape.

M: Well, I thought it might have something, to do with the water saturation of the air. But we're both wrong. The author of this article did extensive research and concluded that. the shapes of snow crystals largely controlled by the temperature of the air. For example, the featherly star-shaped snowflake that everyone thinks is typical occurs only at a specific temperature.

W: Doesn't the relative humidity has anything to do with the shapes？

M: Apparently not. The effect of super saturation is simply to alter the growth rate. The greater the saturation, the faster the snowflakes form.

W: Why, next time when it snows. I'll make a point of taking a close look.

What are the people discussing？

A．Whether or not snowflakes can be analyzed.

B．How snowflakes are formed.

C．What causes a snowstorm.

D．Where the largest snowflakes can be found.

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 comers 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 cloud 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, say, -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-sided plates. If the crystal then drifts into an even warmer pocket of about -5℃, 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, columns, 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.

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 fall on Earth.

C．The statement that no two snowflakes axe alike is confirmed.

D．None of the above.

Section B

Directions: In this section, you will hear 3 short passages. At the end of each passage, you will hear some questions. Both the passage and the questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A, B, C and D.

听力原文：Winter weather has arrived in northern areas of the world. In much of the United States, winter means the return of mow. Snow is a subject of great interest to weather experts. Experts sometimes have difficulty estimating where, when or how much snow will fall.

Snow is a form. of frozen water. It contains many groups of tiny ice particles, called snow crystals. These crystals grow from water panicles in cold clouds. They usually grow around a piece of dust. All snow crystals have six sides, but they grow in different shapes. The shape depends mainly on the temperature and water levels in the air. The shape of a snow crystal may change from one form. to another as the crystal passes through levels of air with different temperatures. When melting snow crystals or raindrops fall through very cold air, they freeze to form. mall particles of ice, called sleet.

When snow crystals stick together, they produce snowflakes. Snowflakes come in different sizes. As many as 100 crystals may join together to form. a snowflake larger than two-and-one-half centimeters. Snow contains much less water than rain. About 15 centimeters of wet snow has as much water as 2.5 centimeters of rain. Each year, the continental United States has an average of 100 snow storms. An average storm produces snow for two to five days. Almost every part of the country has received snowfall at one time or another. Even parts of southern Florida have reported a few snowflakes.

Why are weather experts greatly interested in snow？

(27)

A．Because the shape of snow crystal(晶体) is still a mystery for experts.

B．Because the snowfall is a blessing for human beings.

C．Because experts have different ideas about the formation of snowflakes.

D．Because it is not easy to judge the time or place of snowfall.

听力原文：M: Have you ever looked really closely at the snowflake？

W: Sure, but they usually melt too fast for me to get a close look. Why do you ask？

M: I'm just curious. I was reading an article about the formation of snowflakes, and I real- ized that I had never paid much attention to them before.

W: WeI1, there is a big variety, isn't there？

M: Yes, but they all have one of the three basic forms: hexagonal columns, thin hexagonal plates and the blanching starshaped form.

W: I wonder why are forms different, maybe because ice starts to form. on dust particles with different shape.

M: Well, I thought it might have something, to do with the water saturation of the air. But we're both wrong. The author of this article did extensive research and concluded that. the shapes of snow crystals largely controlled by the temperature of the air. For example, the featherly star-shaped snowflake that everyone thinks is typical occurs only at a specific temperature.

W: Doesn't the relative humidity has anything to do with the shapes？

M: Apparently not. The effect of super saturation is simply to alter the growth rate. The greater the saturation, the faster the snowflakes form.

W: Why, next time when it snows. I'll make a point of taking a close look.

What are the people discussing？

A．Whether or not snowflakes can be analyzed.

B．How snowflakes are formed.

C．What causes a snowstorm.

D．Where the largest snowflakes can be found.

听力原文：W: Have you ever looked really closely at the snowflake？

M: Sure. But they usually melt too fast for me to get a close look. Why do you ask？

W: I'm just curious. I was reading an article about the formation of snowflakes and I realized that I. had never paid much attention to them before.

M: Well, there's a big variety, isn't there？

W: Yeah. But they all have one of three basic forms, hexagonal columns, hexagonal plates, and branching star-shaped form.

M: I wonder why the forms are different. Maybe because ice starts to form. on dust particles with different shapes .

W: Well, I thought it might have something to do with the water saturation of the air, but we were both wrong. The author of this article did extensive research and concluded that the shape of snow crystals is largely controlled by the temperature of the air. For example, the feathery star-shaped snowflake that everyone thinks is typical occurs only at a specific temperature.

M: Doesn't the relative humidity have anything to do with the shapes？

W: Apparently not. The effective super-saturation is simply to alter growth rate. The greater use the saturation, the faster the snowflakes form.

M: Mm, next time it snows, I'll make a point of taking a closer look.

Why has the woman brought up the subject of snowflakes？

A．She has never seen snow before.

B．She is conducting research on snow.

C．She wants to make artificial snow.

D．She has been reading about snow.

All the patients have to pay six dollars for their first visit to the doctor.

A．Right.

B．Wrong.

C．Doesn't say.

A.by

B.till

C.inside

D.within