题目内容
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Ice particles bouncing off hail falling through a cloud acquire______.
A．a negative charge
B．no electric charge at all
C．a positive charge
D．both A and C
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According to Dr. Scorer, Benjamin Franklin first proved thunderclouds are charged with electricity. Later investigation showed that the tops of the clouds have a great positive charge and the bottoms a great negative charge.
When the charges become great enough to break down the insulating properties of the air, lightning flashes carry the electricity within the cloud, or from cloud to cloud, or from cloud to earth. But the question remained: how do the charges develop within the cloud？
To seek the cause, Dr. Scorer and his colleagues at the college first duplicated thundercloud currents in a liquid tank. They found that mixture takes place only at the tops of the clouds.
Next a study of thunderclouds over the North Atlantic showed that lightning occurs only when the air temperature around the cloud is below freezing. Particles at the top of the cloud begin to freeze but those in the remainder of the cloud stay unfrozen although below freezing temperature.
To measure moisture the scientists sent planes equipped with external refrigerated rods into the clouds. The idea was that moisture would freeze on the rods and could later be measured. The men found, however, that some of the moisture particles bounced off the rods. His accidental discovery set the scientists on a new course of action.
In the laboratory, S.E. Reynolds whirled a refrigerated rod through ice particles and found that the particles which bounced off the rod acquired a positive charge. This was the missing link. Without hail and super-cooled clouds, he concluded, there could be no lightning.
This passage is about a______.
A．law
B．hypothesis
C．guess
D．theory
听力原文： You can tell the age of a tree by counting its tings. But these records of a tree's life really say a lot more. Scientists are using tree tings to learn what's been happening on the sun's surface for the last ten thousand years. Each ring represents a year of growth. As the tree grows, it adds a layer to its trunk, taking up chemical elements from the air. By looking at the elements in the rings from a given year, scientists can tell what elements were in the air that year.
Dr. Stevenson is analyzing one element, carbon—14, in rings from both living and dead trees. Some of the rings go back almost ten thousand years to the end of the Ice Age. When Stevenson followed the carbon--14 trail back in time, he found carbon—14 levels change with the intensity of solar burning. You see the sun has cycles. Sometimes it bums fiercely. At other times it's relatively calm. During the sun's violent periods, it throws off charged particles in fast moving streams, called solar winds. The particles interfere with the formation of carbon—14 on earth. When there is more solar wind activity, less carbon—14 is produced. Ten thousand years of tree rings show the carbon—14 level rises and falls about every four hundred and twenty years. The scientists concluded that solar wind activity must follow the same cycle.
(30)
A．To find out the origin of carbon- 14 on Earth.
B．To analyze the composition of different trees.
C．To look into the pattern of solar wind activity.
D．To examine the chemical elements in the Ice Age.
If pollution continues to increase at the present rate, formation of aerosols (悬浮微粒) in the atmosphere will cause the onset (开始) of an ice age in about fifty years' time. This conclusion, reached by Dr. S. Rasool and Dr. H. Schneider of the United States Goddard Space Flight Centre, answers the apparently conflicting questions of whether an increase in the carbon dioxide content of the atmosphere will cause the Earth to warm up or increasing the aerosol content will cause it to cool down. The Americans have shown conclusively that the aerosol question is dominant.
Two specters haunting conservationists have been the prospect that environmental pollution might lead to the planet's becoming unbearably hot or cold. One of these ghosts has now been laid, because it seems that even an increase in the amount of carbon dioxide in the atmosphere to eight times its present value will produce an increase in temperature of only 2℃. which would take place over several thousand years. But the other problem now looms larger than ever.
Aerosols are collections of small liquid or solid particles dispersed in air or some other medium. The particles are all so tiny that each is composed of only a few hundred atoms. Because of this they can float in the air for a very long time. Perhaps the most commonly experienced aerosol is industrial smog of the kind that plagued London in the 1950s and is an even greater problem in Los Angeles today. These collections of aerosols reflect the Sun's heat and thereby cause the Earth to cool.
Dr. Rasool and Dr. Schneider have calculated the exact effect of a dust aerosol layer just above the Earth's surface in the temperature of the planet. As the layer builds up, the present delicate balance between the amount of heat absorbed from the Sun and the amount radiated from the Earth is disturbed. The aerosol layer not only reflects much of the Sun's light but also transmits the infrared radiation from below. So, while the heat input to the surface drops, the loss of heat remains high until the planet cools to a new balanced state.
Within fifty years, if no steps are taken to stop the spread of aerosols in the atmosphere, a cooling of the Earth by as much as 3.5℃ seems inevitable. If that lasts for only a few years it would start another ice age, and because the growing ice caps at each pole would themselves reflect much of the Sun's radiation it would probably continue to develop even if the aerosol layer were destroyed.
The only bright spot in this gloomy forecast lies in the hope expressed by Dr. Rasool and Dr. Schneider that nuclear power may replace fossil fuels in time to prevent the aerosol content of the atmosphere from becoming critical.
The author's main purpose in writing the article is to warn of ______.
A．warm weather
B．hot weather
C．a new ice age
D．a new iceberg
When you look up at the night sky, what do you see？ There are other(26)_____bodies out there besides the moon and stars. One of the most(27)_____of these is a comet.
Comets were formed around the same time the Earth was formed. They are(28)_____ice and other frozen liquids and gases.(29)_____these "dirty snowballs" begin to orbit the sun, just as the planets do.
As a comet gets closer to the sun, some gases in it begin to unfreeze. They(30)_____dust particles from the comet to form. a huge cloud. As the comet gets even nearer to the sun, a solar wind blows the cloud behind the comet, thus forming its tail. The tail and the(31)_____fuzzy atmosphere around a comet are(32)_____that can help identify this(33)_____in the night sky.
In any given year, about a dozen known comets come close to the sun in their orbits. The average person can’t see them all, of course. Usually there is only one or two a year bright enough to be seen with the(34)_____eye. Comet Hale-Bopp, discovered in 1995, was an unusually bright comet. Its orbit brought it(35)_____close to the Earth, within 122 million miles of it. But Hale-Bopp came a long way on its earthly visit. It won’t be back for another four thousand years or so.
A.substances
B.materials
C.particles
D.matters
A.wheels
B.binders
C.particles
D.sands
A.The shape of the dust particles in the air.
B.The relative humidity.
C.The temperature of the air.
D.The geography of the area.
(a) How many particles does the drop collect？
(b) What is the percentage increase m mass of the drop due to these particles？
