PASSAGE 18
Volts from the Sky
Lightning has caused awe and wonder since old times. Although Benjamin Franklin demonstrated lightning as enormous electrical discharge more than 200 years ago， many puzzles still surround this powerful phenomenon.
Lightning is generated when electrical charges separate in rain clouds， though processes are still not fully understood. Typically， positive charges build at the cloud top， while the bottom becomes negatively charged. In most instances of cloud-to-ground lightning， the negatively charged lower portion of the cloud repels negatively charged particles on the ground s surfaces， making it become positively charged. The positive charge on the ground gathers at elevated points.
A flow of electrons begins between the cloud and earth. When the voltage charge becomes large enough， it breaks through the insulating barrier of air， and electrons zigzag earthward. We see the discharge as lightning.
Lightning can occur within a cloud， between clouds， or between clouds and the ground. The first variety， intra-cloud lightning， is the most frequent but is often hidden from our view. Cloud-to-ground lightning， making up about 20 percent of lightning discharges， is what we usually see. Lightning comes in several forms， including sheet， ribbon， and ball. Intra-cloud lightning can illuminate a cloud so it looks like a white sheet， hence its name. When cloud-to-ground lightning occurs during strong winds， they can shift the lightning channel sideways， so it looks like a ribbon. The average lightning strike is more than 3 miles long and can travel at a tenth of the speed of light. Ball lightning， the rarest and most mysterious form， derives its name from the small luminous ball that appears near the impact point， moves horizontally， and lasts for several seconds.
Thunder is generated by the tremendous heat released in a lightning discharge. Temperatures near the discharge can reach as high as 50，000F within thousandths of a second. This sudden heating acts as an explosion， generating shock waves we hear as thunder.
About 2，000 thunderstorms are occurring in the world at any time， generating about 100 light
ing strikes every second， or 8 million daily. Within the United States， lightning strikes are estimated at 20 million a year， or about 22，000 per day. You have a 1-in-600，000 chance of bring struck by lightning during your lifetime. Lightning can strike twice or more in the same spot. The Empire State Building in New York is struck by lightning about two dozen times annually.
You can measure how far you are from a lightning strike by counting the seconds between viewing the flash and hearing the bang， and then dividing by five. This approximates the mileage.
练习：1-4题的要求是，从第一个方框的六个选项A、B、C、D、E、F中选出四个选项为注明的段落各配一个合适的小标题。5-8题的要求是，从第二个方框的六个选项A、B、C、D、E、F中选出四个选项完成每个句子。
1. Paragraphs 2 and 3
2. Paragraph 4
3. Paragraph 5
4. Paragraph 6
A Cloud-to-ground lightning occurring in the U.S.
B Types of lightning
C Cause of lightning
D Differences between thunder and thunderstorm
E Frequencies of thunderstorms occurring in the world and the U.S.
F Shock waves as thunder
5. In most cases of cloud-to-ground lightning， the ground s surface
6. One form of lightning that is ball lightning
7. Cloud lightning looks like a ribbon when its lighting channel
8. Although not fully understanding processes of lightning， man
A occurs most infrequently.
B is shifted sideways by strong winds.
C is often hidden from our view.
D is equipped with a good knowledge of various forms of lightning .
E is estimated at 20 millions a year.
F is positively charged.
KEYS： CBFE FABD
PASSAGE 19
Geology and Health
The importance of particular metals in the human diet has been realized within the past few decades， and the idea that geology might be related to health has been recognized for a number of elements such as iodine， zinc and selenium. For example， soils with low iodine contents produce crops， and animals deficient in iodine. A lack of iodine in the human diet leads to some serious diseases.
The ultimate source of metals within the human body is rocks， which weather into soil， gaining or losing some of their chemical constituents. The crops we eat selectively rem
ove from the soil the elements that they require for growth. The water we drink contains trace elements leached from rock and soil. Thus the geology and geochemistry of the environments have effects on the chemistry and health of plants， animals and people.
So far there is no data to suggest that people living on metal-rich soils experience a potential health hazard. The levels of metals within naturally contaminated soils are generally not high enough to cause serious health problems. Living on metal-rich soils does not represent a health risk unless large quantities of soil are digested or metal-rich dust is inhaled. However， small children are particularly exposed to metal-rich topsoil in playgrounds and gardens. They are also the most likely ones to eat potentially dangerous metal-rich soil.
Heavy metals are persistent; they do not break down to other chemicals in the environment. Industrially polluted sites usually undergo intensive clean-up and rehabilitation because heavy metals are a health concern once they enter the food chain. Some trace metals are alleged to cause cancer and are also known to cause poisoning.
In contrast， naturally contaminated soils have not been subject to risk assessment studies and rehabilitation measures， despite the fact that they frequently possess metal concentrations well above those of such polluted by humans and above environmental quality criteria.
There is a vital need to understand the potential risks and long-term health effects of living on naturally contaminated soils. Future environmental investigations of naturally polluted soils should concentrate on the potential pathways of metals into the food chain and human body. Geologists should be part of such studies as they can provide the essential background information on rock and soil chemistry as well as the chemical forms of heavy metal pollution.
A. No evidence to indicate bad effects of naturally contaminated soil
B. Potential hazards of human contaminated soils
C. Research on channels of heavy metals getting into human food chain
D. Geology and health problems
E. Rocks-the ultimate source of soil pollution
F. Long- term helth effects on children
1. Paragraph 1
2. Paragraph 3
3. Paragraph 4
4. Paragraph 6
A. industrially polluted soils
B. rock and soil chemistry
C. naturally polluted soils
D. the pathways of metals into the food chain
E. the element of iodine
F. the persistence of heavy metals
5. Some serious diseases is connected with deficiency of
6. It is extremely necessary to study the long-term effects caused by living on
7. Geologists are indispensable in the research project on geology and health due to their knowledge on
8. Industrially contaminated sites usually require a thorough clean-up due to
Key： DABCECBF
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