北美考试院 托福写作组 叶晓轩

阅读材料

Hail—pieces of ice that form and fall from clouds instead of snow or rain—has always been a problem for farmers in some areas of the United States. Hail pellets can fall with great force and destroy crops in the field. Over the last few decades, a method of reducing hail, called “cloud seeding,” has been tried. In cloud seeding, the chemical silver iodide is sprayed on storm clouds from an airplane. This makes the clouds produce harmless rain or snow instead of hail. Several pieces of evidence suggest that cloud seeding has been effective in protecting crops from hail.

Laboratory experiments

Experiments in the laboratory support the idea that cloud seeding is effective. Hail usually forms in water vapor that is close to the freezing point. However, when experimenters added silver iodide to cold water vapor in the laboratory, they often observed light snow forming instead of hail pellets.

Evidence from Asia

There is evidence about the effectiveness of cloud seeding from several countries around the world. In some Asian countries, for example, cloud seeding has been successfully used to control precipitation in urban areas. These positive results suggest that cloud seeding should also be effective in protecting fields and farms in the United States.

Local studies

A few local studies also support the value of cloud seeding. One study conducted in a farming region in the central United States, for example, directly monitored crop damage due to hail. The study found that in an area where cloud seeding was used there was reduced hail damage compared to previous years.

听力材料

It’s not clear that cloud seeding is all that effective and there are reasons to question each of the arguments you just read. 

First, it may be true that under laboratory conditions silver iodide creates snow instead of hail. However, in real life, silver iodide can actually prevent any precipitation at all from forming in the cloud, snow, rain or hail. This is a bad thing. Because if you seed all the clouds in areas where it doesn’t rain very often, you ran the risk of causing a drought. In this case the crops simply get damaged for a different reason: lack of water.

Second, it's not clear that positive result of cloud seeding in Asia can be repeated in the United States. The reason is that cloud seeding in Asia was tried in urban areas, in cities. And cities tend to have a high level of air pollution, from car, factory, etc. Surprisingly, pollution particles can create favorable conditions for cloud seeding because they interact with clouds and seeding chemicals. Such favorable conditions for cloud seeding may not occur in an unpolluted area. This means that the cloud seeding method that works in polluted cities may not work in unpolluted farming regions in the United States.

Third, the local study mentioned in the passage isn’t very convincing either. That’s because the study found that hail damage decreased not just in the area where the cloud seeding actually took place, but also in many of the neighboring areas to the east, south and north of the area. So the fact that the whole region was experiencing a reduce number of hail storms that particular year makes it more likely that this was a result of natural variation in local weather, and has nothing to do with cloud seeding.

范文参考：

The reading introduces a kind of technique, cloud seeding, and proposes three pieces of evidence to support that it would work out in protecting crops from hail damage in some areas of the United States. However, the speaker in the listening claims all the evidence to be questionable.

First, the reading offers evidence from the laboratory to support cloud seeding, because this technique successfully induced light snow instead of hail in experiment. By contrast, the speaker indicates this may not be practical in real life. She believes that cloud seeding does rid the hail, but it can prevent all forms of precipitation in the areas with limited precipitation originally, which may further invoke drought. So this technique may save the crops from hail, but it takes a risk of damage caused by the lack of water. Therefore, the speaker deems this evidence to be feeble.

Besides, the reading utilizes some successful cases from Asian urban areas, where the cloud seeding managed to help control precipitation. On the contrary, the speaker again challenges the validity of this proof. She doubts whether these cases could be repeated in the United States or not, due to the totally different air pollution conditions in Asia and America. Those successful cases were all in air-polluted areas, where the pollution particles in the air served as a perfect helping hand to cloud seeding. Hence, since American farming areas are typically unpolluted, it is reasonable to suspect that cloud seeding could not be effective in America. 

Last, the reading refers to the results of some local studies to bolster its viewpoint. It mentions one study in detail, in which cloud seeding reduced the hail damage in a farming region in central United States. Nevertheless, the speaker once again runs counter to this view. She reveals that in this specific study, reduced hail was observed not only in the particular area of the study, but also in its neighboring areas. Thus, there is every reason to believe that the reduced damage could not simply be accredited to cloud seeding, but to the weather variation in this vast region. Consequently, the speaker deems this study cannot prove anything, either.

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