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Mysterious night-shining clouds that light up the polar skies have become more shining and frequent in recent years and climate

admin2015-04-10  41
问题     Mysterious night-shining clouds that light up the polar skies have become more shining and frequent in recent years and climate change may be the culprit, scientists announced. So-called noctilucent clouds, which streak across the sky in sparking colors during polar summers, are ten times brighter than previously believed, according to recent data from NASA’s AIM satellite. The data, collected during two polar cloud seasons, also suggest the formations appear daily, are more widespread, and have started to form at lower latitudes than before.
    Night-shining clouds are made mostly of ice particles that form when water vapor condenses onto atmospheric dust. The AIM mission is the "first detailed exploration of the clouds, which form about 50 miles above Earth’s surface—"literally on the edge of space," AIM principal investigator said. The air there is a thousand times drier than the Sahara, and the temperature can drop to minus 235 degrees Fahrenheit—ideal conditions for creating clouds. A likely explanation for the clouds’ surge is that temperatures in the upper atmosphere have gotten even colder due to the build-up of carbon dioxide in the atmosphere. Yearly emissions of carbon dioxide levels have grown about 20 percent since 1990. Most scientists agree the build-up is causing climate change and heating up Earth’s surface. In the upper atmosphere, however, carbon dioxide does not act like a thick, heat-trapping blanket.
    Instead it forms a thin layer, allowing heat to escape, temperatures to drop, and ice particles to form. These clouds are the mine’s canary of global warming.
    The evidence linking carbon dioxide to changes in night-shining clouds is still tentative. But in all honesty, we don’t know of any other plausible reasons why it wouldn’t be global climate change. If that’s true, that says not only are we changing the atmosphere where we’re living, we’re changing a very remote, rarified part of our atmosphere. Such far-reaching change means that we need to be concerned. If the cloud changes are indeed tied to carbon dioxide build-up, here’s another confirming reason we should strive to do something about it.
    AIM also discovered "dramatic changes" in cloud behavior, said Scott Bailey, AIM’s deputy principal investigator. The data suggest the clouds are highly variable, rotating every five days according to minute shifts in temperature. " These clouds are exquisite thermometers. Now we’re able to see the whole life cycle of a cloud for the first time," Bailey told National Geographic.
What are the ideal conditions for creating clouds?
选项 A、The temperature is as low as minus 235 degrees Fahrenheit.
B、The air is a million times drier than that of Sahara.
C、Extremely dry air and low enough temperature.
D、A mild climate and the build-up of carbon dioxide.
答案C
解析 云形成的理想条件是什么?[A]气温低至华氏零下235度。[B]空气的干燥度是撒哈拉沙漠的一百万倍。[C]极度干燥的空气和足够低的气温。[D]气候温和，且有二氧化碳的累积。由信息提示词creating clouds可定位在第二段第三句话，即The air there is…ideal conditions for creating clouds．破折号前面的内容就是答案，故选[C]。
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