Cloud seeding is a method of artificially causing clouds to produce precipitation （降水） in the form of rain or snow．Cloud seeding has also been used in attempts to modify the severity of hail storms and hurricanes．The effectiveness of cloud seeding remains controversial （有争议的）．but it continues to be used in some regions to try to increase rainfall for agriculture and to build snow packs for water supplies and power production． Cloud seeding was developed by American scientists Irving Langmuir and Vincent Joseph Schaefer during and after World War II．Their work began as an effort to learn more about the buildup of ice on airplane wings, and eventually led them to attempt to create rainfall by releasing several pounds of crushed frozen carbon dioxide into a cloud from an airplane．In this form, the carbon dioxide is called dry ice．On November 13, 1946, the technique appeared to produce snow directly under the cloud; the snow then turned to rain as it fell to the ground． For their experiment, Langmuir and Schaefer selected a supercooled cloud, one in which the water droplets remain liquid in subfreezing temperatures．Their theory was that small grains of dry ice falling through the cloud would cause tiny droplets of water vapor in the cloud to freeze into crystals that attracted more water vapor．Their theory proved to be correct and eventually the crystals became heavy enough to fall from the cloud as snow．As the snow reached the warmer temperatures closer to the ground, it melted and became rain． Another scientist, Bernard Vonnegut, produced a method of cloud seeding using silver iodide （碘化银）．He used particles of silver iodide because its crystal structure resembled that of ice in clouds．Silver iodide also had practical advantages over frozen carbon dioxide It could be stored at room temperature and did not require an airplane as a delivery mechanism．Instead, silver iodide crystals could be fired by cannons （大炮） high into the air, where wind carried them into the clouds． 小题1:According to the text, Langmuir and Schaefer had intended to deal with__ A．how to freeze carbon dioxide B．how to remove ice on plane wings C．how to make weather forecast for the war D how to hide planes in clouds at war 小题2:What is the CORRECT order of the course for cloud seeding with dry ice? a．snow becoming rain in wanner temperatures b．crystals attracting more water vapor c．crystals falling in the form of snow d．crystals becoming very heavy e．freezing tiny droplets of water vapor into crystals f．putting small grains of dry ice in the cloud
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