By Wyclef Jean

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，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 WarⅡ．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 （晶體） 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 （碘化銀）．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．