The Olympic Games in Beijing are less than two weeks away, and the Chinese have been practicing hard.</p><p>At modifying the weather.</p><p>Worried that rain could ruin the opening ceremonies on Aug. 8, the Chinese government has set up anti-aircraft guns in cities outside Beijing that would shoot silver iodide into clouds.</p><p>The hope is the clouds would release rain before it otherwise would at the opening ceremonies.</p><p>It&#8217;s a process called cloud seeding.</p><p>Andrew Detwiler, a professor in the atmospheric sciences department at the South Dakota School of Mines and Technology, said silver iodide is a salt. It is used to help make rain when shot into a cloud.</p><p>&#8220;The silver iodide evaporates and then re-condenses as little tiny smoke particles,&#8221; Detwiler said. &#8220;Micrometers in diameter or less. It&#8217;s not salt like you&#8217;d get out of a salt shaker, it&#8217;s almost invisible smoke. Your hope is you make a few kilograms of silver iodide into millions of these little particles if you can.&#8221;</p><p>Clouds contain water, but rain doesn&#8217;t occur until the moisture in the clouds is heavy enough to fall. Temperatures are colder in the clouds, because they are so far above the earth.</p><p>&#8220;The whole idea with silver iodide is to try to stimulate this freezing process, get a few ice particles to form in this cloud of supercooled liquid droplets,&#8221; Detwiler said. &#8220;Once they do, then the little droplets evaporate and the ice particles grow big and they fall.&#8221;</p><p>The ice particles turn to rain as they get nearer to the earth, where it is warmer. The whole thing is known as the collision-coalescence process, Detwiler said.</p><p>The Chinese government, which reportedly has 7,000 artillery guns, 4,000 rocket launchers and numerous planes across the country for weather modification, also may try to induce rain to break up choking smog.</p><p>Detwiler noted that weather modification has been used in western Kansas, often to cut down on hail.</p><p>Can it really work to stop rain before the opening ceremonies? Detwiler has his doubts.</p><p>&#8220;They have a weather radar that tells them where the cloud is, so they try to get the silver iodide into the cloud at the right temperature level where there&#8217;s the right conditions for the particles to form and grow,&#8221; Detwiler said. &#8220;The real problem is that there is a lot of variability in clouds, so it&#8217;s usually very difficult to tell if you&#8217;re getting it into the right place.&#8221;