Guard at the Tiananmen Square in Beijing on November 1, 2009.
"Everybody complains about the weather," the old saying goes, "But nobody does anything about it." That is, until now. A Nov. 1 snowfall in Beijing — the city's earliest since 1987 — is due, Chinese scientists say to a campaign of "cloud seeding" to encourage precipitation. If true, it's the wettest success yet in a long-standing effort to artificially bring moisture to the parched northern regions of China. So how'd they do it? For those who haven't had a meteorology class since middle school, a quick review of Weather 101: Colder air encourages precipitation, so when the temperature drops at high altitude, water naturally condenses out of the air. Clouds are formed when this moisture, suspended in tiny droplets or crystals, meets a condensation nuclei — small particles of dust or ice that are blown about the upper atmosphere. Without these small particles, clouds can't form.
The method of cloud seeding used by the Chinese involves dosing the atmosphere with silver iodide, an chemical solution either dropped from planes or shot up from the ground. (Other methods use salt or dry ice.) The silver iodide particles supercharges cloud formation, as it acts as excellent condensation nuclei. Once clouds form, they also start a positive feedback effect. As droplets freeze and are added to the cloud, they release their heat, creating an updraft which draws additional moisture from the ground into the atmosphere.
That's the theory, anyway. The effectiveness of cloud seeding is still disputed, because it's difficult to say with any certainty that cloud seeding is responsible for a storm rather than Mother Nature. But if you choose to believe in cloud seeding, the Chinese scientists may have even overdone it. The snowstorm lasted for 11 hours, disrupting flights in and out of Beijing and hampering shipping off the Chinese coast. Still, expect few complaints from the generally dry region: it's the most accumulation the city's seen in a decade, and further proof the Chinese may be becoming the world's best at managing weather. In a 2008 experiment, scientists seeded clouds in advance of the Beijing Summer Olympics, successfully ensuring clear skies for the opening ceremony.
source: Time.com