The summer months are among the rainiest in Beijing, and officials are hoping that rain clouds will sweep in at the right time and wash away the bad air. "Whether pollution is a big problem or not depends on the weather," Streets says. "Pollutants will build up, but they can be dispersed either by rain or wind."
Last year, Beijing's Weather Modification Office announced plans to use a technique called cloud seeding, which attempts to induce precipitation by filling moisture-laden clouds with tiny particles of silver iodide. The particles, dispersed by plane or shot into the air from ground-based rockets, enable water vapor to condense and form raindrops.