The details of how this gets done are mighty impressive, starting with a supercomputer-driven weather tracking system that gives hourly forecasts for the Beijing area, specific to within a kilometer. Once an errant cloud is spotted though, the big guns are hauled out. Literally.
Then, using their two aircraft and an array of twenty artillery and rocket-launch sites around Beijing, the city's weather engineers will shoot and spray silver iodide and dry ice into incoming clouds that are still far enough away that their rain can be flushed out before they reach the stadium.
The obvious implications of technological hubris are dealt with in a smart and balanced way in the remainder of the story, with nods to some of China's other massive technological undertakings like the Three Gorges Dam, and a brief but engaging history of weather control systems across the globe. Worth a read, if only to see what it looks like when you take "designing your environment" to its logical extreme. Full story