The top of China's "bird's nest" Olympic stadium is open to the elements, and therefore the government has ordered the Beijing Meteorological Bureau to make sure it won't rain during the games. The Bureau has already had some success preventing light rain, but heavy rain is harder to control. They'll use two different "seeding" techniques for dissipating droplets in frozen clouds, and dissipating warmer clouds before they start forming water droplets. Beijing's head of weather manipulation, Zhang Qian, explains how.
For cold clouds below zero degrees, we use a coolant made from liquid nitrogen to increase the number of droplets while decreasing their mean size. As a result, the smaller droplets are less likely to fall and precipitation can be reduced. For clouds above zero degrees we use the seeding agent silver iodide to accelerate the droplets' collision and coalescence, producing a downdraft which suppresses the formation of clouds.
China 'will stop the rain' [News.com via Slashdot]