How is seeding clouds to produce rain and dispersing rainmaking clouds possible? How is it done?

Question

China is planning to disperse the clouds on 2008 Olympics to achieve a favorable weather.

Answer 1

The most common chemicals used for cloud seeding include silver iodide and dry ice (frozen carbon dioxide). The expansion of liquid propane into a gas is being used on a smaller scale. The use of hygroscopic materials, such as salt, is increasing in popularity because of some promising research results.

Seeding of clouds requires that they contain supercooled liquid water—that is, liquid water colder than zero degrees Celsius. Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing (heterogeneous nucleation). Dry ice or propane expansion cools the air to such an extent that ice crystals can nucleate spontaneously from the vapor phase. Unlike seeding with silver iodide, this spontaneous nucleation does not require any existing droplets or particles because it produces extremely high vapor supersaturations near the seeding substance. However, the existing droplets are needed for the ice crystals to grow into large enough particles to precipitate out.

Answer 2

During the formation of a cloud,condensation nuclei (tiny salt particles,dust particls,droplets of sulphuric acid etc) play an important role.The atmosphere usually has sufficient condensation nuclei present in the air.The water vapour(when the condensation takes place) become water droplets and become clouds in the presence of the condensation nuclei.The cloud droplets attach themselves with the nuclei.This concept of cloud condesation nuclei has led to the idea of cloud seeding that tries to encourage rainfall by seeding with condensation nuclei appropriately. Thus the nuclei which are used are silver iodide and dry ice(which is frozen carbon-di-oxide).Sometimes salt is added as additional nuclei as supur cooled water is required for the precipitation to occur.
However, the amount of excess rainfall produced in this manner remains as a controversy.