cloud seeding is a common technique to enhance precipitation. cloud seeding entails spraying small particles, such as silver iodide, onto clouds to affect their development, usually with the goal of increasing precipitation. cloud seeding only works to the extent that there is already water vapor present in the air. critics generally contend that claimed successes occur in conditions which were going to lead to rain anyway. it is used in a variety of drought-prone countries, including the united states, china, india, and russia. in china there is a perceived dependency upon it in dry regions, and there is a strong suspicion it is used to "wash the air" in dry and heavily polluted places, such as beijing. in mountainous areas of the united states such as the rocky mountains and sierra nevada, cloud seeding has been employed since the 1950s.