Cloud seeding refers to an attempt to modify the weather in changing the amount of precipitation that falls from the clouds. Through the process, substances are dispersed into the atmosphere that will alter the amount of condensation that takes place within the clouds. It is usually done to increase the amount of snow or rain that falls from the clouds, although it can also be used to suppress fog and hail around airports.
Silver iodide and dry ice (frozen carbon dioxide) are the common substances used in cloud seeding. The use of salt has also shown promising results in producing the desired effect. The liquid used must be much colder than the air within the clouds. Silver iodide, for example, will induce freezing. Dry ice will also help to cool the air to the point that ice crystals will nucleate spontaneously during the vaporization phase. With the use of dry ice, there is no need to use any existing droplets as is needed with the use of silver iodide. However, ice crystals are necessary for the ice crystals to become large enough to cause precipitation to develop.
The pressure of vapor is higher over ice than it is over water. When the ice crystals develop and grow within the clouds, they become heavy enough to fall as precipitation. Snow will result when the temperature of the air is cold enough to sustain it. If it not cold, then the precipitation will fall in the form of rain. Without seeding, the clouds would not produce any precipitation at all. This process is called static seeding.
The chemicals needed for cloud seeding are often dispersed into the atmosphere by aircraft or by dispersion equipment located on the ground. When aircraft are used, the chemicals of silver iodide are sent into the clouds by igniting them and sending them out through the aircraft windows. When ground equipment is used the wind and air carry the fine particles up into the clouds.
There is a controversy about the effectiveness of this technique. It has been shown to be effective in altering the size and shape of a cloud but there is no definitive answer as to how much change it causes in the normal amount of precipitation that would have come from the cloud if it had been left alone. The Weather Modification Association, the World Meteorological Organization and the American Meteorological Association have published documents on this process and state that there is evidence pointing to the fact that cloud seeding does increase the amount of precipitation by about 10%.
During the Olympics of 2008 in Beijing, the Chinese did use the technique of cloud seeding to prevent any rain from falling for the duration of the games. However, since this country limits the amount of information it publishes regarding any of its experiments, there is no documentation to prove that this did take place or whether it did play a role in the weather at the time.