Cloud seeding is a weather modification technique which is used to disperse fog, increase precipitation levels and suppress hail. The process involved in cloud seeding was developed in 1946 and it is actually rather complex to describe. In order to create cloud seeding, scientists have used a couple of chemicals such as silver iodide and dry ice (solid CO2). Liquid propane has also been used. This can produce ice crystals at higher temperatures than silver iodide. The use of hygroscopic materials, such as salt, is becoming more popular too.
Seeding of clouds requires that they contain supercooled liquid water (< 00C). Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing nucleation. Dry ice or propane expansion cools the air to such an extent that ice crystals can nucleate spontaneously from the vapor phase.
In mid-latitude clouds, the usual seeding strategy has been based on the fact that the equilibrium vapor pressure is lower over ice than over water. The formation of ice particles in supercooled clouds allows those particles to grow at the expense of liquid droplets. If sufficient growth takes place, the particles become heavy enough to fall as precipitation from clouds that otherwise would produce no precipitation. This process is known as “static” seeding.
Cloud seeding chemicals may be dispersed by aircraft or by dispersion devices located on the ground (eg: generators ). When released by devices on the ground, the fine particles are carried downwind and upwards by air currents after release. An electronic mechanism was tested in 2010, when infrared laser pulses were directed to the air above Berlin by researchers from the University of Geneva. The experimenters posited that the pulses would encourage atmospheric sulfur dioxide and nitrogen dioxide to form particles that would then act as seeds.