Cloud seeding is a type of weather modification technology that intends to create artificial rainfall by dispersing substances into the air that serve as condensed clouds or ice nuclei, which alter the microphysical processes within the cloud. It is also known by other terms, such as, man-made precipitation enhancement or artificial rainmaking modification.
It works when there are enough pre-existing clouds in the atmosphere. Rain happens when moisture in the air reaches levels at which it can no longer be held. By this technology, it sprays particles of salts like iodide and chloride of silver or potassium, dry ice (solid carbon dioxide), or liquid propane on clouds using a special aircraft, rockets or from dispersion devices located on the ground. These salt particles act as a core (cloud condensation nuclei or ice-nucleating particles) which draw water vapour within the cloud towards them and the moisture latches on, condensing into water droplets leading to the formation of raindrops.
The intent of doing this is to alter the natural development of the cloud to enhance precipitation, suppress hail, dissipate fog, reduce lightning, control air pollution or mitigate drought situation. It can also occur due to ice nucleators in nature, most of which are bacterial in origin.
During 1890s, Louis Gathmann proposed shooting liquid carbon dioxide into rain clouds to cause rain. During the 1930s, the Bergeron-Findeisen process theorized that supercooled water droplets present while ice crystals are released into rain clouds would cause rain. While researching aircraft icing, General Electric (GE)’s Vincent Schaefer and Nobel prize winner Irving Langmuir confirmed the theory.
Schaefer finally discovered the invaluable principle of cloud seeding in July 1946 through a series of events. The first attempt to change natural clouds in the field through “cloud seeding” began during a New York flight on 13 November 1946. Since then, global scientific research and development have made it a popular method of rain enhancement, including in the world’s most arid regions.
Whether cloud seeding is effective in producing a statistically significant increase in amount of precipitation is still a matter of academic debate, with contrasting results depending on the study in question, and contrasting opinion among experts. Its an ongoing process which is improving day by day and overcoming its shortcomings.
Though, there have been several detailed ecological studies that showed negligible environmental and human health impacts.
Human Health – silver iodide can cause temporary incapacitation or possible residual injury to humans and other mammals with intense or chronic exposure resulting from the minute amounts of silver generated by it, which are about one percent of industry emissions into the atmosphere in many parts of the world. Although the toxicity of silver and silver compounds (from silver iodide) was shown to be of low order in some studies.
Environment – Cloud seeding over a biosphere reserve is problematic. Environmentalists are concerned about the uptake of elemental silver in a highly sensitive environment affecting the pygmy possum among other species as well as recent high-level algal blooms in once pristine glacial lakes as summarized by the international Weather Modification Association, since silver iodide and not elemental silver is the material used, the claims of negative environmental impact are disputed by peer-reviewed research.
At least 56 countries have used some sort of cloud seeding, according to the World Meteorological Organization. Experiments of cloud seeding had earlier been tried in Australia, America, Spain and France. In United States, it is occasionally used by ski resorts to induce snowfall.
In United Arab Emirates, the cloud seeding technique led to creation of 52 storms in Abu Dhabi. China using it in an effort to fight pollution before the 2008 Summer Olympics opening ceremony. Till last year, IMD had around 30 successful incidents of seeding. Also, such seeding is routine in Russia and other cold countries where the technique is used to disperse fog at the airports.
Cloud seeding is not new to India and it uses weather modification system to create rainfall during droughts in Karnataka, Andhra Pradesh and Maharashtra and now planning to use it for mitigating the problem of air pollution in North India.
Indian Institute of Tropical Meteorology has been carrying out cloud seeding experiments for several years now. The success rate of these experiments in inducing rains is about 60 to 70 per cent, depending on local atmospheric conditions, the amount of moisture in the air and cloud characteristics. Some private companies also offer cloud-seeding services.