Cloud seeding is an artificial way to create rainfall. It is done either by using artillery to fire shells containing rain-inducing chemicals such as silver iodide into the cloud cover or by dropping chemicals from aircraft. Cloud seeding, a form of weather modification, is a difficult way of attempting to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud. The usual intent is to increase precipitation (rain or snow), but hail and fog suppression are also widely practiced in airports.
Principle of Artificial rain:
In the troposphere, the temperature of the atmosphere descends with altitude. Therefore the clouds in the troposphere are lower in temperature as they rise in altitude and vice versa. When the temperature of a cloud is above 0 °C, the cloud is called a warm cloud; when it is below 0 °C it is called a cold cloud. Inside a warm cloud, small water drops will become large ones through collision and coalescence, and will finally break the buoyancy of the cloud and fall out of its bottom to become rainfall to the ground. Likewise, inside a cold cloud, ice crystals can also grow to a size where they can break the buoyancy and fall out of the cloud's bottom, and when they pass through a temperature of 0 °C they will melt and become rain drops, also providing the ground with rain. When the water drops in a cloud are too small or there are not enough ice crystals to create rainfall, we can use artificial ways to create ice crystals or help small water drops grow, and thereby facilitate the formation of precipitation. This human influenced weather phenomenon is called artificial rain.
The most common chemicals used for cloud seeding include silver iodide, potassium iodide and dry ice (solid carbon dioxide). Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing nucleation.
The largest cloud seeding system is in the People's Republic of China. They believe that it increases the amount of rain over several increasingly arid regions, including its capital city, Beijing, by firing silver iodide rockets into the sky where rain is desired. There is even political strife caused by neighboring regions that accuse each other of "stealing rain" using cloud seeding. The Chinese government has used cloud seeding technology in the past not only to create artificial rain but also to clear air pollution using induced precipitation. Most famously, the technology was apparently used to keep the skies over Beijing blue during the 2008 August Olympics.
Should India emulate China’s approach to tackle air pollution?
Delhi is considering using cloud seeding to tackle the unprecedented smog choking it. It helps increase precipitation and disperses fog. However, caution must be exercised before emulating China’s approach.
First, it isn’t clear how often authorities in Beijing use the technology to clear smog or whether it is effective at times of chart-busting pollution.
Second, the technique is only successful if the conditions are conducive to rain; there must be a cloud there to begin with so that enough moisture could be generated for rain. In this autumn season, when most clouds have already moved towards Himalayas to cause snowing there, it would be difficult. Delhi skies are dry now.
And third, experts warn of secondary air and water pollution as an outcome of the chemicals used in the process – the chemicals used in creating rain and clearing the air could in turn leave behind residues of pollution. With so much pollution in the air already (eg SO2, NO2), if we make it rain right now, it could lead to acid rains.