Artificial rain
The 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. There are many ways to make rain in a warm cloud, e.g. spraying water drops, hygroscopic powder or liquid (sodium chloride), etc, and letting the water drops grow though collision and coalescence to finally become rain. There are also many ways to make rain in a cold cloud, but the most common way is to use dry ice or silver iodide. This is mainly because the temperature of dry ice is -78 °C. Spreading dry ice in a cold cloud lacking ice crystals will rapidly reduce its temperature. Therefore we directly transform supercooled water into ice crystals, which will eventually become rain without the help of ice-crystal nuclei. Silver iodide, on the other hand, is a very effective ice-crystal nucleus. By adding it to a cloud, silver iodide acting as an ice-crystal nucleus can facilitate the solidification of water drops below -5 °C into ice crystals. Through the growing process the ice crystals under the coexistence of water drops, the rain will be formed.
World wide acceptance and application
As per reports from World Meteorological Organization, cloud seeding is being used for more than 42 years now. Currently, this technology is being applied in more than 52 countries.
U.S. alone undertook more than 55 cloud-seeding projects in the last year spending more than $15million. In North America, cloud-seeding is also used for artificial snowfall by big ski resorts.
In 2008, China spent millions of dollars on cloud seeding project to create artificial rain in Beijing before the Olympic Games to reduce the air pollution. In 2011, China spent about $150 million on a single cloud seeding project to make artificial rain. On an annual basis, China creates about 55 billion tons of cloud seeded artificial rain which is expected to increase to about 280 billion tons in the near future.
Impact on Environment
Cloud seeding does not result in any significant negative environmental impacts.
However, silver iodide, one of the chemicals used for cloud seeding, is a hazardous chemical. If artificial rain making techniques are used very often, then this chemical will start accumulating in plants and animals. Too much exposure to this chemical could cause anaemia or argyria. On the contrary, some measurements back since 1950’s mention that the amount of silver iodide deposited even after long cloud seeding project is very low in quantum to be toxic to any life form.
Cloud seeding has been the focus of many theories based on the belief that governments manipulate the weather in order to control various conditions, including global warming, populations, military weapons testing, public health, and flooding.
https://www.cwb.gov.tw/V7e/knowledge/encyclopedia/me031.htm
https://en.wikipedia.org/wiki/Cloud_seeding#Conspiracy_theories