Cloud seeding is a form of weather modification that aims to change the amount or type of precipitation that falls from clouds, mitigate hail, or disperse fog. The primary objective is often to increase rain or snow, particularly in drought-stricken areas, or to prevent precipitation from occurring at undesirable times or locations, such as during major events or to suppress wildfires [1] [2].

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The process involves dispersing substances into the air that act as cloud condensation or ice nuclei, providing a surface for water droplets or ice crystals to form around [2]. Common agents include silver iodide (AgI), potassium iodide (KI), and dry ice (solid carbon dioxide). More recently, hygroscopic materials like table salt (sodium chloride, NaCl) and calcium chloride (CaCl2) are gaining popularity due to their ability to attract moisture [1] [2].

There are two main techniques:

• Hygroscopic seeding is used for warm clouds and involves dispersing salt particles to encourage water condensation and droplet growth [1].
• Glaciogenic seeding is applied to supercooled clouds (clouds containing water droplets below freezing point) and introduces agents like silver iodide to trigger ice production [1].

Dispersion methods include ground-based generators, rockets, and aircraft [1]. Newer approaches involve drones delivering electric charges to stimulate rainfall or infrared laser pulses aimed at inducing particle formation [2].

Cloud seeding has a long history, with initial experiments dating back to the 1940s [1] [2]. Vincent Schaefer discovered the principle in 1946 by using dry ice to create ice crystals in a supercooled cloud [2]. Bernard Vonnegut later discovered that silver iodide could also effectively seed clouds due to its crystalline structure being similar to ice [2].

Globally, cloud seeding is employed for various purposes:

• Drought mitigation and water supply augmentation: Countries like the USA, China, Australia, and the United Arab Emirates use it to maximize water utilization during rainfall [1]. In the US, eleven western states and one Canadian province (Alberta) had ongoing weather modification programs as of 2012, often to increase snowpack for water supply [2].
• Firefighting: Russia has used cloud seeding to fight Siberian forest fires [4]. China has also deployed drones for cloud seeding to combat massive wildfires, such as those in Chongqing and Sichuan [5].
• Hail suppression: In hail-prone agricultural areas, insurance companies fund projects to minimize property damage. Countries like Bulgaria, France, Spain, and parts of Germany and Austria operate hail protection programs using cloud seeding [1] [2].
• Fog dispersal: Cloud seeding can enhance airport visibility by clearing fog [1].
• Enhancing snowfall at ski resorts: Ski resorts leverage cloud seeding for intensified snowfall [1].
• Hydroelectric power generation: Hydroelectric companies use it to boost spring runoff [1].
• Agricultural benefits: Cloud seeding is used to expand agricultural capabilities, particularly in regions with extreme heat or drought [1].
• Event planning: China famously used cloud seeding before the 2008 Beijing Olympics to prevent rain during the opening and closing ceremonies [1] [2].

Despite its widespread application, cloud seeding's effectiveness remains a subject of debate among scientists, with studies offering mixed results [1] [2] [3]. Some studies indicate a 10-15% increase in rainfall [1], while others, like a 2003 US National Research Council report, state that "science is unable to say with assurance which, if any, seeding techniques produce positive effects" [2]. Critics also point out that cloud seeding is least effective during severe drought conditions, as there may not be sufficient moisture in the clouds to begin with [3].

Environmental and health impacts are a significant concern. While proponents argue that the low concentrations of substances used result in minimal impact [2], skeptics raise cautionary flags. Silver iodide, the most common seeding agent, is known to be toxic and is regulated as a hazardous substance under the Clean Water Act [1] [3]. Concerns persist over the potential for bioaccumulation in sensitive ecosystems, and some studies suggest that while overall levels are low, they can exceed health standards in areas with repeated exposure [3]. Human exposure can lead to iodism, causing skin rashes and digestive issues [1]. There are also concerns about increased air pollution due to particulate matter from seeding agents [1].

Furthermore, cloud seeding can have unintended consequences, such as increased risks of flooding and erosion due to excessive rain [1]. There is also the possibility that increasing precipitation in one area could inadvertently trigger a drought elsewhere, as cloud seeding might redistribute rain rather than create new rainfall [1] [3]. This raises complex governance questions about water rights and potential transboundary disputes, as exemplified by China's "Sky River Plan" [3].

The legal frameworks surrounding cloud seeding are incomplete. The Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD) prohibits military use but does not regulate peaceful applications [2]. The question of "ownership" of clouds and artificially induced precipitation remains largely unanswered, leading to potential conflicts [2].

In terms of new artificial rain technology based on recent material chemistry research, the focus is on developing less harmful alternatives to silver iodide and improving the efficiency of seeding agents. Research is exploring the use of negatively charged ions like calcium chloride instead of ice-like crystals [1]. Additionally, advancements in drone technology are enabling more precise and targeted delivery of seeding agents, as seen in China's use of Wing Loong-2H UAVs for cloud seeding during wildfires and droughts [5]. These drones can carry and ignite silver iodide flame bars within clouds to induce rainfall [5]. The development of electric-charge emission instruments and customized sensors on drones, as used by the United Arab Emirates, represents another innovative approach to stimulating rainfall [2].