Artificial way of causing rainfall to reduce air pollution makes use of

  • Silver iodide and potassium iodide   
  • Silver nitrate and potassium iodide   
  • Silver iodide and potassium nitrate   
  • Silver nitrate and potassium chloride  

Answer: (a) Silver iodide and potassium iodide   

Artificial Rainfall
  • Artificial rainfall, more commonly known as cloud seeding, is a technique used to induce or enhance precipitation from clouds. It is increasingly being explored in India as a way to combat severe air pollution, particularly in cities like Delhi, where high levels of particulate matter and smog make air quality dangerous.
  • The process of cloud seeding involves dispersing certain substances into the atmosphere that serve as cloud condensation or ice nuclei. These particles encourage water vapor in the atmosphere to condense into water droplets or ice crystals. When these droplets become heavy enough, they fall as rain.
  • One of the most commonly used chemicals for this purpose is silver iodide (AgI). This compound has a crystalline structure similar to ice, which makes it effective in stimulating the formation of ice crystals within cold clouds. Once seeded, the silver iodide helps initiate the process of condensation and precipitation.
  • In addition to silver iodide, potassium iodide (KI) is also used. It can serve a similar function by acting as a condensation nucleus, though it is less commonly used than silver iodide. Together, these chemicals can enhance the cloud’s ability to produce rainfall, helping to clear out pollutants from the air by washing them down to the ground.
  • This method has been considered by Indian authorities during peak pollution seasons as a way to create artificial rain and thereby reduce the concentration of pollutants in the lower atmosphere.
Cloud Seeding
Types of Cloud Seeding:
  • Static Cloud Seeding:
    • This method involves introducing ice nuclei, such as silver iodide or dry ice, into cold clouds that have supercooled liquid water droplets. 
    • The ice nuclei can trigger the formation of ice crystals or snowflakes, which can grow at the expense of the liquid droplets and fall as precipitation. 
  • Dynamic Cloud Seeding:
    • Dynamic cloud seeding is a technique to artificially stimulate rainfall by enhancing vertical air currents, thereby promoting the formation and growth of rain clouds. 
    • The process is considered more complex than static cloud seeding because it depends on a sequence of events working properly. 
  • Hygroscopic Cloud Seeding:
    • This method involves spraying fine particles of hygroscopic materials, such as salts through flares or explosives into the base of warm clouds. 
    • The particles can act as cloud condensation nuclei and increase the number and size of the cloud droplets, which can enhance the reflectivity and stability of the clouds. 
  • Glaciogenic Cloud Seeding: 
    • It involves inducing ice formation in supercooled clouds by dispersing ice nuclei such as silver iodide or dry ice, leading to ice nucleation and precipitation. 

Applications of the Technique:

  • Cloud seeding is also used to enhance winter snowfall and increase mountain snowpack, supplementing the natural water supply for nearby communities. 
  • Cloud seeding can also be done to prevent hailstorms, dissipate fog, induce rainfall in drought-prone regions, or reduce air pollution.
Challenges in Implementation of Cloud Seeding:
  • Environmental Impact: As artificial rain falls, seeding agents like silver iodide, dry ice or salt will also fall.
    • Residual silver discovered in places near cloud-seeding projects is considered toxic. As for dry ice, it can also be a source of greenhouse gas that contributes to global warming (carbon dioxide). 
  • Temporary Relief: While cloud seeding may provide short-term relief from air pollution by washing away particulate matter, it does not address the root causes of pollution such as vehicular emissions and industrial discharges.
    • Example: Cloud seeding in Lahore, improved AQI from “poor” to “moderate.” However, the effects were short-lived. 
  • Availability Issues: Cloud seeding requires the presence of moisture-filled clouds, which are not always available or predictable.
    • Specific cloud characteristics, including liquid water content and vertical motion, were used to identify clouds with potential for rainfall. 
  • Costly: It involves processes such as delivering chemicals to the sky and releasing them into the air by flare shots or aeroplanes, which involves huge costs and logistic preparation.
    • Example: Cloud seeding in Delhi requires Rs 13 crore to cover the entire aerial area of 1,300 square kilometers.