Question map
Artificial way of causing rainfall to reduce air pollution makes use of
Explanation
Cloud seeding involves injecting substances such as silver iodide, potassium iodide, dry ice (solid carbon dioxide), liquid propane, or salt into clouds to induce precipitation.[1] Silver iodide (AgI) is considered the most common nucleating material used in cloud seeding.[2]
Among the options provided, option A correctly identifies both silver iodide and potassium iodide as chemicals used in the artificial rainfall process. These substances act as nucleating agents that provide particles around which water droplets can condense, eventually leading to precipitation. This technique is employed not only for increasing rainfall in drought-prone areas but also for reducing air pollution by washing away particulate matter from the atmosphere.
The other options (B, C, and D) mention silver nitrate, potassium nitrate, or potassium chloride, which are not the standard chemicals used in cloud seeding operations. Therefore, option A is the correct answer for this UPSC Prelims question.
SourcesPROVENANCE & STUDY PATTERN
Full viewThis is a classic 'Applied Science' question derived from recurring current affairs (Delhi pollution mitigation). While static books explain the physics of precipitation, the specific chemical 'recipe' comes from news reports on cloud seeding trials (e.g., IIT Kanpur's experiments). Strategy: When a tech solution hits the headlines, memorize the specific chemical agents involved.
This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.
- Provides a direct list of substances injected into clouds for cloud seeding.
- Names multiple chemical agents commonly used to induce precipitation (covers both ice and hygroscopic approaches).
- Identifies the single most common nucleating chemical used in cloud seeding.
- Emphasizes silver iodide (AgI) as the primary agent, supporting its prominence in the practice.
- Gives an example of commercial cloud seeding practice using a named chemical.
- Confirms silver iodide particles are released from aircraft to induce rain, linking method to agent.
Defines two removal processes — rain-out (particles incorporated into cloud drops) and washout (materials swept down by falling rain), implying added precipitation can remove airborne particles and gases.
A student could infer cloud seeding aims to increase rain-out/washout and then look for agents that promote droplet formation or growth (i.e., condensation/collection nuclei).
Describes Langmuir coalescence in warm (above-freezing) clouds where droplet collision and fusion produce larger drops — pointing to the importance of processes that enhance droplet coalescence in seeding.
One could test whether seeding chemicals that are hygroscopic (promote droplet growth/coalescence) are used for warm clouds to accelerate this process.
Notes Langmuir precipitation applies only to clouds not extending beyond the freezing point, highlighting a key distinction between warm-cloud and cold-cloud precipitation mechanisms.
A student can use this rule to separate likely seeding agents into those that act as ice nuclei (for cold clouds) versus those that act as hygroscopic/condensation nuclei (for warm clouds).
Explains continuous condensation and particle growth until gravity overcomes air resistance — indicating that adding nuclei that encourage condensation or crystal growth can trigger precipitation.
One could therefore look for chemicals known to serve as condensation or ice-forming nuclei that accelerate droplet/crystal growth in clouds.
Identifies soluble pollutant gases (SO2, NOx) that form acids in water and can be transported and removed by precipitation, showing a rationale for using seeding to reduce such pollutants.
Combining this with the above, a student might search for seeding agents effective at producing precipitation that efficiently scavenges soluble gases and particulates.
- [THE VERDICT]: Sitter/Moderate. Derived from Current Affairs (Delhi Smog/Cloud Seeding news), though Silver Iodide is a standard NCERT-level fact.
- [THE CONCEPTUAL TRIGGER]: Environmental Pollution (GS-3) mitigation technologies. Specifically, the recurring debate on using 'Artificial Rain' to wash away PM2.5 in Delhi.
- [THE HORIZONTAL EXPANSION]: 1. Cold Cloud Seeding agents: Silver Iodide (AgI), Dry Ice (Solid CO2). 2. Warm Cloud Seeding agents: Calcium Chloride, Sodium Chloride (Salt). 3. Role of Potassium Iodide: Used to solubilize Silver Iodide in acetone for ground-based generators. 4. 'Blueskying': China's weather modification program. 5. Voting Ink chemical: Silver Nitrate (to distinguish from Seeding agents).
- [THE STRATEGIC METACOGNITION]: Don't just read that 'Artificial Rain is a solution.' Ask 'What are the ingredients?' UPSC loves asking for the chemical composition of everyday or news-heavy technologies (e.g., chemicals in fireworks, cloud seeding, LED materials).
Precipitation removes gases and particles through rain-out (incorporation into cloud drops) and washout (sweeping of materials below the cloud), the processes that artificial rainfall aims to enhance to reduce air pollution.
High-yield for environment questions: explains how precipitation affects pollutant removal and forms a bridge between atmospheric chemistry and hydrological processes. Useful for answering mitigation-method questions (natural vs. engineered removal) and for evaluating effectiveness of interventions like induced rainfall.
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > EH fr.r"-1 > p. 102
Growth and fusion of cloud droplets (coalescence and Langmuir precipitation) determine when and how rainfall forms, which is the physical basis that cloud seeding seeks to exploit.
Essential for physical geography and disaster-management questions: clarifies which cloud types and temperature regimes support precipitation formation, helps explain limits and operational constraints of cloud-seeding efforts, and links to broader weather modification topics.
- Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 8: Natural Hazards and Disaster Management > cloudburst > p. 70
- Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 17: Contemporary Issues > Cloudburst > p. 28
Sulphur dioxide and nitrogen oxides react with atmospheric moisture to form acids; understanding these reactions is crucial when assessing how precipitation (natural or induced) interacts with air pollutants and acid deposition.
High relevance for environment and ecology topics: connects emissions (industrial, transport) to precipitation chemistry and ecosystem impacts, informs policy-oriented answers on emission controls versus remediation via rainfall, and supports evaluation of unintended consequences of precipitation-based interventions.
- Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > Acidification > p. 8
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 6: Geomorphic Movements > Anthropogenic Solution Weathering > p. 90
The distinction between 'Glaciogenic' and 'Hygroscopic' seeding. Silver Iodide is Glaciogenic (induces ice in supercooled clouds). For tropical 'warm' clouds (above 0°C), which are common in India, Hygroscopic seeding using simple Salt (NaCl) or Calcium Chloride is actually more effective.
The 'Celebrity Chemical' Rule: Silver Iodide (AgI) is the most famous cloud seeding agent taught in basic science. This immediately eliminates options [B] and [D] which list Silver Nitrate (used in voting ink/photography). Between [A] and [C], choose the option where the anion matches (Iodide + Iodide) due to chemical affinity/solubility logic, rather than mixing Iodide with Nitrate.
Link to International Relations & Security: The 'ENMOD Convention' (Environmental Modification Convention) prohibits the military use of weather modification techniques. China's massive expansion of weather modification raises geopolitical concerns regarding water security for downstream neighbors.