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Q85 (IAS/2025) Environment & Ecology › Pollution & Conservation › Air pollution and health Answer Verified

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

Result
Your answer: —  ·  Correct: A
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.

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Q. Artificial way of causing rainfall to reduce air pollution makes use of [A] silver iodide and potassium iodide [B] silver nitrate and p…
At a glance
Origin: Mostly Current Affairs Fairness: Low / Borderline fairness Books / CA: 0/10 · 10/10
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This 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.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
Which chemical agents are commonly used in cloud seeding (artificial rainfall) to induce precipitation to reduce air pollution?
Origin: Web / Current Affairs Fairness: CA heavy Web-answerable

Web source
Presence: 5/5
"Cloud seeding ... by injecting substances such as silver iodide, potassium iodide, dry ice (solid carbon dioxide), liquid propane, or salt into clouds to induce precipitation."
Why this source?
  • 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).
Web source
Presence: 5/5
"Silver iodide (AgI) is considered the most common nucleating material used in cloud seeding."
Why this source?
  • 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.
Web source
Presence: 4/5
"commercial cloud seeding, with silver iodide particles released from aircraft to induce rain."
Why this source?
  • 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.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > EH fr.r"-1 > p. 102
Strength: 5/5
“chemistry and buffering capacity of the oils involved; and the types of fish, trees, and other living things that rely on the water. • . precipitation removes gases and particles from the atmosphere by two processes: • (i) rain-out, which is the incorporation of particles into cloud drops which fall to the ground, and • (ii) washout, which occurs when materials below the cloud are swept down by rain or snow as it falls.”
Why relevant

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.

How to extend

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).

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 8: Natural Hazards and Disaster Management > cloudburst > p. 70
Strength: 4/5
“In brief, the term cloudburst is used to describe sudden heavy, brief and usually unpredictable rainfall. Te hard rain characteristic of a cloudburst is caused by a phenomenon known as Langmuir Precipitation, in which drops of rain fuse together to create large drops as they fall, falling more quickly as they grow. Te Langmuir Precipitation process is applicable only to those clouds which do not extend beyond the freezing point. Te temperature in the uppermost part of the cloudburst seldom falls below 5°C. Te occurrence of precipitation from such cumulus- nimbus clouds involves the coalescence of cloud droplets of diferent sizes.”
Why relevant

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.

How to extend

One could test whether seeding chemicals that are hygroscopic (promote droplet growth/coalescence) are used for warm clouds to accelerate this process.

Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 17: Contemporary Issues > Cloudburst > p. 28
Strength: 4/5
“The Langmuir Precipitation process is applicable only to those clouds which do not extend beyond the freezing point. The temperature in the uppermost part of the cloudburst seldom falls below 5 °C. The occurrence of precipitation from such cumulus-nimbus clouds involve the coalescence of cloud droplets of different sizes. Since the rate of fall of these unequal rain-drops is different, they collide with each other within the cloud, and the larger drops grow at the expense of smaller ones. The size of the rain drops become sufficiently large which fall faster, and results into cloudburst. Sometimes, the rain in cloudburst falls so fast and is large that it is actually scary, fearful and frightening.”
Why relevant

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.

How to extend

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).

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Precipitation > p. 337
Strength: 4/5
“• The process of continuous condensation in the air helps the condensed particles to grow in size. When the resistance of the air fails to hold them against the force of gravity, they fall onto the earth's surface as different forms of precipitation.• Precipitation in the form of drops of water is called rainfall when the drop size is more than 0.5 mm.• It is called Virage when raindrops evaporate before reaching the earth while passing through dry air.• Drizzle is light rainfall with a drop size of less than 0.5 mm. When evaporation of drizzle occurs before reaching the ground, it is referred to as mist.• When the temperature is lower than 0° C, precipitation takes place in the form of fine flakes of snow and is called snowfall.”
Why relevant

Explains continuous condensation and particle growth until gravity overcomes air resistance — indicating that adding nuclei that encourage condensation or crystal growth can trigger precipitation.

How to extend

One could therefore look for chemicals known to serve as condensation or ice-forming nuclei that accelerate droplet/crystal growth in clouds.

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > Acidification > p. 8
Strength: 4/5
“motor vehicles inject large amounts of these chemicals into the atmosphere. In the atmosphere, the chemicals combine with water to form sulphuric acid and nitric acid. Tese droplets may be transported at great distances by wind before they precipitate to the ground (Fig. 6.1). Te main gases involved in the production of acid rains are nitrogen oxides—sulphur dioxide (SO2), nitrogen dioxide (NO2) and nitrous oxide (NO). Te major sources of these gases is due to burning of fossil fuel and industrial processes. Acid rain is a phrase that applies to a process that results in deposition of acid on the surface of the Earth.”
Why relevant

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.

How to extend

Combining this with the above, a student might search for seeding agents effective at producing precipitation that efficiently scavenges soluble gases and particulates.

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