Recently, the Delhi government, in collaboration with the Indian Institute of Technology (IIT) Kanpur, conducted cloud-seeding trials to induce rain as part of efforts to reduce the city’s air pollution.

About Cloud Seeding

• Definition: Cloud seeding is a weather modification technique that aims to enhance precipitation (rain or snow) by dispersing substances into the atmosphere that serve as cloud condensation or ice nuclei.
• Objective: To induce rainfall in moisture-bearing clouds or to augment natural precipitation processes.

Mechanism of Cloud Seeding

• Identification of Suitable Clouds: Only moisture-laden clouds with sufficient water vapour content are suitable.
  • Typically, cumulus or stratiform clouds are targeted.
  • The temperature, humidity, and wind conditions are analyzed before seeding.
• Seeding Agents Used: Chemicals used to trigger condensation or freezing include Silver iodide (AgI), Sodium chloride (NaCl), Potassium iodide (KI), Dry ice (solid CO₂)
  • These materials act as condensation or ice nuclei — helping water vapour to condense into droplets or ice crystals.
• Method of Dispersal:
  • Aerial Method: Chemicals are released from aircraft or drones directly into clouds; Most effective for large-scale cloud systems.
  • Ground-based Method: Generators or rockets release seeding material from the ground; Winds carry the particles upward into the clouds.
• Microphysical Reactions in Clouds: Once the seeding material reaches the cloud,
  • Water vapour condenses (if warm cloud seeding) or freezes (if cold cloud seeding) around the introduced particles.
  • These droplets or ice crystals grow in size by colliding with nearby particles.
• Formation of Raindrops: As the droplets or ice crystals grow larger and heavier, they can no longer remain suspended in the cloud.
  • They fall as precipitation — rain, snow, or sleet — depending on temperature and altitude.

Types of Cloud Seeding (Based on Temperature)

• Cold Cloud Seeding:
  • Used when cloud temperature is below freezing.
  • Agents like silver iodide help form ice crystals that trigger precipitation.
• Warm Cloud Seeding:
  • Used when cloud temperature is above 0°C.
  • Commonly uses salt particles (NaCl) to promote droplet formation and coalescence.

Role in Curbing Air Pollution

• Artificial rain helps wash out pollutants like dust, smoke, and PM2.5 particles.
• Improves air quality temporarily by removing suspended particulates from the lower atmosphere.
• Useful in cities like Delhi during severe post-monsoon smog.
• Forest Fire Control: Helps in managing and suppressing forest fires.
• Also helps in:
  • Reducing temperature and dryness (which worsen pollution).
  • Supporting groundwater recharge and agriculture.

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Challenges and Limitations

• Weather Dependence: Requires suitable moisture-bearing clouds; cannot generate rain in clear skies.
• Limited Effectiveness: Rainfall enhancement is often marginal (≈10–20%).
• Short-term Impact: Pollution reduction lasts only a few days.
• Environmental Concerns: Possible chemical accumulation (e.g., silver iodide) in soil/water.
• High Operational Cost: Aircraft, chemicals, and trained staff make it expensive.
• Scientific Uncertainty: Mixed global evidence on consistent success.

Meteorological Challenges in the Post-Monsoon Season

• The India Meteorological Department (IMD) classifies October to December as the post-monsoon season.
• During this period, rain-bearing monsoon clouds and low-pressure systems that trigger rainfall are absent.
• Winds are generally calm, and atmospheric conditions favour dry weather, except when disturbed by western disturbances at lower latitudes.
• Implications for Cloud Seeding:
• • Cloud seeding requires mature, moisture-laden clouds for success.
  • In the absence of such clouds, even seeding visible clouds may not lead to rainfall.

Past Cloud-Seeding Experiments in India

• Project: Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX)
  • Conducted by the Ministry of Earth Sciences (MoES) in four phases: 2009, 2010–2011, 2014–2015, and 2017–2019.
  • Trials were primarily conducted in southern peninsular India, including Solapur (Maharashtra).
• Key Findings:
  • Rainfall enhancement: Up to 46% at specific locations, with an average increase of 18% over a 100 sq. km area downwind of the seeding site.
  • Guidelines developed: The CAIPEEX results led to the creation of protocols and frameworks for cloud-seeding operations in India.

Global Examples

• USA: Used extensively for drought mitigation and increasing mountain snowpack.
• China: Deployed during the Beijing Olympics (2008) to control rainfall and clear smog.
• UAE: Runs the world’s largest cloud-seeding program for water security in arid regions.
• Thailand: Implements the Royal Rainmaking Project since 1955 for agriculture and drought relief.
• Australia: Conducts seeding in Tasmania to boost rainfall for hydroelectric reservoirs.

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