Recently, a small aircraft known as the Cessna 206H conducted a cloud seeding trial in Delhi amid the recent spike in air pollution.

About Cloud Seeding
Cloud seeding is a type of weather modification that aims to increase the amount or type of precipitation (rain or snow) that falls from clouds.
Here is a breakdown of the concept, how it works, the agents and methods used, and its effectiveness and concerns:
1. How Cloud Seeding Works
Clouds are made of tiny water droplets or ice crystals that form when water vapor in the atmosphere cools and condenses around naturally occurring particles (like dust or salt) called condensation or ice nuclei.

Cloud seeding essentially introduces additional nuclei into existing clouds to enhance the natural process of precipitation:

• Process: A seeding agent, typically consisting of minute particles, is dispersed into a cloud that contains Supercooled Liquid Water (SLW)—water droplets that remain liquid at temperatures below 0∘C.
• Mechanism (Cold Clouds): The most common agents, like silver iodide (AgI), have a crystalline structure similar to natural ice. These particles act as "landing pads" or ice nuclei on which the SLW freezes. These newly formed ice crystals then collide with other water molecules, grow into snowflakes, and eventually become heavy enough to fall as precipitation (rain if they melt on the way down, or snow). This is called glaciogenic seeding.
• Mechanism (Warm Clouds): For clouds entirely above freezing, hygroscopic materials like table salt (sodium chloride) are used. These particles are much larger than natural nuclei and readily absorb water vapor, accelerating the formation of large droplets that collide and coalesce into raindrops. This is called hygroscopic seeding.

2. Seeding Agents and Delivery Methods

Seeding Agent	Type of Seeding	Delivery Method
Silver Iodide (AgI)	Glaciogenic (Cold Clouds)	Released from aircraft flares, ground-based generators, or rockets.
Dry Ice (Solid CO2​)	Glaciogenic (Cold Clouds)	Dispersed from aircraft, as its extremely cold temperature ($ -78.5^\circ \text{C}$) flash-freezes supercooled water.
Sodium Chloride (NaCl), Potassium Chloride (KCl)	Hygroscopic (Warm Clouds)	Dispersed via aircraft (flares, spray) or ground-based systems.
Liquid Propane	Glaciogenic (Cold Clouds)	Expands into a gas, producing ice crystals at relatively warmer temperatures.

 
The most common delivery methods involve:

• Aircraft: Flying into or above the clouds to release pyrotechnic flares or directly spray the agent.
• Ground-based Generators: Burning a solution of the agent (like silver iodide and acetone) so the particles are carried into the clouds by air currents (often used in mountainous areas with strong updrafts).

3. Applications and Effectiveness
Cloud seeding is primarily used for:

• Water Resource Management: Increasing winter snowfall (to augment snowpack and subsequent streamflow) or rainfall in drought-prone areas to boost reservoir levels and support agriculture.
• Pollution Mitigation: Inducing artificial rain to temporarily wash out airborne particulate matter and pollutants from the atmosphere (e.g., in cities with severe smog).
• Hail Suppression: Seeding before hailstorms can increase the number of ice particles, resulting in smaller, less damaging hailstones.

Effectiveness:

• Conditions are Key: Cloud seeding only works on existing clouds with specific microphysical properties (sufficient moisture, supercooled water, and vertical development). It cannot create rain in clear or dry skies.
• Modest Increase: While results vary by project and environment, long-term studies have suggested that successful projects can increase precipitation in targeted areas by an average of 10% to 15% under optimal conditions.
• Difficulty in Proof: It is often challenging to definitively prove that the extra precipitation was a direct result of seeding rather than a natural occurrence.

4. Concerns and Criticisms

• Temporary Solution: For issues like air pollution, the effect is short-lived, as pollution levels quickly rebound if the source emissions are not addressed. Critics argue it is a costly "gimmick" that distracts from long-term emission control.
• Environmental Impact: There are concerns about the long-term ecological impact of accumulating seeding agents, particularly silver iodide, in the soil and water, potentially affecting crops and aquatic life, though most studies suggest the concentrations used are negligible.
• "Rain Theft": A long-standing ethical concern is that by enhancing precipitation in one area, cloud seeding might "steal" moisture from downwind areas that would have naturally received the rain.
• Meteorological Uncertainty: The success of the operation is highly dependent on unpredictable weather conditions, which can lead to costly trials with no rainfall.

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