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Seeding

1. Seeding Process

1.1 Aggregation

Aggregation is the process by which ice particles collide and stick together to form larger particles:

  1. Ice crystals or snowflakes collide in the cloud
  2. They adhere to each other, forming larger aggregates
Aggregation efficiency depends on factors like:
  • Temperature
  • Crystal shape
  • Relative humidity

1.2 Cloud Condensation Nuclei (CCN)

CCN are tiny particles that serve as the foundation for cloud droplet formation.

Characteristic Description
Size Typically 0.2 m in diameter
Relative Size One hundredth the size of a typical cloud droplet
Function Provide surfaces for water vapor condensation

Types of CCN:

  • Sea salt particles from ocean spray
  • Sulfate particles from volcanic activity or oxidation of sulfur dioxide
  • Organic particles from oxidation of volatile organic compounds
  • Soot/black carbon from combustion (e.g., forest fires, engines)
  • Dust particles
  • Clay particles
  • Sodium chloride (NaCl)
  • Ammonium sulfate ((NH4)2SO4)
Key Points:
  • CCN concentrations range from about 100-1000 per cm in the atmosphere
  • Hygroscopic particles like sulfates and sea salt make better CCN than hydrophobic particles like some dust or soot

1.3 Ice Nucleating Particles (INP)

Ice nucleating particles (INPs) initiate ice crystal formation in supercooled water through a process called heterogeneous ice nucleation.

How does this work? And what makes good INPs?

Heterogeneous Ice Nucleation Process:

  1. Contact with supercooled water: INPs come into contact with supercooled water droplets in clouds.

  2. Lowering energy barrier: INPs provide a surface that lowers the energy barrier required for ice crystal formation.

  3. Molecular arrangement: The surface structure of the INP helps orient water molecules into an ice-like lattice.

  4. Critical embryo formation: This leads to the formation of a critical ice embryo on the INP surface.

  5. Growth: Once the critical embryo forms, it grows rapidly into a full ice crystal.

Characteristics of Good INPs:

Crystal structure similarity
  • Effective INPs often have a crystal structure similar to that of ice.
  • Silver iodide (AgI) is a prime example, with a hexagonal crystal structure very close to that of ice.
Surface properties
  • Hydrophilic (water-loving) surfaces can be effective, as they attract water molecules.
  • However, some hydrophobic surfaces with the right structure can also be good INPs.
Size
  • Larger particles (typically >0.1 μm) are generally more effective INPs.
  • This is because they provide a larger surface area for ice nucleation.
Chemical composition
  • Certain minerals like feldspar and kaolinite are known to be good natural INPs.
  • Some biological particles, such as certain bacteria and pollen, can be extremely effective INPs.
Surface defects and irregularities
  • Cracks, steps, or pits on the particle surface can enhance ice nucleation.
  • These features can help anchor water molecules and promote ice-like structuring.
Insolubility
  • Good INPs are typically insoluble in water.
  • This allows them to maintain their structure when in contact with water droplets.
Temperature dependence
  • Different INPs become active at different temperatures.
  • The best INPs can nucleate ice at relatively warm subzero temperatures (e.g., -5°C to -10°C).
Nucleation modes
  • Effective INPs can operate in various nucleation modes (e.g., immersion, deposition, contact).
  • The ability to nucleate ice in multiple modes increases their overall effectiveness.

Types of INP

  • Mineral dust (e.g., clay minerals like kaolinite)
  • Biological particles (e.g., bacteria, pollen, fungal spores)
  • Soot/black carbon
  • Volcanic ash
  • Some organic particles

Important characteristics

  • IN are generally larger particles than CCN
  • IN are much less abundant than CCN, typically less than 1 per liter of air
  • IN are active at higher temperatures than homogeneous ice nucleation

Why AgI?

  • It has a crystal structure very similar to ice.
  • It's insoluble in water.
  • It can nucleate ice at relatively warm temperatures (around -5°C).
  • It's effective in multiple nucleation modes.

1.4 Particle Sizes and Distribution

The effectiveness of particles as CCN or IN depends on:

  1. Size
  2. Chemical composition
  3. Surface properties

Optimal Particle Size for Hygroscopic Seeding

Size Range Reason for Optimality
1-10 microns Ideal balance of size and suspension ability
2-5 microns Considered optimal for some experiments
Why this size range is optimal:
  • Large enough to serve as effective CCN
  • Small enough to remain suspended in the cloud
  • Promotes rapid growth through collision-coalescence

1.5 Over-seeding and Competition Effect

Over-seeding can lead to unintended consequences in cloud formation and precipitation.

Consequences of introducing too many hygroscopic particles:
  1. Increased competition for available water vapor
  2. Narrower droplet spectrum with many small droplets rather than fewer large ones

A study by Cooper et al. (1997) found that seeding with numerous 1 m particles produced high concentrations of drizzle drops rather than fewer larger raindrops.

The Competition Effect:
  • Too many CCN or ice nuclei result in numerous small droplets/crystals
  • Small particles may not grow large enough to fall as precipitation
  • Available moisture is spread too thinly among too many particles

2. Seeding Agents

2.1 Silver Iodide (AgI)

The most common seeding agent for cold-cloud seeding

AgI has been used as a glaciogenic agent for more than half a century. It nucleates the ice phase of pure water, regardless of the specific ice-forming mechanism.

It is relatively insoluble, it is stable enough at high temps to permit vaporization and recondensation to form large numbers of nuclei, and may be dispensed using acetone generators or pyrotechnics from the ground or in the air.

AgI consumption can range from hundreds to thousands of grams per hour during a dynamic cloud seeding operation.

Chemical Properties:
  • Formula: AgI
  • Appearance: Bright yellow solid (often appears greyish due to silver impurities)
  • Molecular weight: 234.77 g/mol
  • Composition: 45.95% silver, 54.05% iodine
Preparation:
  1. Mix a solution of silver ions (e.g., from silver nitrate) with a solution containing iodide ions (e.g., from potassium iodide)
  2. Silver iodide precipitates as a yellowish solid
  3. The reaction can be represented as:

AgNO + KI AgI + KNO

2.2 Potassium Iodide (KI)

  • Often used in combination with AgI
  • Enhances the effectiveness of AgI in some seeding operations

2.3 Hygroscopic Salts

Used for warm-cloud seeding:

Salt Chemical Formula
Sodium Chloride NaCl
Calcium Chloride CaCl2
Potassium Chloride KCl

These salts are highly effective at attracting and holding water molecules

2.4 Biodegradable Alternatives

Emerging alternatives to traditional seeding agents aim to reduce environmental impact of cloud seeding operations.

  • Pseudomonas syringae (bacteria)
  • Certain organic compounds

3. A Positive Sum Game?

Does seeding in one area steal rain from another ('robbing Peter to pay Paul')?

3.1 Extra-Area (Down-Wind) Effects

Take-away: Decades of peer-reviewed work show that well-designed cloud-seeding programs do not reduce precipitation in neighbouring regions; most studies find neutral or modestly positive down-wind gains.

Study Project / Region Down-Wind Finding
DeFelice et al. (2014) Meta-analysis of five winter & summer projects (U.S., Thailand) Precipitation increased +5 – 15 % up to a few hundred km from the target zone
Yorty (2019) – update of Solak et al. (2003) 41-season Utah orographic program +12 % in the target area and similar gains out to ≈ 160 km east; no decreases observed
Friedrich et al. (2020) – SNOWIE Payette Basin, ID Radar-gauge network showed seeded snow spreading over a broad swath down-wind

Basically, there is just a lot of water vapor available. Only 9 % of the moisture passing over Texas actually falls as precipitation.

The target area starts out with far more vapor than it can ever rain out. Routine silver-iodide seeding nudges a small share of this “would-have-evaporated” vapor into ice sooner, so the ridge gains a bit of extra snow while the newly formed crystals keep drifting and will even snow out tens of kilometres down-wind.

  1. Ice-multiplier effect – Seeded ice crystals continue to grow and aggregate while drifting leeward, adding mass outside the target box.
  2. Dynamic feedbacks – Latent-heat release from early precipitation invigorates uplift, sustaining cloud depth farther down-wind.
  3. Moisture recycling – Accelerating ice formation higher in the column leaves ample residual vapour for subsequent clouds rather than depleting it.

3.3 Bottom Line

Across controlled experiments, long-term operational evaluations, and modern radar-tracer campaigns, no credible evidence supports the notion that cloud seeding steals precipitation from adjacent areas. Instead, the practice is consistently neutral-to-positive for down-wind regions.