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Types of Seeding

Glaciogenic Seeding

'Glaciogenic agents are mostly ice nuclei, although some glaciogenic agents are effectively hydrophilic nuclei' (ASCE).

Target: Cold clouds (below freezing) or mixed-phase clouds

Typical Locations: Mid-latitude or mountainous regions

Environment: Most effective in continental environments

Process:

  1. Ice nucleating particles are introduced into supercooled liquid water
  2. Particles initiate ice crystal formation
  3. Promotes ice particle growth through vapor deposition and riming

Materials:

  • Silver iodide (AgI) - most common
  • Dry ice (solid CO2)
  • Liquid propane

Objectives:

  • Increase ice crystal concentration in clouds lacking natural ice nuclei
  • Increase snowfall in winter orographic clouds
  • Accelerate precipitation in convective clouds

Scientific Backing:

  • Strong evidence of microphysical effects (increased ice crystals)
  • Some statistical evidence of precipitation increases in winter orographic clouds
  • Recent field campaigns (e.g., SNOWIE) provided detailed observations

Hygroscopic Seeding

Hygroscopic agents deliquesce (dissolve into their aqueous components), becoming a solution droplet by absorbing water vapor from the air (at relative humidities significantly less than 100%).

Target: Warm clouds (above freezing) or mixed-phase clouds

Typical Locations: Tropical or subtropical regions

Environment: Can be effective in continental or maritime environments

Process:

  1. Large hygroscopic particles (e.g., salt powders) are injected into cloud base
  2. Particles act as cloud condensation nuclei (CCN) to form large cloud droplets
  3. Promotes collision-coalescence process to form raindrops

Materials:

  • Salt powders (NaCl, KCl, CaCl2)
  • Optimal particle size: 2-5 microns

Objectives:

  • Accelerates warm rain process
  • Can potentially increase rainfall in shallow/warm clouds
  • May counteract effects of pollution on precipitation

Scientific Backing:

  • Laboratory and modeling studies show enhanced droplet growth
  • Some randomized field experiments showed increased rainfall (e.g., South Africa)
  • Physical basis is well-established, but large-scale effects still uncertain

Dual-mode Seeding (Combination of Both)

  • Combines aspects of both glaciogenic and hygroscopic seeding
  • Aims to target both warm and cold cloud processes
  • May be used in mixed-phase clouds or in regions with varied cloud types