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:
- Ice nucleating particles are introduced into supercooled liquid water
- Particles initiate ice crystal formation
- 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:
- Large hygroscopic particles (e.g., salt powders) are injected into cloud base
- Particles act as cloud condensation nuclei (CCN) to form large cloud droplets
- 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