Cloud seeding is a long-standing meteorological practice designed to enhance a cloud’s natural ability to produce rain or snow. This weather modification technique involves carefully introducing specific particles into existing clouds that contain supercooled liquid water—water that is below freezing but still in liquid form. The goal is not to create clouds from clear skies, but to make already-formed clouds more efficient at releasing their moisture as precipitation. By increasing the number of ice nuclei, the process encourages water droplets to freeze, grow heavier, and fall to the ground.
Delivery Systems in Action
The “action” of cloud seeding is often seen in the specialized aircraft used to deliver the seeding agents directly into the cloud system. These are typically small, turboprop planes, such as the Beechcraft King Air, that are modified with racks mounted on the wings or the fuselage. Pilots fly these planes into the storm’s updrafts or above the cloud tops, depending on the operational strategy.
The deployment of pyrotechnic flares is the most visually distinct method. These flares, which look like small rockets attached to the aircraft, are ignited electrically, releasing a plume of silver iodide particles directly into the cloud. Ejectable flares drop from the belly, while “burn-in-place” flares are mounted on the wings, creating a visible trail of smoke-like material. The flight path is meticulously tracked to target the most receptive parts of the cloud.
Ground-based generators are small, tower-like structures situated on the windward slopes of mountains. They burn a solution containing silver iodide, releasing microscopic particles as fine smoke into the atmosphere. Operation relies on winds flowing up and over the mountains to carry the seeding plume into the storm clouds. Remote-controlled generators allow technicians to precisely time the release based on real-time wind and temperature conditions.
Core Seeding Agents
The effectiveness of cloud seeding relies on materials that act as artificial ice nuclei. The most widely used agent for cold-cloud seeding is silver iodide (AgI), selected because its crystalline structure closely mimics that of natural ice. This hexagonal shape provides an ideal surface for supercooled water droplets to attach and freeze onto. A single gram of silver iodide can generate trillions of microscopic particles to enhance the freezing process.
This process is known as glaciogenic seeding, effective in clouds where temperatures are between approximately -4°F and 20°F. Dry ice (solid carbon dioxide) is another agent that works by cooling the surrounding air so low that water droplets spontaneously freeze. Dry ice is usually dropped from aircraft and works effectively at warmer temperatures than silver iodide. For warm-cloud seeding, hygroscopic materials like salts (sodium chloride or potassium chloride) are used. These particles attract water vapor, encouraging the formation of large water droplets that combine until they are heavy enough to fall as rain.
Visual Changes in Clouds and Precipitation
For an observer on the ground, the immediate visual effects of cloud seeding are subtle and difficult to distinguish from natural cloud evolution. As supercooled water converts to ice crystals and falls out, a faint clearing or a temporary “hole” can sometimes be observed in a thin layer of seeded cloud. Satellite imagery has occasionally captured a vivid, zigzag track in the cloud deck, tracing the path of an aircraft that dispersed its material. This initial change from liquid water to ice crystals is what radar and specialized instruments detect, confirming the physical effect of the seeding.
The resulting precipitation is visually identical to naturally occurring rain or snow. The induced snowflakes or raindrops are merely the final product of an accelerated natural process. The amount of precipitation generated is an incremental increase, often estimated to be 5% to 15% more than what would have fallen naturally. Since this increase is small and occurs within a larger storm system, the resulting rain or snowfall is visually indistinguishable from the natural weather event. Success is measured through statistical analysis, comparing precipitation data from the seeded area to a nearby control area over a long period.
Current Applications and Emerging Methods
Cloud seeding programs are employed globally to address specific water management and weather-related needs. In the western United States, operations frequently increase winter snowpack over mountainous regions, serving as a natural reservoir for spring and summer water supply. Other applications include suppressing hailstorms to protect agricultural crops and dispersing fog at airports to improve visibility and ensure safe takeoff and landing.
Beyond traditional aircraft and ground generators, new technologies enhance the efficiency and precision of seeding operations. Drones and unmanned aerial systems are beginning to be utilized, offering a lower-cost, more flexible platform for delivering seeding agents into hard-to-reach cloud areas. Experimental research is also investigating the use of liquid propane as a seeding material, which works similarly to dry ice but produces ice crystals at slightly warmer temperatures.