Cloud Seeding

Cloud seeding is a weather modification technique that aims to increase precipitation by injecting specific chemicals—such as silver iodide, dry ice, or hygroscopic salts—into clouds with suitable physical conditions. This method is applied to support agricultural production, enhance water resources, combat drought, or suppress harmful weather phenomena like hail and fog.

Visual illustrating cloud seeding and artificial precipitation. (MGM — Yang H. Ku, 2021)

History

The origins of modern cloud seeding trace back to experiments conducted during World War II by Irving Langmuir and Vincent Schaefer, who studied ice crystal formation. Schaefer observed that dry ice caused moisture in the air to crystallize in a frozen environment. It was later discovered that silver iodide, due to its structural similarity to ice crystals, was even more effective. In 1947, Bernard Vonnegut demonstrated that silver iodide serves as an ideal nucleation agent for crystallization.

This technology was subsequently deployed in countries such as the United States, China, and Russia. The first application in Türkiye was carried out in 1990 by İSKİ in Istanbul, followed by short-term trials in Ankara and İzmir.

Mechanism of Precipitation Formation and Scientific Basis

The process of cloud seeding is explained by the Bergeron-Findeisen theory. According to this theory, ice crystals act as nuclei around which supercooled water droplets in clouds condense. Since the vapor pressure over ice is lower than over liquid water, at the same temperature, condensation begins on ice crystals while water droplets evaporate. This process leads to the growth of ice crystals and their eventual fall as precipitation.

The substances used in cloud seeding trigger crystallization on supercooled droplets in the atmosphere. Silver iodide functions as an ice nucleus between -5 °C and -20 °C, while dry ice promotes crystal formation by cooling the surrounding air. After crystallization, ice particles aggregate and either fall as snow or melt into rain.

Chemicals Used and Their Physical Effects

The most commonly used chemical is silver iodide (AgI), which is the most effective seeding agent due to its structural similarity to ice crystals. Other substances used include:

• Dry ice (solid CO₂): Triggers crystal formation by rapidly cooling the environment.
• Liquid propane: Can form ice crystals at higher temperatures than silver iodide.
• Ammonium nitrate, cadmium iodide: Used as alternative hygroscopic nucleation agents.

Silver iodide produces fewer ice crystals at low temperatures and significantly more at higher temperatures. For example, at -6 °C approximately 10 crystals form, while at +14 °C up to 10¹⁰ crystals have been observed.

System measuring raindrop diameter and a cloud seeding operation conducted by releasing silver iodide smoke from an aircraft. (MGM)

Suitable Cloud Types for Seeding

For artificial precipitation to occur, clouds must already exist naturally; clouds cannot be created, only enhanced where precipitation potential is present. Suitable cloud types can be categorized into three main types:

• Orographic clouds: Formed when air rises and cools over mountain slopes. They have a stable structure and are well suited for seeding.
• Stratus clouds: Low-altitude, layered clouds that can produce light precipitation when sufficiently saturated with moisture.
• Cumulus and cumulonimbus clouds: Formed through convective uplift; they have the highest potential for rain and hail.

Application Methods

Aerial Seeding

Aircraft release seeding agents into clouds from above, directly through the cloud body, or via rising air currents at the base. In this process, silver iodide smoke is emitted through burners mounted on aircraft wings. Silver iodide particles act as condensation nuclei within the cloud, promoting precipitation formation. Additionally, flare systems or spray mechanisms may be deployed from aircraft to deliver seeding materials.

Aircraft-mounted cloud seeding flares and flares containing 50 grams of silver iodide. (MGM)

Ground-Based Seeding

Chemicals are released into the atmosphere via rockets or generators from the ground. Although rocket systems are preferred due to their high accuracy, they are costly. In China, 32,000 personnel and 26 control stations were involved in ground-based seeding operations, successfully altering rainfall patterns during the 2008 Olympics.

Ground-based seeding systems and ground-based silver iodide rocket systems. (MGM)

Application Outcomes and Scientific Evaluations

Cloud seeding can increase precipitation by 5% to 20%. However, its effectiveness depends on numerous factors, including cloud structure, temperature, humidity levels, wind profiles, and timing. Moreover, it is not possible to precisely predict when or where precipitation will occur after seeding.

In some cases, seeding has been observed to increase the risk of hail or sudden downpours. Therefore, careful planning of operations is essential. Additionally, whether seeding causes reduced precipitation in other regions remains a subject of debate.

Environmental and Social Impacts

The environmental effects of chemicals used in seeding—particularly silver iodide—must be closely monitored. Although silver iodide is insoluble in water, it can accumulate over time in soil and waterways. Lack of public awareness and potential legal issues are also among the social dimensions of cloud seeding.

Cloud seeding is a viable weather modification method for increasing precipitation when specific meteorological conditions are met. It is considered a low-cost alternative in arid regions where water management is critical. However, due to its nature as an intervention in natural processes, it must be carefully implemented with attention to scientific measurement, environmental analysis, economic feasibility, and legal regulations.