What Is Cloud Seeding? Understanding Weather Modification

Clouds can be prodded to make more rain and snow. But is it ethical?

Close-up of a turboprop plane sprinkling chemicals into clouds in a blue sky.
A turboprop plane spreads cloud seeding chemicals over skies in Thailand. .

Monthon Wa / Getty Images

Humans might not be able to control the weather, but we can certainly modify it. Cloud seeding is one such type of weather modification. It is defined as the act of injecting chemicals such as dry ice (solid CO2), silver iodide (AgI), table salt (NaCl), into clouds for the sake of altering the weather outcome.

According to the Weather Modification Association, at least eight states practice cloud seeding to boost precipitation, especially winter snowfall. Cloud seeding is a popular tool to cope with the lack of water shortages resulting from droughts and snow droughts, especially across the western United States. However, questions surrounding its efficacy and ethics remain hotly debated. 

History of Cloud Seeding

As ultramodern as cloud seeding sounds, it isn’t a new concept. It was invented in the 1940s by General Electric (GE) scientists Vincent Schaefer and Irving Langmuir, who were researching ways to reduce airplane icing. Icing occurs when supercooled droplets of water residing in clouds hit and immediately freeze onto aircraft surfaces, forming a layer of ice. The scientists theorized that if these droplets could solidify into ice crystals before binding to aircraft, the threat of wing icing could be reduced. 

What Is Supercooled Water?

Supercooled water is water that remains in a liquid state despite being surrounded by below freezing (32 degrees F) air. Only water in its purest form, without sediments, minerals, or dissolved gases, can supercool. It won’t freeze unless it either reaches minus 40 degrees, or it hits something and freezes on it. 

Schaefer tested this theory in the lab by exhaling into a deep freezer, thereby creating “clouds” with his breath. Then, he dropped various materials, such as soil, dust, and talcum powder, into the “cold box” to see which would best stimulate the growth of ice crystals. Upon dropping tiny grains of dry ice into the cold box, a flurry of microscopic ice crystals formed.

Three scientists hover over a freezer chest with cold air spilling out of it.
Schaeffer and his colleagues.

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In this experiment, Schaefer discovered how to cool a cloud’s temperature to initiate condensation and thus precipitation. A few weeks later, fellow GE scientist Bernard Vonnegut discovered that silver iodide served as equally effective particles for glaciation because its molecular structure closely resembles that of ice.  

This research soon garnered widespread attention. The government partnered with GE to investigate how viable cloud seeding might be for producing rain in arid regions and in weakening hurricanes.

Project Cirrus

In October 1947, cloud seeding was put to the tropical test. The U.S. government dropped over 100 pounds of dry ice into the outer bands of Hurricane Nine, also known as the 1947 Cape Sable Hurricane. The theory was that the minus 109-degree-F frozen CO2 might neutralize the heat-fueled hurricane.

Not only did the experiment yield inconclusive results; the storm, which had previously tracked out to sea, reversed course and made landfall near Savannah, Georgia. While it was later shown that the hurricane began to veer west prior to its seeding, public perception was that Project Cirrus was to blame.

Projects Stormfury, Skywater, and Others

During the 1960s, the government commissioned a new wave of hurricane cloud seeding projects. Known as Project Stormfury, the experiments proposed that by seeding a hurricane’s outer cloud bands with silver iodide, convection would grow at the storm’s edges. This would create a new, larger (and therefore, weaker) eye with reduced winds and reduced intensity.

It was later determined that seeding would have little effect on hurricanes since their clouds naturally contain more ice than supercooled water.

From the 1960s to the 1990s, several more programs arose. Project Skywater, led by the U.S. Bureau of Reclamation, was focused on augmenting water supplies in the western United States. The number of U.S. weather modification projects dwindled in the 1980s due to the lack of “convincing scientific proof of the efficacy of intentional weather modification."

However, the Bureau of Reclamation’s 2002-2003 Weather Damage Modification Program, as well as California's 2001-2002 and 2007-2009 historic droughts, sparked renewed interest in cloud seeding which continues to this day.

How Cloud Seeding Works

In nature, precipitation forms when tiny water droplets suspended within clouds grow large enough in volume to fall without evaporating. These droplets grow by colliding and joining with neighboring droplets, either by freezing onto solid particles having crystalline, or ice-like structures, known as ice nuclei, or by attracting onto specs of dust or salt, known as condensation nuclei.

Cloud seeding boosts this natural process by injecting clouds with additional nuclei, thus enhancing the number of droplets that grow large enough to fall like raindrops or snowflakes, depending on air temperatures within and beneath the cloud.

These synthetic nuclei come in the form of chemicals like silver iodide (AgI), sodium chloride (NaCl), and dry ice (solid CO2). All are dispensed into the heart of precipitation-producing clouds via ground-based generators that emit chemicals into the air, or aircraft that deliver payloads of chemical-filled flares.

In 2017, the United Arab Emirates, which conducted almost 250 seeding projects in 2019, began testing new technology in which drones fly into clouds and deliver an electric shock. According to the University of Reading, this electric charge method ionizes the cloud droplets, making them stick to each other, thereby boosting their growth rate. As it eliminates the need for chemicals like silver iodide (which can be toxic to aquatic life), it could become a more eco-friendly seeding option. 

Does Cloud Seeding Work?

Close-up of hands outstretched, catching raindrops.

Elva Etienne / Getty Images

While seeding is traditionally credited with boosting rainfall and snowfall by 5 to 15%, scientists have recently made headway in measuring actual accumulations.

A 2017 Idaho-based winter cloud seeding study used weather radar and snow gauge analyses to parse out the signal specific to seeded precipitation. The study revealed that seeding had produced 100 to 275-acre feet of water—or enough to fill nearly 150 Olympic-sized swimming pools—depending on how many minutes clouds were seeded for.

View Article Sources
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