What Is Cloud Seeding, and Does It Work? Weather Modification Explained

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—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 (more rain, more snow, less fog, less hail)—is one such type of weather modification.

According to the Weather Modification Association, at least eight states, including Arizona, Utah, Wyoming, Colorado, Nevada, California, New Mexico, and Texas, practice cloud seeding to boost precipitation, especially winter snowfall. Despite its popularity as a tool to cope with the lack of water shortages resulting from droughts and snow droughts, especially across the western United States, though, the questions and controversy 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, serendipitously, 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. So, it was theorized that if these droplets could somehow be encouraged to solidify into ice crystals before binding to aircraft, the threat of wing icing could be reduced. 

Schaefer tested this theory in the lab by exhaling into a deep freezer, thereby creating “clouds” with his breath, then dropping 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.

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. 

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

Bettmann / Getty Images

What Schaefer had essentially done was discovered how to cool a cloud’s temperature to initiate condensation and thus precipitation. A few weeks later, Bernard Vonnegut, a fellow GE scientist and brother to famous writer Kurt 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 attention from the government, which 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 when 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 gelid, minus 109-degree-Fahrenheit frozen CO2 might neutralize the heat-fueled hurricane.

Not only did the experiment yield inconclusive results, but 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, thereby creating 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, including Project Skywater, which was led by the U.S. Bureau of Reclamation and was focused on augmenting water supplies in the western United States; and NOAA's Atmospheric Modification Program. 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 It Works

In nature, precipitation forms when tiny water droplets (smaller in size than the diameter of a human hair) suspended within the bellies of 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 as raindrops or snowflakes, depending on air temperatures within and beneath the cloud.

These “man-made” nuclei come in the form of chemicals like silver iodide (AgI), sodium chloride (NaCl), and dry ice (solid CO2), which 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 2021, 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 which headed the project, 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. 

But, Does It Work?

Close-up of hands outstretched, catching raindrops.

Elva Etienne / Getty Images

While the U.S., United Arab Emirates, China, and other countries around the globe routinely seed clouds to supplement their precipitation needs, they've largely done so in good faith. That's because scientists are still determining how to best distinguish seeding-induced precipitation from naturally occurring rain and snow within the same storm.

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 was able to do just this by using 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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