What is cloud seeding? Changing the Weather Explained

Humans may not be able to control the weather, but we sure can change it. Cloud seeding – the act of injecting chemicals such as dry ice (solid CO2), silver iodide (AgI), table salt (NaCl), in the clouds with the aim of modifying the weather conditions (more rain, more snow, less fog, less hail) – is one of these types of weather modification.

According to the Weather Modification Association, at least eight states, including Idaho, Utah, Wyoming, Colorado, Nevada, California, North Dakota and Texas, practice cloud seeding to increase precipitation, especially winter snowfall. Despite its popularity as a tool for coping with water scarcity resulting from droughts and snow droughts, particularly in the western United States, questions and controversy surrounding its effectiveness and ethics remain hotly debated. .

History of cloud seeding

As ultra-modern as cloud seeding may seem, it is not a new concept. It was invented, by chance, in the 1940s by General Electric (GE) scientists Vincent Schaefer and Irving Langmuir, who were researching ways to reduce aircraft icing. Icing occurs when supercooled water droplets residing in clouds strike and immediately freeze on aircraft surfaces, forming a layer of ice. Thus, it was hypothesized 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 freezer, creating “clouds” with his breath, then dropping various materials, such as dirt, dust, and talc, into the “cold box” to see what would stimulate the best. the growth of ice crystals. By dropping tiny grains of dry ice into the cooler, 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 air below freezing (32 degrees F). Only water in its purest form, without sediments, minerals or dissolved gases, can supercool. It won’t freeze unless it gets to minus 40 degrees, or it hits something and freezes on it.


Schaeffer and his colleagues.

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What Schaefer had essentially done was figure out how to cool the temperature of a cloud to initiate condensation and hence precipitation. A few weeks later, Bernard Vonnegut, a fellow GE scientist and brother of famed writer Kurt Vonnegut, discovered that silver iodide served as an equally effective particle for glaciation because its molecular structure closely resembles that of ice. .

This research quickly caught the attention of the government, which partnered with GE to investigate the viability of cloud seeding for producing rain in arid regions and weakening hurricanes.

Project Cirrus

In October 1947, cloud seeding was put to the tropical test when the U.S. government dropped more than 100 pounds of dry ice into the outer bands of Hurricane Nine, also known as Hurricane Nine. Cape Sable Hurricane of 1947. The theory was that the gelid, minus 109 degrees Fahrenheit of frozen CO2 could 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. Although the hurricane was later shown to have begun to veer west before it was seeded, the public perception was that Project Cirrus was to blame.

Stormfury, Skywater and other projects

During the 1960s, the government commissioned a new wave of hurricane cloud seeding projects. Known as Project Stormfury, the experiments proposed that by seeding the outer cloud bands of a hurricane with silver iodide, convection would develop at the edges of the storm, creating a new, larger eye. (and therefore weaker) 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 through the 1990s, several other programs emerged, including Project Skywater, run by the U.S. Bureau of Reclamation and focused on increasing water supplies in the western United States; and the NOAA Atmospheric Modification Program. The number of weather modification projects in the United States declined in the 1980s due to the lack of “compelling scientific evidence for the effectiveness of intentional weather modification”.

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

How it works

In nature, precipitation forms when tiny water droplets (less than the diameter of a human hair) suspended in the belly of clouds grow large enough to fall without evaporating. These droplets grow by colliding and joining with neighboring droplets, either by freezing on solid particles with crystalline or ice-like structures, called ice nuclei, or by attracting dust or salt particles , called condensation nuclei.

Cloud seeding stimulates this natural process by injecting clouds with additional nuclei, increasing the number of droplets that grow large enough to fall as raindrops or snowflakes, depending on temperature. air inside and under 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 distributed to the heart of precipitation-producing clouds via ground-based generators. that emit chemicals into the air, or aircraft that launch chemical-filled flare payloads.

In 2017, the United Arab Emirates, which carried out nearly 250 seeding projects in 2019, began testing a new technology in which drones fly through clouds and deliver an electric shock. According to the University of Reading, which led the project, this method of electrical charging ionizes the droplets in the cloud, causing them to stick together, increasing their growth rate. Since it eliminates the need for chemicals like silver iodide (which can be toxic to aquatic life), it could become a greener stocking option.

But, does it work?

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While the United States, United Arab Emirates, China, and other countries around the world routinely seed clouds to supplement their precipitation requirements, they have largely done so in good faith. That’s because scientists are still figuring out how best to distinguish seeding-induced precipitation from natural rain and snow during a single storm.

While seeding is traditionally credited with increasing precipitation and snowfall by 5-15%, scientists have recently made progress in measuring actual accumulations. A 2017 study of winter cloud seeding based in Idaho was able to do just that by using weather radar and snow gauge scans to analyze the signal specific to seeded precipitation. The study found that the seeding produced 100 to 275 acre feet of water, enough to fill nearly 150 Olympic-sized swimming pools, depending on how many minutes the clouds were seeded.