Project Stormfury was an attempt to weaken tropical cyclones by flying aircraft into them and seeding with silver iodide. The project was run by the United States Government from 1962 to 1983.
Theoretically, the silver iodide would cause supercooled water in the storm to freeze, disrupting the inner structure of the hurricane. This led to the seeding of several Atlantic hurricanes. However, it was later shown that this hypothesis was incorrect. In reality, it was determined most hurricanes do not contain enough supercooled water for cloud seeding to be effective. Additionally, researchers found that unseeded hurricanes often undergo the same structural changes that were expected from seeded hurricanes. This finding called Stormfury's successes into question, as the changes reported now had a natural explanation.
The last experimental flight was flown in 1971, due to a lack of candidate storms and a changeover in NOAA's fleet. More than a decade after the last modification experiment, Project Stormfury was officially cancelled. Although a failure in its goal of reducing the destructiveness of hurricanes, Project Stormfury was not without merit. The observational data and storm lifecycle research generated by Stormfury helped improve meteorologists' ability to forecast the movement and intensity of future hurricanes.
Cloud seeding was first attempted by Vincent Schaefer and Irving Langmuir. After witnessing the artificial creation of ice crystals, Langmuir became an enthusiastic proponent of weather modification. Schaefer found that when he dumped crushed dry ice into a cloud, precipitation in the form of snow resulted.
With regard to hurricanes, it was hypothesized that by seeding the area around the eyewall with silver iodide, latent heat would be released. This would promote the formation of a new eyewall. As this new eyewall was larger than the old eyewall, the winds of the tropical cyclone would be weaker due to a reduced pressure gradient. Even a small reduction in the speed of a hurricane's winds would be beneficial; as the damage potential of a hurricane increased exponentially with its winds, a slight lowering of wind velocity would have a large reduction in potential destructiveness.
Due to Langmuir's efforts, and the research of Schaefer at General Electric, the concept of using cloud seeding to weaken hurricanes gathered momentum. Indeed, Schaefer had caused a major snowstorm on December 20, 1946 by seeding a cloud. This caused GE to drop out for legal reasons. Schaefer and Langmuir assisted the U.S. military as advisors for Project Cirrus, the first large study of cloud physics and weather modification. Its most important goal was to try and weaken hurricanes.
In the sense of weakening hurricanes to reduce their destructiveness, Project Stormfury was a complete failure because it did not distinguish between natural phenomena in tropical cyclones and the impact of human intervention. Millions of dollars had been spent trying to do the impossible. In the end, "[Project] STORMFURY had two fatal flaws: it was neither microphysically nor statistically feasible."
In addition, Stormfury had been a primary generator of funding for the Hurricane Research Division. While the project was operational, the HRD's budget had been around $4 million (1975 USD), with a staff of approximately 100 people. Today, the HRD employs 30 people and has a budget of roughly $2.6 million each year.
However, Project Stormfury had positive results as well. Knowledge gained during flights proved invaluable in debunking its hypotheses. Other science resulted in a greater understanding of tropical cyclones. In addition, the Lockheed P-3's were perfectly suitable for gathering data on tropical cyclones, allowing improved forecasting of these monstrous storms. Those planes are still used by the NOAA today.