Wouldn't it be nice if we could just drop some "rain seeds" into a cloud and watch the water pour out? Well, actually we can to some extent. While cloud seeding can be used to disperse fog at airports, suppress hail, or even change winds, its most common use is to increase precipitation over areas in need of rain.
In order to understand how cloud seeding works, it is first necessary to understand what makes rain in the first place. As we all know, warm air has moisture in it just look at the fog created when you breathe on your glasses. The air near the surface of the earth is warm relative to the air above it. As the warm air rises, it cools and the water in it condenses and forms cloud droplets around dust or salt particles. When groups of cloud droplets are big enough, their weight sends them earthward in the form of rain, snow, or hail depending on the air temperatures. If not, then they just remain as puffy white clouds that we admire on a sunny day.
The first discovery that drawing and enhancing moisture from clouds was possible came in World War II at General Electric when two scientists, Irving Langmuir and Vincent Joseph Schaefer were trying to determine what made ice buildup on airplane wings and thus be able to prevent it. Indeed our airplane de-icing fluids, a descendant of those first experiments, do just that. Those same scientists also released several pounds of dry ice into a cloud from an airplane. It appeared that their efforts caused snow which then turned to rain on the way to the ground. They could make rain! Bernard Vonnegut, also of GE, used silver iodide in replicating the experiments. Silver iodide is easier to use than dry ice because it can be stored at room temperature and can also be fired by ground cannons.
So what happens inside seeded clouds? Other particles are introduced that serve as the nuclei of cloud condensation. The more condensation created, the greater the potential for precipitation. The three types of cloud seeding are: STATIC MODE, DYNAMIC MODE, and HYGROSCOPIC SEEDING. STATIC MODE adds super cooled substances (usually silver iodide and dry ice) into existing clouds. The crystalline structure of silver iodide is similar to that of ice and stimulates "freezing nucleation," which simply means that bubbles, crystals, or droplets form. Dry ice cools the air so much that ice crystals form during the vapor phase. The goal is to create large enough ice crystals which will melt and fall as rain. DYNAMIC MODE uses even more seeding material; the release of warm air as the particles form into precipitate creates updrafts and taller clouds capable of holding more moisture. HYGROSCOPIC seeding involves adding salt crystals into the cloud which hopefully grow into larger particles that cause precipitation to form. All three methods of seeding can be accomplished with airplanes or from ground rockets.
A lot of work went on with cloud seeding in the 1950's and most of it was unsuccessful. It is still being experimented with as a way of taking strength out of hurricanes and also of delivering much-needed moisture to drought-stricken areas. It is not without controversy though as some scientists feel that it is immoral to empty clouds of moisture over one area when they might have traveled to another area equally in need. It is also difficult to assess whether the efforts are successful as no one can know how much rain would have fallen from the cloud if it had not been seeded.
One of my all time favorite comic books when I was little involved Donald Duck, along with Huey, Duey, and Luey, involved in starting a cloud seeding operation. The imagery of them carving out perfect cloud squares or rectangles with their bi-plane and then shoving them over the farm needing moisture comes into my mind every time I fly. Just a comic book story? With science and imagination, anything is possible.