Technology can make it rain. Conditions must be right, and the effort sometimes fails, but it is possible to change the weather. Cloud seeding happens every day.
The success of cloud seeding depends on an understanding of the dynamics of rainfall. It turns out that clouds can be full of moisture without bringing rain. For rain to fall, the formless water vapor in clouds needs nuclei, cores, to collect upon.
Seeding tries to provide nuclei. Each nucleus may be an ice crystal, silver iodide (which is shaped like ice crystals), dry ice, or salt, but without a core of some kind, water vapor won’t form droplets.
Even after each droplet forms, there still must be many droplets to make rain.
Cloud seeding is a complicated endeavor, but in essence, it introduces new cloud condensation nuclei into clouds. There are three basic methods of cloud seeding: static, dynamic, and hygroscopic.
The static mode of cloud seeding tries to optimize the concentration of ice crystals in clouds. If it’s successful, snow or hail will fall, or melt to rain. The process needs clouds containing supercooled water, which is water that is below freezing temperature but has not frozen because it lack a nucleus to coalesce around.
Static seeding drops silver iodide particles into clouds. This method is believed to work best on clouds that are continental, meaning not formed over water, and preferably orographic, meaning formed by the uplift into cooler regions associated with air flowing over mountains.
The dynamic mode, on the other hand, seeds puffy cumulus clouds, in an effort to make them grow taller. It tries to enhance the vertical air currents within the clouds in order to make the clouds process more rainwater.
This method uses salt particles to seed clouds. They are introduced into the base of likely clouds. These then attract water by vapor deposition and increase in size, to the point that they can serve as nuclei for droplet formation. This imitates a process found above some industrial smokestacks, where soot serves as nuclei.
Nuclei are added to clouds by various means, including lighted silver iodide flares carried by aircraft to the inflow areas of clouds, and seeding materials launched into clouds by artillery.
Cloud seeding has been used to try to prevent precipitation too. Airports use it to dissipate fog. Before the Beijing Olympics, Chinese authorities used cloud seeding to ensure clear skies for the events. Experiments were done in the fifties and sixties to see if it was possible to downsize hurricanes by making them rain away their enormous energies. Apparently, the attempts were unsuccessful.
It is often hard to tell if cloud seeding worked, or if rain would have fallen anyway. That is one of the controversies about making rain. Its opponents say the technology lacks validity, because no one can tell how well it worked or if it had any affect at all.
Another objection to cloud seeding is that the systems it deals with are so complex and unknowable. It is impossible to predict the long-range results of changing the weather. Others believe that effective cloud seeding is a kind of theft, stealing water from those downwind.
Still, crops need rain, and rain-making can be seen as a kind of irrigation. Since cloud seeding is done, it plainly must be a cooperative venture, done with respect for all human laws as well as the laws of nature.