Controlling the weather is a popular bit of science fiction. Be it Storm's powers in the X-Men franchises, or high-tech satellites in movies like "Geostorm" or the 1998 film "The Avengers," in which Sean Connery plays a mad scientist bent on holding the world hostage by selling countries their weather.

In reality, controlling the weather is impossible. We can, however, influence it, and we've been attempting to do it for some time. Various efforts, from hail cannons to stopping hurricanes, have been tried or proposed over the years, to say nothing of the incidental effect of our use of fossil fuels. Perhaps the most popular attempt at geo-engineering, however, is cloud seeding.

Cloud seeding is the process of combining different kinds of chemical agents — including silver iodide, dry ice and even common table salt — with existing clouds in an effort to thicken the clouds and increase the chance of rain or snowfall. The chemicals are either shot into the clouds or released by flying near and into the clouds.

At least 56 countries have engaged in some sort of cloud seeding as of 2016, according to the World Meteorological Organization. These efforts range from the United Arab Emirates trying to meet an increasing demand for water to China using cloud seeding in an effort to stop the 2008 Summer Olympics opening ceremony from getting doused with rain.

The merits of cloud seeding are debatable, however, with a long history of inconclusive results. Some recent studies may be changing that story, though.

Making it rain

Efforts at cloud seeding date to at least the late 1800s, but it wasn't until after World War II that the process gained any traction.

Vincent Schaefer was working at General Electric in 1946 when he and Irving Langmuir were discussing ways to seed clouds while dealing with the problem of airplane icing. The work ended up giving both men keen insight into the properties of clouds. Wanting to test some of the theories in a lab instead of on mountains, Schaefer developed a "cold box," or a freezer in which he could produce a breath of air that would form droplets of water from the condensation. Schaefer added dry ice to further lower the temperature, so that when he breathed into it, ice crystals rapidly formed. It would later be discovered that any substance with a temperature of minus 40 degrees Fahrenheit or Celsius (they're the same) ended up producing the effect.

Schaefer's work provided one way to seed clouds by cooling them down a great deal. Bernard Vonnegut was working on another way. Vonnegut hypothesized that it might be possible to nudge ice crystals in the cold box to form around the particles whose crystal structure was similar to that of ice. The result would be a template upon which water molecules would arrange themselves in an ice crystal arrangement. Vonnegut tried a few different substances until he found that silver iodine worked to induce the freezing process in water droplets.

Clouds drop heavy rainfall Cloud seeding is often intended to speed up the water cycle. (Photo: serkan senturk/Shutterstock)

So why is all this cooling important for cloud seeding? It has to do with the water cycle and what triggers precipitation to fall.

When water evaporates and rises into the atmosphere, it's in gas form. Over time, this water vapor cools and condenses to become clouds. However, to condense, the vapor needs something to attach itself to. Drops of water or ice crystals do nicely for this. As more water condenses onto these other droplets, the droplets get heavier and, eventually, they fall as rain. If the cloud is at or below the freezing point of water, and the layers of air the precipitation falls to are similarly at or below the freezing point, you get snow.

Cloud-seeding enterprises intending to create rain or snow basically try to spur on the condensation of water and nudge the water to glom onto these introduced particles, like silver iodine, or to form more ice crystals by cooling the clouds' temperatures.

Inducing rain isn't the only aim of cloud seeding. In an effort disperse fog, airports may seed clouds with salt because salt lowers the freezing point of ice; it's why we use salt on icy roads.

Does cloud seeding work?

A technician fits cloud seeding canisters onto an airplane. A technician fits canisters containing silver iodine, sodium chloride and potassium chloride onto a small aircraft for a cloud-seeding experiment project in India. (Photo: Manjunath Kiran/AFP/Getty Images)

It's been difficult to gauge the effectiveness of cloud-seeding efforts.

After Schaefer and Vonnegut's work yielded strong results, the prospect of cloud seeding was on the rise. The U.S. military used cloud seeding extensively in an effort to extend monsoon season during the Vietnam War. But expectations for cloud seeding were probably more than it could reasonably deliver.

"There was all this excitement extrapolated into very optimistic claims about how effective it would be," Daniel Breed, a meteorologist at the National Center for Atmospheric Research (NCAR), told FiveThirtyEight in 2014. "You're looking for a small signal in a very large range."

More recent science has been split on the issue. A 2010 Israeli study published in Atmospheric Research declared that cloud seeding wasn't an "effective mechanism for precipitation enhancement." A National Academies panel in 2003 determined there was "no convincing scientific proof" that cloud seeding worked, and it called for more rigorous evaluation of cloud seeding's merits.

The Wyoming Weather Modification Pilot Project, a six-year, $13 million study that concluded in 2013, also wasn't able to provide definitive answers. This study focused on two adjacent mountain ranges in southern Wyoming, the Medicine Bow and Sierra Madres. Both ranges are often hit by the same storms, so researchers seeded some clouds in one range and left the other range unseeded, as a control.

The study showed that seeding produced around a 5 to 15 percent increase in precipitation, but the researchers discarded instances where seeding may have drifted into the control clouds or when not enough seeding agents were released. The result is a lack of statistical significance on a scientific level, but is still one that may be attractive to those looking to boost water levels.

"If you can get a 10 or 15 percent impact on every storm you seed, and 30 percent of the snowpack comes from those storms, then yeah — those numbers are competitive," Terry Deshler, an atmospheric scientist from the University of Wyoming who worked on the project, told FiveThirtyEight.

In late January, a study published in the Proceedings of the National Academy of Sciences yielded "direct, quantifiable observations of cloud seeding for increased snowfall," according to the National Science Foundation, which supported the project. Researchers flew one plane in laps around potential clouds between two ground-based radars in southwestern Idaho while dropping canisters of silver iodine. The same plane also flew through the clouds while it streamed silver iodine from its wings. A second plane, loaded with cloud measuring equipment, flew a perpendicular path to the first plane to collect readings.

Sensors initially didn't show any results, and then the silver iodine-laced lines of the first plane appeared. Water droplets were colliding with the silver iodine, freezing and eventually getting large enough that they would drop.

The researchers caution that this doesn't prove that cloud seeding works as a whole, however, only that their methods are a sound way to observe and measure whether or not cloud seeding works. Questions remain, too, whether cloud seeding is financially viable.

"Does it make enough snow to make an impact on a water budget?" Katja Friedrich, an atmospheric scientist at the University of Colorado in Boulder and one of the researches involved in the study, wondered to Science. "We still have to answer those fundamental questions."