Meteorologists are often asked by folks who read of drought here or elsewhere—“Why not just seed the clouds to produce rain or snow?” Discussions of cloud seeding have emerged in recent days in the wake of reports that China plans to attempt cloud seeding plans as sections of that country continue in the grip of record and debilitating drought.

What isn’t realized is that cloud seeding HAS LIMITATIONS. It’s not a cure for drought. For cloud seeding to work, you must first have to have clouds to seed. Droughts develop because such clouds AREN’T present. You can’t seed clouds which aren’t there. Nor can you seed a clear sky and expect rain to magically fall.

There’s also the question of how much rain or snow can actually be coaxed from clouds which ARE “seeded”. A 2017 study on seeding snow clouds conducted by scientists from the University of Colorado Boulder, University of Wyoming, and University of Illinois at Urbana-Champaign put the increase of snowfall from clouds seeded during a 2017 research project at 10 to 15%.

In a release addressing cloud seeding from the National Center for Atmospheric Research (NCAR) in Boulder, Colorado at the time, the following appeared: “As far back as the 1940s, scientists demonstrated that injecting certain types of particles into clouds could induce ice to form and grow around them until they fell out of the clouds. But measuring what effect, if any, cloud seeding had on measurable rain or snow proved very difficult. Researchers compared the amount of precipitation from randomly seeded clouds with similar clouds that were not seeded, but such statistical analysis produced mixed results, partly because natural precipitation is so variable that it is difficult to pick out the signal from the noise. Other work has indicated that cloud seeding can boost precipitation at specific locations but left open the question of whether the increase in precipitation extended across significant areas.”

Sarah Tessendorf, a scientist at the National Center for Atmospheric Research (NCAR) and co-author of a new paper about research on the effect on snowfall from seeded clouds at the time and cautioned in the NCAR release that successfully producing precipitation requires the presence of clouds.

The results are also dependent on such atmospheric factors as local winds.

Even when cloud seeding enhances precipitation, there are additional factors that will determine if it is a cost-effective approach to increasing snowpack or replenishing reservoirs.

“The seeding produces ice, and that ice can form snow but is it enough additional snow to make it cost effective?” she asked. “For water managers, the bottom line is the amount of snowpack that you’re building over the whole winter and how much runoff it will generate. We are looking into some promising approaches to address those bigger questions, but we still have plenty of work to do to get there.”

While the 2017 study involved snowfall, many parallel issues exist when it comes to attempts to produce or enhance precipitation of any kind.

Read the full release on the cloud seeding research here.