Q. Why can’t we start a cloud seeding program in dry years to make sure we get enough rain?
A. This question pops up whenever we have a dry year here. The answers:
- We can.
- But it doesn’t work well in our truly dry years, because cloud seeding requires CLOUDS – which were in unusually short supply last winter.
- And it’s not cheap, either.
Q. How does cloud seeding work?
A. In the 1940s, Irving Langmuir and Vincent Schaefer performed an experiment on cold clouds inside a home freezer. They found that if they dropped tiny chunks of dry ice into the cloud, some of the “supercooled droplets” could be turned into ice crystals. The ice crystals grow rapidly, finally reaching a large enough size that they fall out of the cloud. The result is precipitation.
The supercooled droplets are the key to this. Small cloud droplets can remain liquid even when the temperature is as low as 40 degrees below zero. Aircraft flying through clouds can experience severe icing when droplets freeze upon contact with the fuselage or wings. Winter fogs also contain supercooled droplets that freeze on contact with objects such as fences, overhead wires, and vehicles.
During winter cloud seeding operations, aircraft fly above storm clouds and drop dry ice into them. If the clouds are already producing some precipitation, seeding can be very effective in increasing the snow pack over mountains. Long-term winter seeding programs in California and Utah have demonstrated that the process works well, producing increases in precipitation on the order of 10-20% higher than if no seeding were done.
Other substances besides dry ice can be used as seeding agents. The most common is silver iodide (AgI), a compound whose crystal structure closely resembles that of ice. Some scientists think it “fools” the supercooled droplets into freezing when they encounter AgI.
Utah experimented with using an unusual seeding agent, propane gas, in the Wasatch Mountains. The scientist in charge of Utah’s project, Arlin Super, a retired professor of meteorology with 30 years of experience in cloud seeding, said the propane method appeared to increase snowfall by at least 7 percent. And propane seeding (from ground generators rather than aircraft) is far cheaper than dry ice or silver iodide seeding.
Summertime seeding is tricky, and not as effective, probably because there are fewer supercooled droplets.
Q. Do they use cloud seeding for other things besides rain and snow making?
A. Yes. Seeding can be used to clear up fog. This is practiced at some airports and air bases, including Fairchild AFB in Spokane. It is sometimes used in the Midwest to try to reduce hail.
The National Hurricane Center at one time tried to seed of hurricanes to reduce their intensity. This was called Project Stormfury. Unfortunately, it failed to modify hurricanes significantly, probably because there is already an abundance of ice crystals in hurricane rain systems, so the production of new ice particles through cloud seeding has little if any effect.
Q. Is there anything we can do in dry years?
A. Forget about cloud seeding. But how about conservation? Conservation is a good idea every year, because the typical Oregon summer is warm and dry. Even in those wet years when we wouldn’t even THINK about doing cloud seeding, conservation makes sense.
Ref: Lyons, Walter A. The Handy Weather Answer Book. Visible Ink Publishers, Detroit, 1997.