Summer 2001

Let There Be Rain: An Interview with William Cotton

The past and future of cloud seeding

Anna Grace and William Cotton

In 1946, the long, colorful history of unscientific rainmaking came to an end with the first successful experiment to induce a cloud to rain. The promise of cloud seeding was never fulfilled on a large scale, however, and funding for experiments in the US has dried up. Dr. William Cotton, currently a professor at the Department of Atmospheric Science at Colorado State University, conducted research in the 1970s on cloud seeding. Anna Grace talked to him over the phone about the physics and the potential future of cloud seeding.

Anna Grace: What is cloud seeding is and how does it work?

Dr. William Cotton: The basic concept of cloud seeding principles used today originated in the late 1940s with Laeimer and Schaffer’s work. It involves the introduction of some form of ice nuclei or another agent that will cool the air to a point where ice crystals form at warmer temperatures than will normally occur in nature. There are two different microphysical approaches. In the case of super-cooled clouds, if you enhance the concentration of ice crystals by introducing either silver iodide or dry ice, the ice crystals will grow at the expense of cloud droplets and very rapidly consume the liquid water in the cloud. This can sometimes lead to—and this is a matter of debate—enhanced precipitation.

In warm clouds in which the ice phase isn’t active, the approach is to introduce hygroscopic materials like salt particles. These salt particles take on water vapor at saturation levels which are below 100% relative humidity and, basically, they swell. The main precipitation process in these warm clouds is called collision coalescence, in which slightly larger droplets collide with smaller droplets and get bigger and fall faster and collide with other little droplets until you get precipitation. The idea is that they can become big enough that they can initiate collision coalescence faster than nature does it.

What criteria determine whether a cloud is a good candidate for seeding, and how are radar technologies used to determine these criteria?

They’re looking for clouds that are fairly mature, tall and wide, and have the potential of containing a fair amount of liquid water content. What the radar sees are basically precipitation particles. The radar return, or reflectivity, is a strong function of the diameter of the particles. It’s something like to the sixth power to the diameter of the particle. So they’re seeding clouds that are at least wet enough to produce some radar-detectable precipitation.

Has cloud seeding demonstrated promise in reducing drought conditions?

Recent experiments in South Africa and Mexico and Thailand seem very encouraging. In South Africa and Mexico, they’re seeding clouds using pyrotechnics, which is a new delivery-device for hygroscopic material that they can stick out on the wing of an airplane and that’s quite cost effective. The introduction of this device has increased the interest in cloud seeding because you don’t have to take a C130 and load it with thousands of pounds of table salt-like particles. Instead you carry lower mass loads but get a fairly high yield from somewhat smaller particles. The evidence suggests that they’re still larger than nature’s cloud condensation nuclei, and therefore you can get a jump start on getting droplets large enough to initiate collision coalescence. These experiments have shown that they do increase rainfall. But they don’t entirely know how they’re doing it, because it appears that the main responses are not on the individual clouds that they seed but actually on neighboring clouds. They think that seeding actually increases the downdrafts and the outflow from the seeded clouds which then triggers stronger developing neighboring clouds. But it’s pretty much conjecture at this point.

Can they determine where the rain is actually going to fall?

No, they can’t. No one’s carried out a specific experiment to see whether carrying out either hygroscopic seeding or silver iodide seeding over an area can really enhance rainfall enough to be of value to a particular customer—to some reservoir operator, or farmers, or whoever.

Once you’ve carried out a cloud seeding experiment, is it possible that you could rain out a cloud and prevent it from raining in another region?

Oh, I guess that’s somewhat possible. I mean, the cloud lifetimes that they’re working with are usually thirty-five to forty-five minutes at the most. But if you do increase rainfall from that cloud enough, it still might effect the rainfall four or five miles away.

What are you working on now?

I’m doing research on how changes in aerosol content in the atmosphere affect the optical properties of clouds and the likelihood that they might increase the reflective radiation from the earth system, thereby causing a cooling effect on the climate as opposed to a warming effect on greenhouse gasses. I haven’t had any weather modification related research in about 20 years. There hasn’t been funding for it. I basically had some funding for it when I when I worked on a major weather modification experiment in Salt, Florida, in the late 1970s. And then when I came to CSU in 1975, I had some funding for weather modification, another five or eight years, but the funding basically dried out. There’s no national research program in weather modification in the US. In fact the work that is being done is all being funded essentially by the South African government, but they’ve now stopped funding their program. And now there’s the Mexican program. So we’re going abroad to get funding.

Whenever I mention weather modification, people respond with concern for the environment. Could cloud seeding pose a threat to the environment?

No, I don’t think so. The amounts of materials are so small, whether you’re dealing with silver iodide or with hygroscopic materials, and they’re not a whole lot different from what already exists in the cloud, that compared with all the other things that are being added to the environment, I don’t think these things make much of a difference. You know, people in the science community refer to weather modification as the pimple on the butt of nature.)

William Cotton is a professor at the Department of Atmospheric Science at Colorado State University.

Anna Grace is a writer and performer who lives in New York.

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