It was not what they’d expected. They had thought the chemical would disperse much earlier, and lower down in the cloud. We had this conceptual model, Boe explains, and what it said basically was that if we release this stuff in the updraft it will rise--logically--and spread, so that by the time it reaches the cloud top it will be pretty well dispersed over a large area. It’s so turbulent in the cloud, and everybody knows that mixes things up, right? Well, wrong. It didn’t do that. When we found the tracer it wasn’t in concentrations of twenty parts per trillion, it was in two hundred or two thousand parts per trillion. It was a whole lot more concentrated, and there was a whole lot less mixing going on in the lower levels of the cloud.
This was a major revelation. Mixing, of course, is what brings the silver iodide into contact with supercooled water in the cloud to get the ice crystal process started. Finding that the mixing was delayed meant that seeds were delayed in forming ice crystals as well, and that gave Boe and company new insights into why some clouds they had tried to seed produced huge hailstorms anyway. It also told them something about the right kind of clouds to seed if they wanted to prevent this.
The centerpiece of nearly all big thunderstorms is a main, monster-size thundercloud, perhaps 40,000 or 50,000 feet from base to top. This main cloud is surrounded by several smaller feeder clouds that will eventually merge into the main cloud, becoming part of a giant hailstorm or rainstorm. The updrafts in the feeder clouds are relatively gentle, but the updraft in the main storm cloud is a fierce gale, often blowing faster than 100 miles per hour. If you place artificial seeds in it, Boe says, they shoot right to the top before they begin to grow, and that doesn’t do any good. The strong updraft will simply keep bouncing them around in the cloud top; they’ll never grow big enough to drop out of the cloud and take some of the cloud’s water content with them.
If you try to attack the main updraft where the hail actually grows, Boe says, that’s too late, because there’s just so much energy being released there. If you look at the thermal dynamics of it, it’s on about the same scale as a small atomic bomb. It’s hard to win an argument with an energy release of that scale, so you’ve got to go downscale, where the microphysics first get started. And that happens in the feeder clouds.
The name of the game, Boe says, is to use seeding to drain off energy from the feeder clouds before it’s funneled into the main cloud. What we try to do is convert the supercooled water to ice sooner, in the younger clouds, Boe continues. There are four reasons why we want to do that. One is that when water vapor condenses to form water, or when liquid water freezes to form ice, heat is given off. That heat, if the release occurs in the main updraft, will simply add buoyancy to the main updraft and make it stronger. If we release it instead in the smaller clouds that are full of supercooled liquid water, it will strengthen their milder updrafts. That deprives the main updraft of some of the energy it might otherwise obtain. The second reason is that if we start the ice forming sooner, the particles won’t be hail. They may be what I call popcorn snow: little, lightweight snow particles, pellets that are still big enough to fall out of a cloud with a gentle updraft. And that causes a premature rainout of the cloud, a cloud that would otherwise not precipitate at all.
The third reason is that if we create a whole bunch of these moderately sized ice particles that are big enough to fall out of a cloud but that don’t quite get there, and instead they do merge with the main updraft, there are tens of thousands of millions more of them than there would have been naturally. That’s a lot of additional precipitation mass the updraft must support. The physical loading on the updraft, the additional mass, has got to slow it down. And the slower the main updraft is, the smaller the maximum size of the particles it can produce. A hundred-mile-per-hour updraft can grow a hellacious hailstone. A fifty- mile-per-hour updraft will produce one that is way smaller. And so you’re limiting the maximum size on your hail particle. The fourth reason is that rain you basically force on the clouds earlier falls over a broader area and is gentler. So you’re less likely to have an area under the main updraft where you have torrential rain--or so we hope.
The next step, of course, is to demonstrate how much of the water a seeded cloud finally drops can be credited to seeding, and that work is being carried out in the mountains of Nevada by researchers using a different sort of tracer technology. Led by Joseph Warburton of the University of Nevada’s Desert Research Institute in Reno, these investigators are looking at winter snow clouds. They are trying to find an alternative to old-fashioned cup methods of measuring seeding’s effect-- that is, with ground-based gauges that compare how much rain or snow fell during a seeded period versus a nonseeded period.
As you can imagine, there’s lots of natural variability among clouds and storms irrespective of whether you’re seeding them or not, says Warburton. In addition, he says, gauges are notoriously unreliable for measuring precipitation because they don’t take into account how shifts in wind can affect a rain or snow level reading at a specific site.
Warburton began exploring alternative measuring methods in the mid-1980s. He selected a tracer, indium oxide, that does not nucleate ice but can, like so many other stray particles in the atmosphere, be washed out of clouds through rainfall or snowfall. He released equal parts of this tracer and silver iodide into snow clouds. After snow fell, his crew rushed to remote sites by ski and helicopter, collected samples of the fresh snow, and carried them back to a clean lab for analysis with atomic absorption spectrophotometers. These instruments can measure how much of a chemical exists in a sample right down to parts per trillion, Warburton says.
In 1985 Warburton found ten times more silver iodide than indium oxide in the snow samples. That meant that more silver iodide than indium oxide took part in snow creation. It wasn’t a surprising result, but nevertheless it was the first time it was shown, Warburton says. So now we have evidence that silver iodide is in fact taking part in the process the way we thought it did.
While Warburton was studying snow clouds in Nevada, several hundred miles away in Texas cloud physicist William Woodley was using a sophisticated computer program for analyzing radar images of clouds to provide firm evidence that seeding with silver iodide does indeed make clouds rain. And this January he reported to a meteorological meeting that he had done just that.
The clouds Woodley seeded weren’t connect-ed to big storm systems; they were simply halfhearted collections of water vapor scudding across the Texas sky. By dumping silver iodide seeds into these clouds, Woodley caused ice crystals to form, and the heat given off in the process strengthened the clouds’ updraft. Warm air rises higher, and that makes clouds bigger. Unlike human beings, Woodley says, the fatter a cloud is, the longer it lives. Bigger clouds hold more water, and that means there is more water to freeze into crystals. The stronger updraft keeps those crystals aloft for a longer time, so they grow bigger and are more likely to fall out of the cloud as rain (the clouds are too small to start with to create hail).
Woodley used ground-based radar to track both seeded and unseeded clouds. Denser objects give off stronger radar echoes, and so Woodley was able to sweep the clouds with radar every five minutes and determine which ones were growing fatter. The software, developed in 1987 by Daniel Rosenfeld of Hebrew University in Jerusalem, allowed him to zero in on a particular cloud in a clump of clouds and follow its progress. What he found was that seeded clouds merged twice as often as did unseeded clouds. Not only that, but the radar indicated that seeded clouds produced more than twice as much rain as did their unseeded counterparts. It’s sort of like the old cliché: United we stand, divided we fall, says Woodley. The more clouds merge, the more rain they’re going to produce.
With renewed confidence that seeding works, researchers are now concentrating on figuring out the most cost-effective ways of doing it and, armed with a sharper knowledge of seeding’s physics, narrowing down the range of candidate clouds. Boe would dearly like to know exactly how much seeding agent he needs to use in a cloud to get it to turn its water loose. For seeding to work, it has to be timely, it has to be spatially correct-- on the correct cloud, the correct location--and it has to be with enough seeding agent to alter the microphysics of the cloud, Boe says. To get all three of those right at once is not easy.
Even though all the answers aren’t in, the results so far have begun to redeem weather modification’s spotty reputation. In 1989 the World Meteorological Organization counted 118 weather modification projects operating in 32 countries, compared with 80 such projects in 1983. The science as a whole hurt itself in the early days, Boe says. I guess when you have something that works as dramatically as Vince Schaefer’s experiment did, you think, ‘Wow, this is really neat, and it’s just a matter of time till we figure out how to do this with all clouds.’ And clouds turned out to be a whole lot more complicated than anybody thought, and so a lot of false hopes were raised. When somebody promises you something and doesn’t deliver, you become skeptical, and that’s what happened to weather modification.
Today people better appreciate what we can do, and you almost never--at least not from any of my friends--hear broad claims of ending droughts and so on. Now when somebody inquires about whether they should start a program, we say, ‘Well, let’s look at your area, do a feasibility study, and then see if it’s worthwhile.’
The first features seeders look for are clouds with good updrafts and long-enough lives. Then they have to determine whether or not the water in these clouds is supercooled, which can be ascertained by a plane on a fly-through. If there is supercooled water in the clouds, then it’s worth considering a program, but it’s clearly not going to be cure-all.
In North Dakota, says Boe, we’re seeing an increase in precipitation as well as a decrease in hail fall, but it’s not huge. It’s maybe ten percent. So the technology has limited applications. But it’s still beneficial to get an additional inch of rainfall that’s worth tens of millions of dollars in crops.