Cloud Seeding Meeting Spawns
Lively Debate At Littlefield

LITTLEFIELD, Texas — What was to be an informational meeting on weather modification here turned into a lively debate between two severe storm scientists, a consultant working for several of the weather modification programs in Texas, and a crowd of ranchers and farmers who have seen little rain this year.

Billy Tiller, a farmer, rancher and former banker here, organized the meeting, inviting Dr. Edwin Kessler, retired director of the National Severe Storm Laboratory in Norman, Okla.; Dr. William L. Woodley, president of Woodley Weather Consultants; and Dr. Charles Doswell, a senior research scientist with the Cooperative Institute for Mesoscale Meteorological Studies at the University of Oklahoma to speak about weather modification.

"I think we understand that we have a very controversial issue here," said C.E. Williams, general manager of the Panhandle Ground Water Conservation District, based at White Deer in the Texas Panhandle, more than 100 miles north of here.

The Panhandle Ground Water Conservation District completed its third season of cloud seeding this week. The High Plains Underground Water District, which has been seeding clouds on the Texas South Plains for four years now, halted its program on Aug. 9, almost two months early, because of irate farmers and ranchers in Lamb and Bailey counties.

Woodley, a veteran weather modification scientist and consultant for the High Plains Underground Water District, based in Lubbock, acknowledges that Bailey County has indeed had below average rainfall for the past four years, but he said changes in the cloud seeding program, not doing away with the program, may change that.

"The challenge we have is to prove one way or another and substantiate how much good we're doing or we're not," Williams said.

That may be easier said than done, however.

"One of the things about science that most people don't understand," Doswell said, "is that science rarely provides you with black and white answers. In fact, it almost never does. If you want science to help make your decision for you in a black and white way, it's probably not going to be very helpful."

It's not his job, Doswell said, to make the decision, only to provide information.

"It's my job as a scientist to be skeptical," Doswell said, "and to try to keep an open mind."

An appropriate way to use science, Doswell explained, is to use it as part of the information that decision-makers have to deal with.

"I have not made up my mind for or against weather modification in an absolute sense," Doswell said. "I do have an opinion."

While his opinion is at the very least skeptical, he presented a consensus viewpoint of the scientific community on weather modification, noting that neither science nor weather is a democracy.

"Science doesn't get decided by majority vote," Doswell said. "Science is only right or wrong as an empirical fact. Does it fit the observations or does it not? That gets to be rather a complicated issue. It doesn't matter what the vote is."

Doswell admits that he is not an expert on weather modification — his specialty is severe storms — but he said he does know something about clouds.

"Clouds form as the result of updrafts," Doswell said. "Updraft are essentially chunks of air that are going up."

Air going up is headed toward lower pressure.

Doswell explained that as air is expelled from a spray can, the can gets cold. The air is expanding and going into lower pressure. In the process, the air cools.

"That cooling increases the humidity," Doswell continued.

Until condensation begins, the water vapor content remains the same, he said.

"At least, that's what we tend to assume."

That increases relative humidity.

He likens the process to a glass of iced tea on a hot summer's day. Moisture forms on the outside of the glass, leaving a puddle of water.

"That's essentially the same sort of process," Doswell said. "The cold glass of tea lowers the temperature of the air and that produces condensation on the outside of the tea glass, which then creates the puddle."

Once the air is cooled and humidity reaches 100 percent, condensation begins, Doswell said.

In the atmosphere, as opposed to a tea glass, water droplets form on condensation nuclei.

"These are particles that are floating around in the air," Doswell explained.

These form nuclei that attract water and become cloud droplets.

"You can get cloud droplets that form before the relative humidity reaches absolutely 100 percent," Doswell said.

A single cloud six miles in diameter and six miles tall, a cylindrical cloud, conservatively, can contain as much as 200 million gallons of condensed water.

"That sounds like a lot of water," Doswell said, "and in fact it is, but you have to understand that cloud droplets are really small."

A typical raindrop is two millimeters in diameter. A typical cloud droplet is about 20 micrometers.

"That's pretty small compared to a raindrop," Doswell said. "It's important to understand that, because if we want precipitation from a cloud, if we want raindrops, we've got to somehow convert all the cloud droplets into droplets big enough to fall out of the cloud."

Cloud droplets, he said, are so small that they're too small to fall as rain. They're suspended in the atmosphere.

"On a typical day, when you look at fair weather clouds, little fluffy fair weather clouds, you're never going to get any rain out of those clouds," Doswell said. "That's just impossible."

Those cloud droplets have to be converted into rain droplets.

"In the tropics, a substantial portion of clouds may well be above the melting point, 32 degrees Fahrenheit," Doswell said. "Such clouds can still produce rainfall in the tropics. It's pretty rare to get that kind of cloud here in West Texas."

In West Texas, the clouds that produce rain are typically tall clouds, and a significant portion of the clouds are above the melting level.

The freezing point of water, he points out, is not necessarily 32 degrees Fahrenheit.

"If you take absolutely pure water," Doswell said, "and lower its temperature slowly, you can reduce its temperature to well below 32 degrees Fahrenheit."

By the time the temperature reaches minus 40 degrees Fahrenheit, it is so cold the water will freeze.

"Super-cooled water is what we call the water when its temperature is lower than the melting point, 32 degrees Fahrenheit, but it's above the homogeneous nucleation point, minus 40 degrees Fahrenheit," Doswell said.

It's the presence of super-cooled water that plays a big role in the formation of rain.

"Super-cooled water is up there in the tall clouds," Doswell said, "and when super-cooled droplets are present, those water droplets have to be taught how to freeze."

Here is where the potential for cloud seeding lies, he said.

"When super-cooled water is present in a cloud and there is also some ice present in that cloud," Doswell said, "it turns out that the ice actually grows at the expense of the super-cooled water."

The molecules in ice are bound together more tightly than they are in liquid water.

"That's why it's ice," Doswell said. "They have a crystalline structure."

That structure holds the water molecules in more tightly than in liquid water.

"If you have liquid water present at the same time as super-cooled water, both of them have molecules that are coming in and going out of those particles all the time," Doswell said, "but the rate at which particles are moving in and out of water is actually larger than with ice."

Some of the water molecules come out of the water droplets and end up adding themselves to the ice particles, he said.

"In other words, most rainfall starts out as particles of ice," Doswell said.

He said scientists believe that this is the mechanism that produces particles that are large enough to fall.

"Once they get large enough to fall," Doswell said, "they can sweep up smaller droplets and other ice particles and continue to grow."

He said it is similar to a droplet of water on a window. When it gets large enough to run down the pane, it doesn't go straight down, but will move over to absorb other droplets as it runs down the pane, making the drop larger.

"That's the way precipitation forms," Doswell said.

The basic building block of a thunderstorm, Doswell said, is the cell.

"Every thunderstorm is made up of one or more thunderstorm cells," he explained.

Thunderstorms begin with a towering cumulus cloud, a thunderhead, which is an updraft.

"All the air in this cloud is going up," Doswell said. "There's no precipitation falling."

In the mature stage, there are updrafts and downdrafts. In the mature stage, it begins to rain. There can also be lightning and hail.

In the dissipating stage, the rain continues to fall, but it's diminishing. The winds are all downdrafts.

The rain, Doswell said, comes with the downdrafts. The formation of rain signals the beginning of the end of the thunderstorm.

Normally, the life cycle of a thunderstorm cell, he said, is about 20 to 30 minutes, but a supercell can last hours.

The typical thunderstorm is made up of many cells.

"If you're going to get significant rainfall," he said, "you're going to have to have more than one cell. If you only have one cell, all you get is what that one cell was able to produce, which typically isn't a lot."

These cells can recycle. As the downdrafts reach the ground, the rain-cooled air can form a pool, mix with warm moist air and continue to redevelop as new thunderstorms.

In cloud seeding, silver iodide particles are put into the clouds and act as ice crystals to form the nuclei of raindrops, but they must be used at the stage where there is super-cooled water.

"Otherwise," Doswell said, "there is no effect."

The clouds have to be tall enough to have super-cooled water.

As to whether seeding works, Doswell said there is little doubt that seeding influences clouds.

Whether or not it increases rainfall, he said, is another matter.

Every thunderstorm, Doswell said, is unique.

"They're all different."

While he has studied clouds all his life, Doswell said neither he nor anyone else knows a great deal about them.

"We scientists have worked long and hard to come by what we don't know," he shrugs. "It's a complex machine."

Doswell said there is a huge amount of variability in thunderstorms.

There have been calls for double-blind tests in rain enhancement studies, but Doswell said that would be difficult.

Not all clouds are alike. Not all clouds are seeded, though typically, most clouds are seeded if possible.

A double-blind study would require comparative clouds to be seeded, one with silver iodide and one with a placebo. Neither the pilot nor the ground director would know which.

The storms would have to be consistent. They would need to appear at the same time, under the same conditions, each year. Even then, it would take five to 10 years to run a study.

Such an experiment would be expensive.

Doswell said there have been two large federally funded tests. The results were inconclusive.

"There was no evidence of a strong signal either for or against rainfall enhancement," Doswell said.

The federal government eventually got out of the weather modification business.

Doswell said cloud seeding must be done at a time and place where clouds are close to raining by themselves.

"Seeding focuses on marginal situations," he said.

Drouths are beyond cloud seeding, Doswell contended, because the conditions aren't available to produce rain with or without seeding.

Doswell subscribes to the 'Robbing Peter to pay Paul' theory, in which if it rains at one point, someone downwind will not get any rain.

Citing papers by Dr. Edwin Kessler, the retired director of the National Severe Storm Laboratory in Norman, Okla., Doswell agreed with Kessler that rain is a zero sum game. There is only so much water, and seeding water vapor to make it rain one place robs someone somewhere else of that water.

"If it falls here," Doswell said, "it couldn't fall there."

Doswell also subscribes to the theory of unforeseen consequences.

He said there may be side effects to cloud seeding of which no one is aware.

"It is inconceivable to have effects without side effects," he said.

Doswell said the track record of humans interrupting nature has not been good, but he adds that there is no proof of negative side effects, just as he said there is no proof of beneficial effects to weather modification.

"At the moment, the consensus in the scientific community is that there's no conclusive evidence of the efficacy of seeding," Doswell said. "End of story. That is the scientific consensus. Now, if that changes as the result of further research, so be it."

Doswell said he would like to see more research on cloud seeding.

"If you want to conduct an evaluation on a specific program that's going on here," Doswell said, "I suggest you develop a plan that includes proper randomizations and be prepared to carry out probably several years of trials before you begin to get significant results. That's what it's going to take. If it hasn't been done to date, then that has to be implemented immediately and carried on in a carefully constructed program. At the end of five years, you may have a result for what's going on here with this particular program."

"I'm willing to do what it takes to substantiate it, one way or the other," Williams said, "but I'm not willing to totally shut down the deal, because even if there's a chance of two-tenths of an inch of rain, I've been a farmer for about 10 years myself, and I know how valuable rain can be at certain times."

The organizer of the meeting here, Billy Tiller, said he doesn't think weather modification programs will get full support until ranchers and farmers are allowed to vote on whether they want such programs in their counties.

Tiller admitted that he didn’t attend when the High Plains Underground Water District held public meetings before they started their weather modification program.

Tiller wants a vote on the High Plains Underground Water District's program.

"A public hearing is not good enough," Tiller said. "If a public hearing was good enough, we'd be electing our public officials at a public hearing. That's never going to happen. We need to put this thing to a vote."

"One of the things I love about American democracy is we have a chance to express ourselves through things like votes," Doswell said, "but people aren't always right when they make decisions by majority vote."