Scientists Seed Clouds to Make Rain

ByLee Dye

Feb. 7, 2001 -- Critics may scoff and skeptics may laugh, but rainmakers are finally emerging from the backwaters of science.

For more than half a century scientists have experimented with wringing more water out of clouds and dumping that priceless moisture on the parched earth. Most have failed, or at least produced inconclusive results, but a mounting body of evidence now suggests that if done properly, cloud seeding can be effective.

Some areas, like Texas, have held in there despite the skeptics' sneers, and today cloud seeding is done over about a third of the Lone Star state, a region of some 45 million acres.

Texas claims to produce more rain, over a wider area, and for a longer duration because of its weather modification program, according to George Bomar, who wrote his doctoral dissertation on the subject in 1974. But why, then, hasn't the technology flooded the world?

"I think up until now the limiting factor has been our inability to engender substantive proof that the technology works, at least in the eyes of some in the meteorological community who remain skeptical," says Bomar, who administers the rain enhancement program for the Texas Natural Resource Conservation Commission.

But over the past few years the prestigious National Center for Atmospheric Research (NCAR) has been compiling evidence in hopes of getting to the bottom of the matter.

Encouraging Results

A team of scientists from the center spent three years seeding clouds in the drought-stricken northern Mexican state of Coahuila to see if they could finally answer the question of whether it works.

The answer seems to be a clear "yes," although support for a fourth and conclusive year of the project was dropped by the Mexican government because the drought came to an end. You might say it was cancelled because of rain.

Here's what the researchers found: Rainfall from seeded clouds lasted longer than rain from unseeded clouds, the rainfall covered a larger area, and total precipitation was higher, sometimes even doubled. And in many cases results began just 20 minutes after the seeding.

"We are very encouraged by the results," says lead scientist Roelof Bruinties of NCAR. The work in Mexico followed on the heels of a research project in South Africa in the early 1990s that reported similar results.

Bruinties is now in the United Arab Emirates conducting a three-month feasibility study to determine whether conditions there are right for a cloud seeding program.

"They have had a drought for the past four years and contacted us to conduct some research here in the possibility of rainfall enhancement via cloud seeding," Bruinties said by e-mail. "The field program started the beginning of January and will last until the end of March."

In addition to providing a scientific basis for evaluating cloud seeding, the NCAR project uses different elements as "seeds" to enhance rain formation in clouds, and that could turn out to be even more important than the scientific documentation.

Timing Is Everything

In nearly all other cases, tiny grains of silver are used as seeds. But the NCAR group is using a mixture of sodium, magnesium and calcium chloride, and it appears this type of seeding works better than silver in drought stricken areas.

Before rain can form in a cloud, tiny specks of dust attract moisture from the surrounding area, growing into raindrops. Cloud seeding, basically, consists of adding additional specks of dust that are very proficient at coalescing with other droplets to form rain.

But it's not as easy as it sounds. Only certain clouds, early in their formation, are useful, and timing is everything.

"We believe the two critical criteria for successful seeding are timing and targeting," says Texas' Bomar. "There's a window of opportunity there that may be only a few minutes long."

If the cloud isn't precisely downwind from the target area, the rain will fall in the wrong place anyway. And if the seeding is done at the wrong time, or on the wrong cloud, it may cause "cratering," or large holes in the cloud that cause it to fall apart. So an errant effort can destroy the clouds that might otherwise produce rain.

The seeds usually are released from an aircraft via pyrotechnic flares as the plane flies through the updraft below the cloud. Texas has 16 such aircraft, scattered around the state. The seeds are carried by the updraft up into the cloud, increasing in size as they gain moisture on the way up.

That also increases the efficiency of the cloud system itself, causing it to collect even more moisture from the atmosphere. The process produces a larger cloud that is slightly taller, thus increasing the convective processes that draw the seeds and the moisture into it.

Clouds Not Included

Over the years, cloud seeding has been haunted by a couple of questions. Some were concerned that adding chemicals to clouds would pollute the earth, but the national Weather Modification Association insists that the amounts are so low as to be insignificant. The amount of silver used in seeding a cloud is less than 0.1 micrograms per liter, about 1/500th the concentration deemed acceptable by the U.S. Public Health Service.

Still others are concerned that producing rainfall in one area will come at the expense of areas farther downwind, the old "robbing Peter to pay Paul" syndrome.

Bomar admits that not enough research has been directed at that, but most experts insist that wringing rain out of a cloud will not cause adverse effects elsewhere.

"We've seen no evidence that anyone is being deprived of the usual or normal amount of rainfall in or near a cloud seeding target area," Bomar says.

That's because the amount of moisture that comes out of a cloud during a rain storm is really only a tiny fraction of the moisture that's available in the atmosphere.

He likens it to someone scooping a coffee pot of water out of a lake.

"You're not going to notice much of a decline in the water level of the lake," he says, because the coffee pot is really only a drop in the bucket.

So the message in all of this is that cloud seeding apparently works, but there's a catch. First you've got to have clouds. Sometimes, during the worse droughts, there aren't any clouds at all.

And no one, so far, has figured out how to make a cloud.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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