Scientists Seed Clouds to Make Rain

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 ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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