Tuesday, July 11, 2000

Cloud seeding questions arise
George Flickinger

Does cloud seeding work? If so, how much rain does it make? What problems does it create? Everyone wants to know these answers about cloud seeding, yet it’s hard to answer some of the simplest questions on the issue.

Unlike a recent letter to the editor claimed, I have not commented on the air or taken a stance on cloud seeding. But I will offer my insight on the problems of measuring weather modification changes, the lack of “seedable” clouds and questions that may never be answered about cloud seeding.

Like most people, I’m probably for cloud seeding if it works. Research shows that cloud seeding does increase the rain, but how much?

Proving that dropping chemicals into a cloud actually increases its rainfall is nearly impossible to measure because we don’t know what the cloud would have done without our intervention.

Common sense suggests that if a seeded cloud produces more rain, then the seeding is a success. But rain areas naturally intensify and diminish throughout their life cycle anyway. How do we know we caused it to change?

Our current technology can’t predict if the changes occurred due to man or environment.

Common sense then might suggest if two similar rain clouds develop, one could be seeded and the other left alone, like a “test” and a “control” subject.

But even similar rain clouds can’t be compared because each forms in a slightly different environment. The two storms could live independently or interact with each other. Either way, the storms have different life cycles.

Comparing similar weather patterns on different days is also difficult because the exact dynamics of the atmosphere will not be duplicated again. Weather that seems the same from day to day is always a little different.

Here’s another seeding problem: Most of the clouds in the Big Country that do produce rain are unsuitable or difficult to seed.

Stratiform rain clouds (long clouds that stretch across the sky) could be considered the clouds of choice for seeding because they are not linked to severe weather. But these clouds are rare during warm months and typically produce only light to moderate rain.

The rewards are lower from stratiform clouds, but so are the risks.

Most of our rain falls from convective clouds (puffy, crisp clouds forming in pockets of moist, rising air). These clouds can be difficult to seed because the updrafts and wind shear can be hazardous to pilots, and they can also develop into storms.

For cloud seeding to work, the cloud has to be near the precipitating stage. On many summer days the skies are clear or the clouds fail to produce rain on their own— you can’t seed what’s not there.

Many “what if” questions about cloud seeding will likely go unanswered.

Cloud seeding has been proven to suppress hail development. But what if a rain area is seeded, and the rain’s remnants leave a temporary atmospheric boundary (like a cool front or outflow boundary) for later weather?

The next day, the boundary could be a focal point for new convection that could develop into thunderstorms. If those storms produce damage, who is responsible? Was the severe weather partially initiated by man, or would it have formed anyway?

I have never heard or read of a cloud seeding project that contributed to future severe weather, but it can’t be ruled out.

What if a seeded storm briefly produces more rain, then dissipates quickly or changes course slightly? Some residents could claim their water was stolen.

Another problem: Too much rain can fall from a seeded cloud. I’ve only heard of one case like this: A Sterling County farmer got a half a year’s worth of rain in one day from a seeded storm. The 7-inch rain flooded his property.

Though weather modification is a growing business, many questions are unresolved and may never be answered. The same thing can also be said about the weather.

George Flickinger is chief meteorologist at KTXS-TV.