With water increasingly scarce in its parched and heavily populated northeastern plain, China has become the world's leading rainmaker, using aircraft, rockets and even antiaircraft guns to seed the clouds for precious moisture. The hunt for rain has become so intense that rival regions sometimes compete for clouds sailing across the sky.

Provincial, county and municipal governments in 23 of the country's 34 provinces have set up what they call weather modification bureaus assigned to regularly bombard the heavens with chemicals in hopes of squeezing out more rainfall for demanding farmers and thirsty city dwellers among China's 1.3 billion residents.

The heavy cloud seeding is a dramatic example of China's increasing difficulty in finding enough natural resources -- from aluminum to oil and rubber -- as its economy expands rapidly and its huge population consumes more goods. Imports of commodities from neighboring countries have shot up in recent years, for instance, and government oil company officials have traveled to Sudan and other distant lands in search of more fuel.

Hu Zhijin of the Weather Modification Research Center at the Chinese Academy of Meteorological Science said official statistics show that 30 modified aircraft, 6,900 antiaircraft guns and 3,800 rocket launchers, some mounted on trucks, were used repeatedly in attempts to change the weather across China's driest areas in 2003. The effort is just as sustained this year, in response to drought conditions across a wide swath of the country, he said.

The result, he added, is that rainmakers across China have accumulated more hands-on practice than their counterparts elsewhere, wringing water from the clouds for season after season in intense government-sponsored programs. "They have been doing it for a long time," he said. "They do it every year. They have more experience, and they invest more funds in it."

Ban Xianxiu, director of Liaoning province's Weather Modification Bureau, was at ease in a recent conversation discussing the fine points of rockets vs. antiaircraft guns for coaxing rain out of the clouds, which he and his staff do for a living. The guns do better with small, fat clouds, he said, while rockets can spread chemicals over a wider area. Planes do so over a wider area still, if the clouds blow in as flat layers.

The Liaoning bureau, 350 miles northeast of Beijing, has salted the clouds about twice a month so far this year, he said during a visit to the capital. But for all their skill, he cautioned, China's rainmakers can work their magic only if the weather cooperates, by sending damp clouds near drought-affected areas.

"We can only modify the weather," Ban said. "We can't create it."

The resort to artificial rain has become so frequent in the drought-plagued northeast, experts and local officials reported, that some cities and counties have quarreled over gray clouds that drift in with the promise of moisture while rainmakers stand poised below with their weapons at the ready.

Five major regions of Henan province, about 400 miles south of Beijing, all seeded clouds around the same time last month during a bad dry spell. But one of them, Pingdingshan, got most of the rain that finally fell July 10, mostly because it was in the path of prevailing winds. Pingdingshan measured more than four inches of rain, local meteorologists reported, while the nearby city of Zhoukou recorded only a little over an inch.

Foul, cried Zhoukou's meteorological officials. They said the Pingdingshan Weather Modification Office repeatedly seeded clouds that, if nature had been allowed to follow its course, would have scudded along to other places -- such as Zhoukou -- before delivering their rainfall.

Nonsense, replied the Pingdingshan office. "We didn't grab the clouds away from other cities," declared the office director, who gave his name only as Wang.

"What we are doing is quite a scientific thing. And we reported our cloud-seeding schedule to the provincial government. I believe other cities also did so," Wang said in a telephone interview. "The water vapor resource is not like water resources in a river, which could be intercepted from points upstream. Or it is not like a cake -- if I have a bite, others get only a smaller piece. Besides, clouds change while floating in the sky, so it is quite complicated."

As do their counterparts elsewhere, Chinese rainmakers spread silver iodide or liquid nitrogen in moist clouds to produce ice crystals, which turn into rain as they fall to warmer air below. The chemical products can be sprayed from a plane or shot into the air with rockets -- sometimes 100 at a time -- or fired up by the antiaircraft guns using special shells roughly similar to fireworks, Hu explained.

Such science is widely known, he said, and was applied in the United States and elsewhere as early as the 1950s. But because of its severe weather problems, China has put the know-how to practical use more often than other countries in recent decades. So far, he added, no signs have emerged that frequent cloud seeding harms the environment, but scientists are keeping an eye out.

As the practice spread, the Chinese central government in March 2002 handed down a directive regulating weather modification. Mainly, it mandated cooperation and information-sharing by provinces, counties and cities, and barred cloud seeding by unofficial groups.

The director of the Weather Modification Bureau in Jilin province, north of Beijing, said his staff files an annual cloud-seeding plan in line with the directive, but that the clouds' mobility and the uneven distribution of water vapor within them make the enterprise unpredictable at best. Nevertheless, he said, his office frequently is in touch with neighboring provinces on cloud-seeding plans in the area, along the North Korean border.

"Sometimes we even have combined action with them," said the official, who insisted on being identified only as Zheng. "Although the effect would be great if several provinces work together when there is a huge cloud system in the sky, in reality, we seldom do that."

Nanjing and five other cities in Jiangsu province, 550 miles southeast of Beijing, organized joint cloud seeding on July 25, producing a much-needed rain during a hot spell, and another round has been planned for this week. The operation was designed mainly to save electricity, which also is scarce, by bringing down temperatures and easing pressure from air conditioner use.

"The purpose was to cool down the cities," Bai Kawa, director of the Jiangsu provincial Weather Modification Office, said in a telephone conversation. "The hot weather has been going on for days. We launched rockets to seed the clouds. The result was satisfying."

Hu said his years of research have showed that even the best efforts of China's rainmakers produce only a 10 percent or 15 percent increase in rainfall. In addition, he said, the vagaries of nature, such as wind direction and velocity, mean the effect of cloud seeding on any given locality is difficult to predict.

Yiwu City, south of Shanghai in Zhejiang province bordering the South China Sea, has been forced to cut electricity and water supplies to sweltering residents every other day this summer, city officials said. Seeking to alleviate the situation, its meteorologists tried to seed the clouds 10 times during May and June, with mixed results.

An official with the weather bureau said the meteorologists succeeded in bringing down one heavy rain. But in a dry July, as temperatures have risen even higher, the sky has been uniformly blue, making further cloud seeding impossible, said the official, who declined to be identified.

The shortages also have produced tugging among regions and cities on water resources other than rain. Beijing and Tianjin, a large port city 80 miles to the southwest, are competing for water from the Juhe River, the China Youth Daily reported last week. Meanwhile, the central government has begun work on a huge canal designed to push water from southern rivers to the parched north -- a plan that has not made everybody in the south happy.

Researcher Zhang Jing contributed to this report.

In China's northeast province of Liaoning, recurrent drought can turn the wheat crop brittle. The Weather Modification Bureau there has salted clouds about twice a month this year.