To draw rain from the heavens, ancient Chinese once used to offer their best in sacrifice. Apart from oxen and sheep, legend has it that beautiful virgin girls were also thrown into rivers to please the water dragon, who would then show mercy and release the rain.

Today, Chinese meteorologists believe that cloud seeding, either by using artillery to fire shells containing rain-inducing chemicals into the cloud cover or by dropping the chemicals from aircraft, is a more scientifically effective approach.

Cloud seeding has been often used to ease the drought situation in North China.

Despite the diversity of opinions on the feasibility of the technology, primarily because of the difficulties in assessing its results, the prevailing opinion is that it has reached a relatively advanced stage of application, and it can be considered one of the technologies capable of contributing to the augmentation of freshwater supplies in arid regions.

In a bid to address Beijing's growing water shortages, the city has decided to take every possible measure to increase water supplies, including precipitation inducement technology, Zhang Qiang, an official with the Beijing Office of Weather Modification, told China Daily.

Zhang said the city uses specially modified Military Aviation Xia Yan IIIA and An-26 aircraft for this purpose.

In the first half of the year, Beijing meteorologists carried out several cloud seeding operations. Aircraft, rockets, artillery shells, meteorological balloons and mountain-top-based devices have been employed to scatter silver iodide particles into gathering clouds to induce precipitation in the form of rain or snow over the city.

According to statistics, precipitation last year was increased by 25 per cent.

"Although it may be an exaggeration to attribute all of the increase to cloud seeding, we believe it has worked," said Zhang.

In order to make more rain and expand water reserves for the Chinese capital, experts and rain-making workers have been stationed upstream of the Miyun and Guanting reservoirs this summer. "Cloud seeding shells will continue to be fired into the skies above Beijing in the hope of breaking a prolonged dry spell," Zhang said.

Nationwide application

Cloud seeding has been widely used in other parts of China as well.

According to statistics from the China Meteorological Administration (CMA), from January to the end of June, China used 227 aircraft putting in 529.8 hours of flight time in cloud seeding operations in 15 provinces and regions, covering an area of about 1.66 million square metres, and scattered chemicals in clouds using 12,464 rockets and 66,000 large-calibre artillery shells in 30 provinces and cities, producing 10 billion cubic metres of water.

"I believe that nearly every time it has rained in Beijing or northern China this spring, cloud seeding technologies have been used," said Hu Zhijin, an expert with the Institute of Artificial Rain under the China Meteorological Administration.

According to a report by Qin Dahe, the top official of the China Meteorological Administration, from 1995 to 2003, 23 provinces around the country conducted precipitation enhancement (PE) operations using 4,231 flights of aircraft with a total flying time of 9,881 hours; and many provinces employed artillery and rockets for their PE work. The total area of operation covered more than 3 millions square kilometres and precipitation was about 2,100 billion cubic metres.

In 2003 alone, more than 3,800 rocket launchers, about 7,000 antiaircraft guns and many airplanes were used for various weather modification operations to enhance precipitation in more than 1,800 counties throughout China. More than 35,000 people were involved.

Last year, 413 million yuan (US$49 million) was injected into the programme.

Addressing water shortages

At the national level, the Second National Co-ordination Meeting on Weather Modification was held on February 27, 2004.

The meetings are held to organize, coordinate and guide weather modification activities across the country. Many ministries and committees took part in this meeting, held at the China Meteorological Administration.

"The widespread use of this technology in northern China reflects the seriousness of the water shortage problem in this part of the country," Hu said.

The average annual rainfall in China's arid north is only 65 per cent of the world average, while per capita water resources in the region are only about 40 per cent of the world average.

One study reveals that northern China is hit by drought in 28 out of every 30 years, 11 of which see drought of severe proportions.

"The growth in population and the development of the economy have led to a high increase in demand for water. This, combined with severe drought, has led to water shortages, a fall in ground water levels and the frequent drying up of rivers, including the Yellow River," Hu said.

Meteorological officials said cloud seeding efforts to enhance rainfall are being used in drought-stricken northern China every time clouds amass.

Years of drought in North China have even sparked reports that some towns were complaining of other towns stealing their rain by seeding clouds as they passed.

"So co-operation in such cases is essential, and may require the issuance of legislation to regulate the legal aspects of cloud seeding," Hu said.

According to the CMA, China has already issued regulations on clouding seeding to promote co-operation between provinces and regions.

But up to now, only Beijing and Hebei Province have co-operated in carrying out cloud-seeding operations financed by the Beijing municipal government, which they have done twice since last year, according to a source at the Beijing Office of Weather Modification.

Expensive or not?

The seeding process requires the use of a weather radar system and an aircraft equipped with instruments for meteorological recording and seeding operations or artillery to fire the silver iodide shells. Qualified radar technicians, engineers, meteorologists and pilots are also necessary.

Cloud-seeding technology is generally considered an expensive process, so it can only be continued as a regular practice if it is efficient and effective.

However, Hu says that "cloud seeding is cheaper than other methods that the government is implementing to solve the water shortage problem, such as the South-North Water Diversion project that will carry water from the Yangtze River basin to Beijing and the North."

"In one dry season only 2-3 million yuan (US$241,800 to US$362,760) was needed to carry out the cloud seeding programmes," Hu said.

In short, one cubic metre of man-made rain costs 0.2 yuan, according to statistics provided by Zhang Qiang.

'Immature' technology

Because cloud seeding is an "immature" technology, as Hu says, it has also generated a certain amount of debate.

Some people are afraid that large-scale seeding operations might have produced and continue to produce environmental problems.

Hu reassured China Daily that so far, there is no evidence to prove that chemical cloud seeding causes environmental pollution.

"Because silver iodide is very effective, 1 gram of it can produce 100,000 billion ice crystals over a very wide area, so the quantity of silver iodine used is relatively tiny," Hu said. And nitrogen and calcium chloride, contained in cloud seeding materials, exist naturally in the air.

Materials used for cloud seeding include silver iodide (in the form of pyrotechnic), azotic cooling liquid, dry ice and propane-colourless gas.

Scattering the material from above the cloud produces better results than doing it from below. Seeding is typically done using either aircraft or ground generators, with the goal of facilitating optimal distribution of the seeding material among the cloud components containing the largest portion of supercooled water.

Hu also said that evaluation of the cloud-seeding effect is very difficult, especially for the operations aimed at mitigating drought disasters.

But the expert added "the main problem is that because rainfall is so variable from day to day and from place to place, it's incredibly difficult to tell if you've achieved anything or not."

In order to try and settle the matter, scientists from the Chinese geological department are now studying soil sediments from the operation area for traces of silver iodide.

"At present, cloud-seeding technology is still in its developing stage. Many scientific-technical problems have not been completely resolved. However, we do have the scientific tools and techniques to make any necessary advancements," Hu said.

Other applications

Chinese scientists are also researching how to expand the technologies to decrease high-temperatures in cities, disperse fog, suppress hail from ruining crops and reduce frost and air pollution.

"Man-made efforts to influence the weather are an important way of using modern science and technology to reduce or prevent disasters, and are already receiving a high level of attention in our country," Qin said during a national conference in Beijing.

"Cloud seeding or artificial rain-making, hail prevention and fog-dispersal techniques will also be used to help improve ecosystem and control forest fires, secure freeway transportation and the success of key social events like the Beijing 2008 Olympic Games and the 2010 Shanghai World Expo," Qin said in the meeting.

(China Daily July 26, 2004)