The Hindu : How effective is cloud-seeding?

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Wednesday, Nov 19, 2003

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How effective is cloud-seeding?

By N. Gopal Raj

Although cloud-seeding is several decades old, its effects remain unproven and even controversial.

DURING THE recent south-west monsoon, several States felt it necessary to try cloud-seeding as a way of increasing rainfall. Karnataka, Andhra Pradesh and Maharashtra were concerned about drought conditions, and Tamil Nadu wanted more water for Chennai city. But although cloud-seeding is several decades old, its effects remain unproven and even controversial.

Cloud-seeding can be traced back to experiments carried out in the mid-1940s by a few scientists at the General Electric Laboratory in the United States. Subsequently, as one recent report remarked, "the combination of a few overly enthusiastic scientists, an active press and a receptive populace (especially in drought-prone areas) quickly resulted in a worldwide commercial industry devoted to cloud-seeding, and an era of great interest and concern among governmental and scientific organisations." But the inability to demonstrate beyond doubt that seeding was beneficial led to growing scepticism and commercial projects declined to one-fourth of their peak by the mid-1950s. However, even today more than two dozen countries have operational weather modification programmes, which primarily involve cloud-seeding.

Cloud-seeding involves scattering fine particles in clouds to accelerate natural processes which lead to rain or snow. Broadly, there are two types of cloud-seeding. With "warm clouds", scattering of a chemical such as common salt induces water droplets to coalesce, become bigger and heavier, and thereby fall as rain. When the clouds are "cold", with their tops below freezing temperatures, water can still remain liquid. Dropping fine particles of silver iodide or dry ice can then promote ice formation, which too leads to rain or snowfall. These chemicals are scattered by aircraft or, if the clouds are low enough, using ground generators.

Cloud-seeding can work only if there are suitable rain-bearing clouds in the region. "The energy involved in weather systems is so large that it is impossible to create artificially rainstorms or to alter wind patterns to bring water vapour into a region," pointed out the executive council of the World Meteorological Organisation (WMO) at its meeting a few years ago. The most realistic approach to modifying weather was to take advantage of situations in which a relatively small human-induced disturbance in the system could substantially alter the natural evolution of atmospheric processes. Moreover, as the council noted, the expected effects of seeding are almost always within the range of natural variability and therefore not simple to detect.

"There is still no convincing scientific proof of the efficacy of intentional weather modification efforts," concluded a committee appointed by the U.S. National Research Council (NRC) in a recent report. In some instances there were strong indications of induced changes, but this evidence had not been subjected to tests of significance and reproducibility. The committee blamed poor research funding for the inadequate understanding of atmospheric processes involved in weather modification.

The dilemma in weather modification is that while on the one hand, little funding is available for research, on the other, people are willing to spend money applying unproven technologies, observes Roelof Bruintjes of the National Center for Atmospheric Research, U.S., and a member of the NRC committee. "We know that human activities can affect the weather, and we know that seeding will cause some changes to a cloud. However, we are still unable to translate these induced changes into verifiable changes in rainfall, hail fall, and snowfall on the ground, or to employ methods that produce credible, repeatable changes in precipitation," he said at a workshop in Bangalore in October.

Cloud-seeding as a technique for enhancing rainfall has been examined for a long time and, under ideal conditions, it can lead to increase in rainfall, observes J. Srinivasan, chairman of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science. "But the primary issue that we must be concerned with is whether the higher rainfall that may occur with cloud-seeding is commensurate with the high expenditure that is involved in this method." A large amount of work has been done in this field in Australia and the U.S., but it has not clearly shown that cloud-seeding could be an economic method for harvesting water, he says.

But worldover, users of cloud-seeding technology do not seem to require such rigorous proof and droughts make Governments ready to try desperate measures. "You must remember that this was the third successive year of drought in Karnataka and there was acute distress in the State," points out V.S. Prakash, who heads the Government's Drought Monitoring Cell. Cloud-seeding was not seen as a panacea to drought. But it was felt that if rainfall could be augmented, cloud-seeding would be worth attempting, Mr. Prakash told The Hindu. Accordingly, cloud-seeding was tried State-wide, using aircraft, with the highest priority given to the drought-prone interior districts.

"If a dying man is given a glass of water, how do you cost it," asks Vijay Gore, Karnataka's Development Commissioner. Although interior Karnataka ended the monsoon with a rainfall deficit of over 25 per cent (which classifies it as a drought), he still feels that the cloud-seeding exercise was worthwhile. When suitable clouds were seeded, fairly good rains had been received. Cloud-seeding cost the Government only Rs. 10 crores, a paltry amount compared to the Rs. 1,500 crores spent on drought relief during the last financial year, he points out.

These sentiments are echoed in Maharashtra as well. Although cloud-seeding was started in seven drought-prone districts only from early September, towards the tail-end of the monsoon, the feedback from the ground was quite positive, according to a senior official of the State's Irrigation Department involved in the project. As a result, the Government would be inclined to continue the experiment next year as well, starting cloud-seeding early in the monsoon. Tamil Nadu resorted to cloud-seeding using ground generators in order to try to increase the water levels in the reservoirs providing water to Chennai city.

During the south-west monsoon period, from June to September, there were only about 35 days that were conducive to such seeding, according to an official source. Since the results of the seeding suggested "possible positive effects", the seeding would continue during the north-east monsoon too, he added. Tamil Nadu has tried cloud-seeding several times since the mid-1970s. The last attempt was in 1993. Individual storm clouds can give rainfall increases of 20 per cent or more when seeded, claims Weather Modification Inc., the U.S. company which has been in the business for four decades, and which undertook cloud-seeding in Karnataka, Maharashtra and one district of Andhra Pradesh. Although the rainfall increases over a wide area would be less, many scientific papers suggest increases of upwards of 10 per cent, according to Bruce Boe, the company's chief meteorologist. The company usually uses multiple aircraft for large areas such as Karnataka, but this time it was a bare bones operation. With only one or two aircraft available, opportunities for seeding would have been missed, he told The Hindu.

Cloud-seeding should not be an activity carried out only during a bad monsoon, argues K.T. Sebastian of Agni Aviation Consultants, Indian partners in the seeding operations. "We do not know enough about the structure of clouds, including the cloud condensation nuclei around which droplets form and the droplet size distribution during the monsoon season when India receives most of its rain, or about the dynamics within clouds which leads to rainfall to judge whether cloud-seeding would be useful in the Indian context," says G.S. Bhat of the Indian Institute of Science.

Nor is enough known about aerosols, fine particles in the air which can be natural or the result of human activity, and their variation over the seasons. While aerosols are essential for the formation of water droplets in the cloud and thus for rain, high aerosol levels can produce too many tiny droplets reducing the efficiency with which water in the cloud turns into rain. Carefully designed experiments, involving a good deal of expensive instrumentation, are needed before one can say that cloud-seeding is actually beneficial, he adds.

Dr. Bruintjes too believes that India ought to conduct a "sustained research effort" to understand fundamental scientific questions relating to clouds, precipitation and cloud-seeding. Sustainable use of atmospheric water resources and mitigation of the risks posed by hazardous weather are important goals, including in India where water stresses would be a recurring issue, he points out.

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