Weather modification: let it rain!
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CTA. 2006. Weather modification: let it rain!. Spore 122. CTA, Wageningen, The Netherlands.
Permanent link to this item: http://hdl.handle.net/10568/48046
Internet URL: http://spore.cta.int/images/stories/pdf/old/spore122.pdf
In many ACP countries with growing populations, water scarcity is a major obstacle to agricultural development...
In many ACP countries with growing populations, water scarcity is a major obstacle to agricultural development. For decades, scientists have grappled with the challenge of producing rain by artificial means, and new technology has brought them closer to their goal. But does it really work? By the year 2025, two-thirds of the world s population will have to cope with water stresses, according to the United Nations Environment Programme (UNEP). Increasing demands for fresh water due to rapid population growth, coupled with recent periods of severe drought, have focused the spotlight on Africa. Some 66% of the continent is now classed as desert or drylands and 3% of agricultural GDP is lost annually to soil and nutrient loss in sub-Saharan Africa. But the picture in parts of the Caribbean and Pacific is hardly more encouraging. In the International Year of Deserts and Desertification, the challenge of developing new water sources for large areas of the South has become a matter of great urgency. Some scientists believe that modern technology may have the answer. What is needed, they say, is to make more rain. In order to coax precious water out of the skies, experts have developed techniques which involve showering clouds with chemical particles, releasing rain to irrigate crops or replenish aquifers. Interest in rainmaking was spurred by the Sahelian drought crisis of 1968-1974, but weather modification to give it its official name has been around for some time. Nearly all modern attempts to modify the weather involve cloud seeding, using rockets, planes, cannons or ground generators to fire particles into clouds to encourage water vapour to gather round them and make them fall as rain. Cloud seeding dates back to the late 1940s when two US scientists accidentally discovered that supercooled water droplets could be converted to ice crystals using either dry ice or silver iodide. Trials in the atmosphere soon followed, and operational cloud seeding programmes began in about 1950. In more recent times, improvements in observational facilities and modelling capabilities have allowed more detailed examination of precipitation processes and offered new opportunities. The development of new equipment such as aircraft platforms with microphysical and air-motion measuring systems, radar, satellites and automated rain gauge networks has brought rainmaking closer to reality. Too good to be true? According to the World Meteorological Organization (WMO), cloud seeding projects are now being conducted in more than 40 countries, including Australia, China, India, Russia and the US. In the ACP region, South Africa leads the field, with Burkina Faso emerging as a more recent player. Senegal has started on a rain enhancement programme and in Zimbabwe, cloud seeding has been regularly used to help agriculture during inadequate natural rainfall periods. How it works Cloud seeding works on the principle of stimulating clouds with substances to release rain. Cold clouds and warm clouds require different seeding methods. Cold cloud seeding, also known as glaciogenic seeding, involves firing silver iodide into clouds to trigger the formation of ice crystals. Warm cloud seeding, or hygroscopic seeding, was developed by South African scientists, who experimented with firing salt crystals using flares strapped to an aircraft wing. 1. Aircraft or rockets spray substances (e.g. silver iodide or dry ice) into clouds to encourage tiny vapour droplets to coalesce. 2. These droplets of supercooled water (liquid below freezing) coalesce into snow and melt as they fall. 3. Heat released as the droplets freeze boosts updrafts, which pull more moist air into the cloud. On the face of it, the advantages of cloud seeding are clear. According to the WMO, analyses have shown that the technology can produce significant economic benefits. A South African team which has conducted a long-term study of cloud seeding claims the technique can increase the mass of water droplets in a cloud by up to 60%. The team carried out experimental seeding programmes in the drought-stricken Limpopo province and a cost-benefit analysis concluded that the benefits of cloud seeding outweighed the costs by a ratio of 1:7, with sizeable increases in rain-fed agricultural yields, such as maize, pasture and timber. But in spite of its apparent potential, cloud seeding remains a controversial practice, and in some scientific circles, there is scepticism that such methods can offer a viable means of increasing water resources. Many experts agree that fundamental questions need to be answered to provide a sound scientific basis for the technology. The Kenya government attempted cloud seeding back in the 1960s, but abandoned it after achieving disappointing results. One factor hampering widespread use of cloud seeding in the South is the high start-up cost. The technology requires sophisticated equipment, monitoring procedures and materials, including at least three airplanes for each operation, as well as meteorological radar and air sounding equipment a computer system and data analysis software, a rain gauge network and automatic weather stations. In Zimbabwe, where aircraft were mainly supplied by the farming community, a major problem has been a lack of sufficient planes. No clouds, no rain But in order to make rain, you also need suitable cloud formations. Cloud seeding cannot be carried out on just any old cloud. And most areas that are short of rain are also short of clouds. No clouds, no seeding, and therefore no rain. According to Italian agrometeorologist Pasquale Steduto, in order to have rain, you need a cloud in an incipient condition. This technique has not taken off because in arid and semi-arid conditions clouds are lacking, and therefore the seeding does not help. Seeding the skies in South Africa Among ACP countries, South Africa has played a key role in developing operational techniques for cloud seeding, and has produced some of the most encouraging results. The National Precipitation Research Programme (NPRP), formed in the 1990s, developed the hygroscopic flare for seeding convective storms, which suck in humid air from close to the Earth s surface. It also developed radar technology for cloud seeding, now being used in more than 20 projects in six other countries. Experiments conducted between 1991 and 1997 found that seeded storms translated into an average increase of between 20% and 48% in the average annual runoff in 13 different catchments over the eastern Highveld and escarpment. The model also showed average increases of 22% in timber yields due to cloud seeding. Now known as the South African Rainfall Enhancement Programme (SAREP), the South African team is setting up a network of weather radars to provide the final link in the chain of proof the evidence that rain from seeded clouds actually falls on the ground where it is needed. Cloud seeding also requires a considerable degree of fine tuning. The idea is to increase the number of water droplets in a cloud, but quantity alone is not enough only clouds with droplets of varying sizes will actually produce rain. And precipitation mechanisms can differ dramatically from one location to another. A report from the May 2005 meeting of the WMO s working group on weather modification warned: Cloud structure can vary widely from region to region. Seeding results obtained in one geographic area cannot be automatically assumed to apply to another area. One of the main drawbacks of the technique is lack of control over where the rain will fall, a fundamental consideration, especially for small island states. What is the point of generating extra rain if it is going to fall into the sea? Scientist Roelof Bruintjes of the National Center for Atmospheric Research (NCAR) says Increasing the rainfall is just one aspect. We also have to consider what the impact would be. Every cloud has a silver lining The government of Burkina Faso has estimated that cash earnings from agriculture have increased by 10 to 15% since it began its cloud seeding programme in 1998. The additional rainfall created has helped to fill reservoirs, allowing irrigated cultivation to continue during the dry season. As a result, imports, particularly of cereals, have dropped and in some parts of the country fresh tomatoes are now available all year round. With technical assistance from the National Center for Atmospheric Research (NCAR), Burkina Faso has set up software systems to support cloud seeding efforts. The software is used to display and analyse radar data about cloud systems and precipitation, thereby guiding cloud seeding operations and helping scientists evaluate the results. Burkina Faso s cloud seeding programme has proved so successful, that the Inter State Committee Against Drought in the Sahel (CILSS) is hoping to find backing for a US$60 million ( 50 million) programme to extend cloud seeding to its other eight member countries. The US-based NCAR has been in the forefront of weather modification research in Mexico and South Africa. The centre has refined a technique to increase the size of particles in clouds and promote the coalescence of water droplets. Called hygroscopic seeding, this technique uses flares mounted on aircraft to seed clouds with small salt particles which, however, do not affect the quality of the resulting rainwater. In the air, water droplets bond to the particles and grow large enough to fall out of the cloud as rain. More research Evaluation of results remains a major challenge. Scientists need to be able to provide proof of cause and effect that it really is cloud seeding which has produced a specific downfall of rain. Some have their doubts, among them Rene Gommes, Senior Agrometeorology Officer at FAO. The problem is, no one knows whether it really works or not. Unless you carry out extensive testing in exactly the same place for, say 20 years, you will never be able to say that it would not have rained anyway. While giving a cautious welcome to weather modification initiatives, the WMO wants to see far more research and scientific evaluation. It is crucial to recognise that the current technology is immature , it warns. It also calls for greater cooperation between countries to spread the burden of funding costly technologies. Economic, social and legal aspects must also be taken into account, and international guidelines will be needed, especially when cloud seeding operations are taking place close to borders. If a cloud is seeded in one country, the rain may fall in another. There is still unresolved debate over whether producing rainfall by artificial means in one region will rob another of its natural share. The practice recently caused friction between neighbouring states in China, and, on one occasion, prompted drought-ridden Mali to accuse Niger of stealing its rain.
- CTA Spore (English)