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1.1.14 Cloud Seeding

Technical Description

This technique involves the beneficial modification of convective rainfall to increase rainfall production efficiency. Typically, only about 30% of the atmospheric water vapour entering Southern Africa reaches the ground as precipitation (Mather and Terblance, 1993). This approach encourages efficient raindrop formation through a collision - coalescence process which is enhanced or accelerated by the introduction of hygroscopic nuclei into a storm updraft at cloud base. Silver iodide crystals are most commonly used as nuclei for raindrop formation, and are released (by rocket or aeroplane) into an appropriate cloud formation.

The requirements for the evolution of precipitation from water vapour have been summarised as:

1. The air must be cooled, or vapour must be added, until the air becomes saturated.
   
   
2. The air must contain particulate nuclei to enable the phase transition from vapour to water or ice to occur without great supersaturation.
   
   
3. Some of the water particles formed must grow large enough to fall out of the cloud.

Extent of Use

Rainfall augmentation is extensively used in Zimbabwe, and also in Egypt.

Operation and Maintenance

This is an expensive technology to operate, requiring sophisticated equipment, control and monitoring procedures, and materials, including a cloud-seeding aeroplane, a measurement and monitoring plane, and a communications plane for experimental and monitoring purposes; aircraft maintenance and hanger facilities; meteorological radar and air sounding equipment; a computer system and data analysis software; a rain gauge network and automatic weather stations; and suitable cloud formations. Also, optionally required are structures for increased rainfall storage to make optimal use of the additional rainfall generated through this technology.

Level of Involvement

Personnel with various skills and from different disciplines (e.g., meteorological cloud physicists, radar operators and technicians, computer operators and technicians, and engineering technicians) are required to operate and maintain sophisticated measuring systems like laser imaging probes. Further, implementation of this technology involves a lot of forward planning and requires coordination and application of skills and knowledge developed over several years. Implementation of this technology is mainly by governments because of the costs and skills involved. However, beneficiaries (e.g., forestry, water and agriculture agencies) need to be involved in planning so that design, implementation and measurements associated with this technology (i.e., rain and stream gauging, radar operations, etc.) can be smoothly undertaken, and best use made of the artificial rainfall.

Costs

It is estimated that the cost of water produced is about $1.50/m3/season/ha (United Nations, 1985). This cost is made up of scientific equipment and hardware costs; flying costs for cloud seeding (capital and operational, including maintenance or hire charges); salaries for scientists and pilots; the cost of seeding agents and flares; and, software costs (for experimental and monitoring purposes).

Suitability

Cloud seeding is suitable in regions where water resources are seriously limited, and, particularly, where agriculture is a major commercial activity. Cloud seeding provides additional water to crops during periods when little or no rainfall would otherwise occur. However, cloud seeding can only be practised when weather conditions are favourable.

Advantages

The advantages of cloud seeding include:

• Few adverse environmental impacts
  
  
• Political attractiveness
  
  
• No long-term effects on weather patterns
  
  
• No physical structures to remain if programme is discontinued.

Disadvantages

The disadvantages of cloud seeding include:

• Weather-dependent success (clouds must be present)
  
  
• Lack of trained personnel (the required expertise is not available in many developing countries; adequate training may take several years to develop)
  
  
• Many benefits will accrue to all in the project area, regardless of expectations and desired participation (a lack of control over where the rain will fall)
  
  
• The belief that the rain would have fallen anyway.

Environmental Benefits

There are no expected environmental effects associated with this technique. The localised disturbances in rainfall patterns may even cause some limited harm.

Cultural Acceptability

There are no cultural problems.

Further Development of the Technology

Better indicators of impact of cloud-seeding need to be developed and implemented.

Information Sources

Contacts

G.K. Mather and D.E. Terblance, National Precipitation Research Programme, Water Research Commission, Private Bag 824, Pretoria, South Africa, Tel. +27 12 705 925, Fax +27 12 705 925

Ministry of Transport and National Supplies, Department of Meteorological Services, Post Office Box BE150, Belvedere, Zimbabwe, Reference File - National Cloud Seeding Season 1992/93.

Bibliography

Knapp, H.V., A. Durgunnogln, and S.A. Changnon 1988. Effects of Added Summer Rainfall on the Hydrological Cycle of Midwestern Watersheds. The Journal of Weather Modification, 20(1).

McNaughton, D.L. 1980. Cloud Seeding in Zimbabwe and Some of its Effects on SR52 Maize Growth. PhD Dissertation, Faculty of Agriculture, University of Zimbabwe.

National Academy of Sciences. s.d. Weather and Climate Modification Problems and Prospects. Vol. II Research and Development, National Research Council, Washington, DC.

United Nations 1985. Non-conventional Water Resources Use in Developing Countries. Proceedings of the Interregional Seminar, Willemstad, Curacao, Netherlands, Antilles: 22-28 April 1985. United Nations Natural Resources/ Water Series No. 22, New York.

WMO (World Meteorological Organisation) 1979. Register of National Weather Modification Projects. WMO, Geneva.

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