Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation
1.1.14 Cloud Seeding
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:
- The air must be cooled, or vapour must be added, until the air
- The air must contain particulate nuclei to enable the phase
transition from vapour to water or ice to occur without great
- 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
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
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.
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).
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.
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.
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
- 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.
There are no expected environmental effects associated with this
technique. The localised disturbances in rainfall patterns may even cause
some limited harm.
There are no cultural problems.
Further Development of the Technology
Better indicators of impact of cloud-seeding need to be developed and
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.
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.