INTRODUCTION   

Water is one of the most basic commodities on earth sustaining human life. In many regions of the world, however, traditional sources and supplies of ground water, rivers and reservoirs are either inadequate or under threat from ever increasing demands on water from changes in land use and growing populations. In many countries water supplies frequently come under stress from droughts and increased pollution in rivers, resulting in shortages and an increase in the cost of potable water. Ground water tables have been steadily decreasing in many areas around the world where groundwater is one of the primary sources of fresh water. This is particularly evident in the southwest U.S. and Mexico. To help alleviate some of these stresses, cloud seeding for precipitation enhancement has been used as a tool to help to mitigate dwindling water resources.

While many countries conducting weather modification activities are located in semi-arid regions of the world, several countries in the tropics such as Indonesia, Malaysia, India, and Thailand are also involved in weather modification activities. Although these countries receive a relatively large amount of rainfall, a 5% below normal rainfall year translates into a drought for them due to their infrastructure and agricultural practices that are more water intensive than in other parts of the world. Weather modification activities to enhance water supplies have been conducted for a wide variety of users including water resource managers, hydro-electric power companies, and agriculture.
 

  Only a small part of the available moisture in clouds is transformed into precipitation that reaches the surface. This fact has prompted scientists and engineers to explore the possibility of augmenting water supplies by means of cloud seeding. If more water could be transformed into precipitation, the potential benefits appear very attractive.
The ability to influence and modify cloud microstructure in certain simple cloud systems such as fog, thin layer clouds, simple orographic clouds, and small cumulus clouds, has been demonstrated and verified in laboratory, modeling, and observational studies Although past experiments suggest that precipitation from single-cell and multi-cell convective clouds may be increased, decreased, and/or redistributed, the response variability is not fully understood. It appears to be linked to variations in targeting, cloud selection criteria, and assessment methods. The complexity of atmospheric processes and specifically cloud and precipitation development has prevented significant progress in developing a cloud-seeding technology that can be tested and verified in a repeatable manner with the level of proof required by the scientific community.

The fact that many operational programs have been ongoing and have increased in number in the past 10 years indicates the ever increasing need for additional water resources in many parts of the world including the U.S. It also suggests that the level of proof needed by users, water managers, engineers and operators for the application of this technology is generally lower than what is expected in the scientific community. The decision of whether to implement or continue an operational program becomes a matter of cost/benefit risk management and raises the question of what constitutes a successful precipitation enhancement program. This question may be answered differently by scientists, water managers or economists, and will depend on different factors depending on who answers the question. This difference is illustrated by the fact that although scientific cloud seeding experiments have shown mixed results based on the level of proof required by the scientific community, many operational cloud seeding programs are still ongoing. However, it also emphasizes the fact that the potential technology of precipitation enhancement is very closely linked to water resources management. It is thus important that the users of this potential technology are integrated into programs at a very early stage in order to establish the requirements and economic viability of any program. In addition, the continued need for additional water and the fact that most programs currently ongoing in the U.S. and the rest of the world are operational programs emphasizes the need for continued and more intensive scientific studies to further develop the scientific basis for this technology.
 

In the past weather modification activities were often initiated in times of a drought when desperate water needs exist. In many cases, the programs were discontinued when the drought was over. Apart from the question of whether these programs are successful or not, the more relevant question is whether cloud seeding should be initiated during drought conditions at all due to the limited number of clouds available for seeding. A better approach that has been adopted by some operational programs is to view the technology as a longer-term water resources management tool. It may be better to continue seeding during non-drought years in order to build up water supplies for the future.
 

Weather modification research requires the involvement of a large range of expertise due to both the multifaceted nature of the problem and the large range of scales that are addressed. The large- and meso-scale dynamics determining the characteristics of the cloud systems down to the small-scale microphysics determining the nucleation and growth characteristics of water droplets and ice particles all form part of the chain of events of precipitation development. This events are shown in the next figure:
 

Although our knowledge of the individual aspects in the chain has significantly increased in the past twenty years there still exist major gaps about certain physical processes.  Precipitation initiation and development can proceed via several physical paths as shown in the last figure, involving various microphysical processes which proceed simultaneously but at different rates, with one path becoming dominant because of its greater efficiency under given atmospheric conditions. The efficiency with which clouds produce rain at the surface varies greatly. Precipitation efficiency, defined as the ratio of the rate of rain reaching the ground to the flux of water vapor passing through cloud base, can range from zero in non-precipitating clouds to greater than unity for short times, in very intense, time-dependent, convective systems. Some of the earliest studies showed that ordinary thunderstorms transform less than 20% of the in-flux of water vapor into rain on the ground. The principles of most, if not all, precipitation enhancement hypotheses are rooted in these efficiency factors which, in general, seek to improve the effectiveness of the precipitation evolution path. The seeding conceptual model (physical hypothesis) describes how this is accomplished by the seeding intervention, and specifically how the initiation and development of precipitation in seeded clouds differs from that in unseeded clouds and may affect the dynamics of the cloud.
Precipitation formation mechanisms can differ dramatically from one location to another, and at one location, depending on the meteorological setting. Precipitation growth can either take place through coalescence or the ice process or a combination of the two as ahown in the last figure. In clouds with tops warmer than 0oC precipitation can develop by means of the coalescence process. Clouds are further categorized as either continental or maritime, which describes the degree of colloidal instability. However, when cloud tops reach temperatures colder than 0oC, ice develops and precipitation can develop through a different set of paths, as is displayed in the last figure.

The number concentration and size spectrum of cloud droplets can also vary dramatically, depending on the cloud condensation nucleus (CCN) size distribution. A maritime droplet spectra will consist of fewer particles but more large drops than in a continental spectrum
Field experiments conducted in combination with theoretical and numerical modeling efforts based on the development of new instruments and advanced computer systems have shown (Klimowski et al., 1998), and continue to offer the greatest opportunity for providing the understanding necessary to successfully assess precipitation enhancement potential and evaluate such experiments.
 
 

In many cases, convective precipitation development through collision-coalescence can be thought of as a two stage process involving production of large embryos that have the potential to grow into raindrops, and the subsequent growth of this embryos to precipitable sizes. There are many different ways in which large drops can be created or introduced into convective clouds. The existence of the embryos however does not necessarily mean that the cloud will develop precipitation. Some clouds have lifetimes that are too short to permit precipitation development, while others entrain so much dry air that they simply do not have enough liquid water to fuel raindrop production. In order to enhance collision-coalescence and the ice process, cloud seeding techniques have been developed over the years, in which the main feature is the introduction of material that can act as cloud condensation nuclei or ice nuclei. And from there, enhance processes like the collision-coalescence described above, and the ice growth process.