The Rundown on Rainmaking

21 April 2011 FDI Team

 

Summary

Australia’s susceptibility to drought has affected its farming capacity in the past and continues to do so now. This is despite the extensive research on weather modification and the development of technology designed to ameliorate water shortages. Artificial rainmaking has the potential to form a significant component of water resource management. Worldwide, scientists have attempted to induce rain artificially using various techniques in order to ease drought conditions. These include laser technology that triggers the formation of water droplets in the air, and cloud seeding, which involves the introduction of chemicals into clouds in an attempt to stimulate the precipitation process. 

 Analysis

An average rain cloud contains eight million tonnes of rainwater, meaning there is a vast quantity of untapped rain in the sky. This is why artificial rainmaking techniques appear to be an obvious solution to water shortages around the world. In 2010, the World Meteorological Organisation reported that 24 countries were participating in a total of 80 cloud seeding projects.

Cloud seeding was discovered in 1946, when scientists realised that dropping particular substances into clouds could trigger rainfall. Typically, cloud seeding is the dispersion of silver iodide aerosols or frozen carbon dioxide (dry ice) throughout the upper part of clouds. 

There are three different types of cloud seeding methods: static, dynamic and hygroscopic. The static and dynamic methods fall into the category of cold cloud seeding, which is typically applied to cumulus congestus clouds. These clouds are produced by strong updrafts and are usually taller than they are wide. The hygroscopic method, referred to as warm cloud seeding, is aimed at convective clouds, which are smaller and stretch several kilometres across.

Cold Cloud Seeding

Static cloud seeding seeks to increase precipitation by scattering silver iodide into the cloud formation. Silver iodide has a crystalline structure similar to that of ice and hence the moisture in the clouds condenses around the molecules and become heavy. These water droplets fall from the sky as rain or, in some cases, snow. While silver iodide acts as an ice-forming agent by providing additional nuclei for water vapour condensation, dry ice lowers the temperature as it evaporates. In turn, this works to increase the amount and rapidity of the ice forming.

Dynamic cloud seeding involves the injection of a much larger amount of silver iodide crystals than in the static method so as to cause glaciation of the cloud. The heat released from the freezing adds buoyancy and enhances the vertical air currents with the aim of yielding increased precipitation. Although more complex than the static mode, dynamic cloud seeding is capable of producing more rain.

Cloud seeding chemicals are dispersed either by light aircraft or by devices on the ground. Targeting with aircraft is more efficient and accurate, but it is also the more expensive option. When released by ground-based generators that burn the granular form of the chemical, the fine particles are carried downwind and upwards by air currents after release. Another means of delivery is that of rocket launchers, which have been widely used by China.

Artillery used to seed clouds in China

Source: asianpacificpost.com

Warm Cloud Seeding

Pyrotechnic flares emitted by aeroplane

Hygroscopic seeding affects warm cloud processes. It involves the use of water absorbing particles, usually sodium chloride, that are spread through the lower parts of clouds. They attract water droplets which induces the coalescence process, producing rain. The salts are usually delivered by pyrotechnic flare technology. It was recently discovered, however, that the optimal size of hygroscopic particles is larger than what can be dispersed by current flare technology.

Source: iceflares.com

The Limitations of Seeding 

There are a number of drawbacks associated with cloud seeding. In most cases, there is no control system in place that can determine whether a cloud would have rained even without being seeded. Also, particular conditions are required for cold cloud seeding to be effective.

The temperature of the clouds must contain a significant amount of super cooled water that is below freezing point, but not frozen, approximately between -10°C and -20°C. The wind must also be below a certain speed so as to not cause dispersion of the cloud. Warm clouds need to maintain an updraft of moist air. The depth of clouds is another significant factor so as to allow sufficient time for the ice particles to grow before falling. Given that the potential for rainfall is strongly dependant on the dynamics of the clouds that are being seeded, cloud seeding is not a cure for drought.

There is also much scepticism in the field. Researchers in Israel recently analysed fifty years’ worth of data and ultimately proclaimed cloud seeding as an ineffective method for increasing precipitation. They argue that cloud seeding is only successful when performed on orographic clouds, which are formed over mountains and short-lived.

Above and beyond Rainmaking

There are other uses for cloud seeding besides rainfall enhancement. It has been used as a way of suppressing hail in order to reduce the damage to crops, and also fog in order to clear the air around airports. It has even been implemented to reduce the intensity of hurricanes, and, as demonstrated by China prior to the 2008 Beijing Olympics, to clear away air pollution and smog. Also worth noting are reports of the Russian Government’s use of the technology to seed clouds containing radioactive particles over Belarus.

Cloud Seeding in Australia

The general characteristics of many areas in Australia, coupled with inconclusive results from several experiments, led CSIRO to abandon cloud seeding as a field for scientific exploration in 1984. It is, however, still being tried and tested by other bodies as means of increasing rainfall and mitigating the impacts of drought. There are currently three major cloud seeding trials underway in Australia, two of which have concentrated on providing additional capacity to hydropower systems.

In 1964, the Hydro-Electricity Commission of Tasmania began trialling cloud seeding as a means of increasing the runoff rates into the mountainous hydro-electric catchment area. From initial experiments conducted, the Bureau of Meteorology estimated a 30 per cent increase in rainfall over approximately 2,500 square kilometres in the autumn months. Due to these positive results, cloud seeding operations have been commissioned in the region ever since.

The New South Wales Government has been operating a cloud seeding project in the Snowy Mountains for the Hydro-Electric Scheme. This particular trial uses land-based aerosol generators that shoot miniscule particles of silver iodide, initiating the formation of ice crystals, which in this case fall as snow, not rain. Late last year, the Natural Resources Campaign concluded that the cloud seeding technology had increased snowfall by 14 per cent. The operation has succeeded in adding a tracing element (indium sesquioxide) which can determine how much snow has accumulated due to the seeding. The trial commenced in 2004 and, although there is ample evidence to suggest a significant increase in snowfall, it is

not yet known whether this has resulted in commensurate run-off into the dams. Thus, the trial has been extended until 2014.

Economic Benefits

Cloud seeding is a costly technology. The first experiments conducted showed that cloud seeding would need to be conducted over a wide area for an extended period of time for it to have any worthwhile economic impact. However, if an area contains clouds suitable for seeding, then investment into the augmentation of rainfall can provide great economic benefits, and potentially greater food and energy security. 

The United States invests approximately US$600,000 each year into cloud seeding which has produced an average annual output of 50 million tonnes of water for drinking and farm water supplies. This equates to a cost of 1.3 cents per tonne of water. The North Dakota Cloud Modification Project has supposedly increased summer rainfall by five to 10 per cent, which yields an increased crop production worth $US8.4 million to $US16 million annually. The hydro-electric project in Tasmania is estimated to be worth about three times the cost of the cloud seeding programme.

Laser Technology

Seeding clouds with lasers has also been investigated as a way of stimulating precipitation on demand.1Short pulses of infrared laser rays sent through humid air into clouds can trigger artificial lightning. The discharge of lightning increases the temperature of the atmosphere to such an extent that the molecular bonds between nitrogen and oxygen are broken. This endothermic (heat-absorbing) reaction creates a plasma containing charged particles that act as condensation nuclei allowing raindrops to grow, not unlike silver iodide crystals in cloud seeding. This scientific occurrence is evidenced by the high incidence of lightning flashes during hail storms.

The lasers can be generated on the ground, at cloud level and also in space. In space, solar energy is used to produce high power intensity laser beams on a solar power satellite or a space station. These laser beams are transferred to a particular ground station and released in the surrounding atmosphere where they act as an agent for artificial lightning.

Some scientists are sceptical of the feasibility of lasers to trigger rain, stating that typical atmospheric conditions do not allow sufficient humidity for it to work. The researching team have reported that it will take several more years to fully develop rainmaking laser technology. 

Other Rainmaking Technologies

Rain can also be made to fall by applying the same science for how rain is naturally created. By collecting solar energy on the ground and using it to heat the surrounding atmosphere, it will produce an ascending air current. This air current takes moisture vapour from seawater

up into the sky, causing condensation to form cloud particles, and water is accordingly precipitated.

A process for the artificial growth of clouds and causing them to condensate has also been proposed. It is believed that this can be achieved through the pulverisation of smaller clouds with water to form larger clouds.

Conclusion

As population and urbanisation exponentially increase and the impacts of climate change become more severe, water resource management is an ever-increasing concern. Artificial rainmaking technologies have come a long way since before the middle of the twentieth century. The objective of current rainmaking methods, particularly cloud seeding, is to make clouds more effective in dispensing the rain that they hold.

Although drought has often been the impetus for the implementation of cloud seeding technology, cloud seeding cannot be relied upon as a short-term response. It does not produce changes in long-term weather patterns or climate, and it is a costly operation. Also, cloud seeding does not work everywhere. Suitable conditions must be verified in order for it to be a cost-effective means of increasing rainfall. 

Artificial rainmaking is an area in need of continual research and operations that have produced positive results are a sign of hope for many. On the other hand, it could take some years before it provides the reliable and consistent results desired by drought-affected agricultural producers. Meanwhile, the evidence to date shows that the technology should not be abandoned. Cloud seeding projects in both Tasmania and the NSW Snowy Mountains indicate successes which, if sustained, have the potential to provide some certainty to rainfall patterns. 

Brooke Jones

Researcher

FDI Global Food and Water Crises Research Programme

1Chang, A., ‘Lasers Bring Home the Rain’, Strategic Weekly Analysis, Vol. 1, № 38, 12 October 2010, Future Directions International: Perth.

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