CLOUD SEEDING - FREQUENTLY ASKED QUESTIONS-4


Prof.T.Shivaji Rao,

Director, Centre for Environmental Studies,

Gitam University, Visakhapatnam-530 045

After the cloud seeding experiments were conducted by Vincent Schaefer near New York in November 1946, many arguments for and against cloud seeding have been advanced by the scientists, social workers and the general public. While some of them are in favour of cloud seeding operations many are strongly against them. However the scientists and technical experts working in the companies associated with cloud seeding operations are arguing that the operations will be highly successful if only they are conducted on scientific lines, the other scientists from the universities and research organizations, government officials and some politicians have been arguing that the cloud seeding operations are not that successful and hence public money should not be misused for the purpose. Moreover they emphasize that the environmental assets will be adversely affected in the long run. Further when cloud seeding experiments are conducted in one region or state the other places downwind will not get their share of normal rainfall. They argue that the eco-systems in nature will be adversely affected and nature’s balances will be upset in course of time. Under these adverse conditions cloud seeding operations cannot make a success unless public sympathy and cooperation is obtained. Hence it is necessary for the proponent of cloud seeding operations to analyse the various arguments and doubts raised by the scientists and other people and provide realistic information to clarify the doubts for enlisting their cooperation for the success of the cloud seeding operations. Hence some of these problems are highlighted in the following pages and the necessary scientific information is also furnished to clarify the misgivings on the subject.

Can the clouds belonging to one area be stolen by the others?

There is a feeling among the people that if the cloud seeding operations are conducted in one region or state the other downwind regions or states will be deprived of their share of normal rainfall. This apprehension is not correct because it is not based on true facts. In a

typical precipitation cap-cloud ascending over a mountain, if it is assumed that the ascending water vapour mass is cloud-free, about 20% of the water vapour in the ascending air mass condenses and forms a cloud. Out of this about 20% comes down as rainfall from the cloud. Hence only 4% of the atmospheric water vapour amounting to 100(0.20 x 0.20) = 4% is thus removed. If it is assumed that the cloud seeding operations increase the rainfall by 20% then the additional water vapour removed from the ascending cloud vapour in the sky works out to 100 (0.20 x 0.04) =0.8% and this is a relatively insignificant amount and hence only a trivial reduction in the total atmospheric reservoir of water vapour would occur in the region downwind of a target area. According to scientists of South Africa about 6 million cubic meters of moisture passes over the country every day in its sky out of which 5% gets precipitated as rainfall.  Out of this rainfall 60% is returning to the sky through evaporation from the rivers, lakes and plants. It means that 40% of the rain amounting to 2% of the atmospheric moisture (1,20,000 m3) is left in South Africa. But cloud seeding operations produce about 25% additional rainfall. Out of this additional rainfall only 40% remains over land while 60% again returns to the atmosphere due to evaporation. Thus cloud seeding operations are capable of squeezing only a very small fraction of the atmospheric moisture flow in the skies. This inconsequential small fraction of the atmospheric moisture obtained through cloud seeding operations cannot cause adverse impacts on the normal rainfall anticipated in the areas downwind of the target area selected for the operations.

Clarifying doubts if cloud seeding implies the syndrome of “Robbing Peter to pay Paul”,Bomar of Texas,  under the web site: admits thus

 http://abcnews.go.com/print?id=98859

 No evidence  is found that anyone is being deprived of the  normal amount of rainfall in downwind areas due to cloud seeding in an upwind target area.It must be understood that the amount of moisture that comes out of a cloud during a rain storm is really only a tiny fraction of the moisture that's available in the atmosphere. Experts  compare the phenomena to someone scooping a coffee pot of water out of a big lake. No body can notice much of a decline in the water level of the  big lake, because the coffee pot is really some thing like a small drop in the bucket"

This comparison should be more like someone is drawing a glass of water from a massive river of moisture that continuously flows in the sky from the upwind to the downwind side of an area as influenced by the constant additions and deletions of moisture based on local weather conditions and topographical features like forests,hills,lakes and arid areas.

According to  Australian experts[see website: 

http://svc097.wic010v.server-web.com/files/SPET_QA.pdf

 A common misconception regarding cloud seeding is to consider the atmosphere a static pool of cloud water passing over the earth, which is a limited steady state supply of water. With this conceptual model, it very easy to argue that because this supply is limited and we remove a percentage of the water in the form of precipitation from the atmosphere through cloud seeding in one area, there will be less available to fall atother (downwind) locations because a larger fraction of this fixed supply of water was removed in another (upwind) location.Fortunately, the atmosphere does not behave in this simplistic manner. Clouds are systems that continuously process moist air. They are created when tiny water droplets form when cooling rising air ascends.

Precipitation data from a number of cloud seeding projects in the USA have been examined in detail for evidence of extra area effects. There are no statistically significant indications of rainfall/snowfall decreases downwind from any long term cloud seeding projects.

Depending upon its size the life of a cloud may be 30 minutes to 60 minutes. If we are not able to use the cloud in proper time for seeding operations and squeeze its water content in the form of precipitation this cloud may dissipate as moisture. Hence there is no guarantee that this cloud will remain in tact for such a long time beyond its life time to give rain in another region downwind of the target area. Based upon local environmental conditions like meteorology and topography the cloud downwind of a mountain may descend and get dissipated without giving rain in the rain shadow area. However if the area downwind contains good forests or mountains the moisture gets replenished again and the newly formed cloud may provide more rainfall to the areas down wind of the target areas.

Will the chemicals used for cloud seeding cause damage to public health and environment?

Chemicals like Sodium Chloride, Calcium Chloride, Silver Iodide and dry ice are used as nuclei to seed the warm clouds and cold clouds. It is necessary to know what will be their short term and long term effects on the environment. Estimates made in USA indicate that due to the use of silver iodide for cloud seeding operations in 1977 the amount of silver iodide worked out to 1500kgs.  According to EPA reports of 1973 about 360mg of silver enters into the atmosphere over USA. In the rain water samples obtained by cloud seeding operations silver concentration is estimated at 10-12 that is one part per billion (ppb). Since AgI is not soluble in water it does not readily get into the sea but tends to be deposited in the soil and the bottom of stream beds. The research investigations conducted up to 1977 indicate that this chemical has not posed any threat to the ecological systems. Silver has been detected at 10 nano-grams to less than 0.1 micro grams per litre in the rain water samples collected from the cloud seeding operations.The US authorities specified drinking water standards with the safe concentration of silver at 50 micrograms per litre. The values of silver in the precipitation samples in the Montana project was 1/50 to 1/500 of the limiting values set by the US public health service for silver in drinking water.

Similarly the iodine concentration in the rain water samples collected at the end of cloud seeding operation is found to be far lower in concentration than that found in the common iodized table salt used by the people.

During the experiments conducted for 11 years at Baramati, warm cloud seeding operations were done by sprinkling common salt powder into the clouds from the aeroplanes. The analysis of the rain water samples collected after the experiments indicated the concentration of chloride at 10mg/litre while sodium was present at 4.5 mg/litre. Hence these chemicals are not at all harmful to the public health and the environment.

Scientific investigations indicated that even when the chemicals from the silver contaminated water samples entered into the soil and then into the green vegetation and from there into the insects, birds and animals, it was found that there were not adverse effects in the food chains and food webs of the ecological systems. Moreover the ecological systems in nature are subject to dynamic changes and yet remain in bio-dynamical equilibrium. When compared with the changes that occur in nature, the chemicals used in cloud seeding operations do not cause any changes that upset the ecological balances in nature.

Moreover some states like New South Wales are conducting cloud seeding operation experiments to fight the adverse impacts anticipated due to the global warming. Because of the adverse effects of El Nino several states in South India have been facing drought conditions and the conditions have become so serious that several farmers have been resorting to suicides due to failure of crops. This critical situation calls for remedial action by providing ample supplies of water for drinking, food production, hydro-power generation and environmental improvement. Since the river courses, tanks and lake beds have virtually dried up, the only course available to the Government is to make use of the sky water in the form of clouds by resorting to cloud seeding operations on an extensive scale.

 Social problems :

Most of the cloud seeding operations have been started with the fond hope of obtaining many economic and social benefits. For instance fog dissipation by cloud seeding is done at several airports to minimize the economic losses to the airline companies and the traveling public, because the interruption to flight schedules causes great inconvenience to the businessmen who travel by air. Since the impacts of these operations are highly localized, there is not a serious debate on the employment of cloud seeding for dissipation of fog in the air ports and on railway tracks and major roads in metropolitan cities like New Delhi. Economic studies made so far demonstrate that augmentation of annual rainfall provides the greatest economic benefits. Additional water from cloud seeding has great economic value for agriculture, hydropower generation and production of timber and pulp wood, low- flow augmentation for restoring water quality in river courses subjected to domestic and industrial pollution and for sound maintenance of forests, wildlife and eco-systems. Additional rainfall obtained by cloud seeding has a great direct impact upon crops, ground water and environment in arid and semi-arid regions. Irrigation and other water management systems are perhaps the ideal areas for the application of cloud seeding operations.

The seeding of monsoon clouds that pass over the forests and mountains result in precipitation augmentation whose effects can be predicted with great accuracy. In some places the additional water stored in the reservoirs is passed through percolation beds into suitable underground aquifers where the water is not only virtually immune to evaporation losses but also can be subsequently recovered and reused by pumping. Some hydro-power generating firms have found it highly economical to conduct orographic cloud seeding operations only for the sake of producing additional electricity at a very inexpensive cost. Moreover such water used for hydro- power generation in the Snowy mountains of Australia is immediately made available for development of irrigation.

Computer programmes may be run to determine the timings for the commencement of dry crop operations during different seasons and also the periods during which cloud seeding operations have to be planned to obtain specified amounts of rainfall to maintain adequate soil moisture to maximize the crop yields in dry farming areas of Rayalaseema and Telangana districts of Andhra Pradesh and other drought prone areas in different parts of the country. Hail suppression by cloud seeding drastically reduces the costs of damage caused to human and animal population, agriculture crops and other properties.

Social Aspects :

Even where cloud seeding operations confer substantial economic benefits and where there are no over riding ecological and environmental disruptions, the attitudes of people to implement this new technology vary greatly. Among the people perhaps about 10% are innovators who are very receptive to almost any proposed technological changes which in their opinion always provide new opportunities for development. But there are also some persons who would not like to tolerate any technical changes whatever. In between these proponents and opponents of cloud seeding, there is a great majority of people who occupy an intermediate position, neither actively promoting introduction of new technology nor actively opposing it.

Some people who blindly believe in rituals for promoting rainfall do not believe in the scientific experiments on cloud seeding. The opinions of many people about cloud seeding are based on the views of influential people who may be termed as “opinion makers”.

Even among the scientific academies of different countries there is no unanimity about the scientific and economic aspects of cloud seeding. While some scientists of the National Academy of USA are raising some doubts about cloud seeding operations, many scientists of the Chinese Academy of Sciences are promoting cloud seeding operations on an extensive scale to make available substantial extra water by cloud seeding for mitigating the damage caused to public health, agriculture and the environment. The US Government which used to sanction every year about $20 million as research grants since 1970s has virtually stopped such grants during the recent years. Perhaps the US Government must have felt that adequate research work has already been conducted to transform the science behind cloud seeding into a useful technological tool for conducting cloud seeding operations. In China about 35,000 workers are engaged in cloud seeding operations which are reported to be yielding substantial additional rainfall at an inexpensive cost. The king of Thailand is reported to take personal interest in encouraging cloud seeding operations for promoting public health, economic growth and national prosperity.

The scientists of the Indian Institute of Tropical Meteorology (IITM) conducted warm cloud seeding experiments for a 11-year period during the Southwest monsoons from 1973-1986. The results of these experiments were evaluated by International experts like Dr.P.Koteswaram, former Director General of the Indian Meteorological Department (IMD) and the results demonstrated 24% increase in the annual rainfall. These experiments were conducted in a rain shadow area at Baramati with an annual rainfall of 350mm. The results of this research work were published in the international journal of Weather Modification Association in April 2000. In the light of the successful results of cloud seeding operations conducted in several countries during the last 4 decades the people of India must come forward to debate on the advantages and disadvantages of cloud seeding operations in the light of their own environmental conditions. The Indian scientific community including the Indian Meteorological Department must come forward to improve the cloud seeding methodologies as adopted by other advanced countries like Australia, USA, Israel, Thailand and China and suggest suitable modifications to make the technology more effective and intune with the local meteorological and topographical features of different regions of the country.

Frequently Asked Questions :

How is cloud seeding used for making artificial rains?

Cloud seeding is a new technique used to treat the natural clouds with chemicals for squeezing more water in the form of rainfall or snowfall. The operation will be highly effective only if suitable clouds are present in the atmosphere during the experiments.

What are clouds?

Clouds contain water droplets and often a few ice crystals. When air filled with moisture is raised by the heat of the Sun into higher elevations in the atmosphere the pressure gradually decreases and the air gets cooled and consequently the water vapour condenses over dust particles to become water droplets which are visible as clouds to the naked eye.

When was this cloud seeding technology discovered by the scientists?

Cloud seeding by injecting artificial substances into the clouds was discovered in the experimental laboratories of General Electric Company on 12-7-1946 by Vincent Schaefer who sprinkled dry ice crystals at –78oC into the super cooled droplets of the artificial clouds produced in his laboratory experimental chamber when the ice crystals were found to transform by freezing the super cooled water droplets into ice crystals that precipitated as snowfall on the bottom of the cloud chamber. Thus the scientific basis for artificial precipitation was established.  Subsequently on 13-11-1946 Schaefer along with his research director Dr.Irwing Langmuir, Nobel Laureate conducted a field experiment over a stratiform cloud above Mount Greylock to the East of Schenectady, New York. The cloud was 14,000ft. high with a temperature of –20oC. He sprinkled 3 pounds of dry ice along a line about 3 miles long over the cloud and within about 5 minutes the whole cloud turned into snow and the snow fell down to about 2000ft below the cloud without touching the ground before evaporating. By using silver iodide particles which are almost identical to the ice particles the cloud drops were nucleated at a threshold temperature of –4oC unlike the temperature threshold for natural freezing or crystalisation nuclei of –15oC to –20oC. Subsequent researches demonstrated that in nature the predominant natural nuclei for the initial formation of ice nuclei were clay mineral particles active at about -15oC or lower. After transforming the science of cloud seeding into a new technology for practical applications, Langmuir until his death in 1957 is reported to have been insisting on harnessing this new technology to squeeze more water from the skies for changing the arid and drought prone areas in the Southwest of United States into green pastures and agricultural fields. The scientific and industrial research organization (Common wealth CSIRO) of Australia also conducted cloud seeding operations in February 1947 and reported that the first case of documented man-made rain occurred near Bathurst in Australia. Due to positive research findings the first cloud seeding experiment in Australia was conducted in 1964 by the Hydro- Tasmania and CSIRO.

What are the principles behind cold cloud seeding operations?

The theory behind cold cloud seeding states that the amount of rain produced by a cold cloud is directly proportional to the number of naturally occurring ice nuclei present in it. The number of ice nuclei naturally available in a cloud is usually far lower than the optimum number needed for effective rain formation. Hence artificial rain making by cloud seeding operations seeks to increase the number of nuclei that are naturally present by injecting into the cloud some artificial cloud condensation nuclei in the form of silver iodide particles. The concept is that the cloud droplets form small ice crystals on the surface of the natural ice nuclei and also on artificial nuclei like silver iodide particles. Ice crystals falling through the cloud collide and coalesce with more cloud droplets and grow in size. Ultimately when these ice crystals grow into ice flakes and fall from the cloud to the earth they melt as they pass through the melting point and fall as rainfall or snowfall, if the ambient temperature is low enough.

What chemicals are used for cloud seeding operations?

Three types of substances like dry ice, silver iodide and hygroscopic salts are used as seeding agents. Silver iodide is very similar in structure to the naturally occurring ice. Water vapour and droplets in the cloud deposit on the surface of the artificial crystal and the ice crystals continue to grow as if they are the naturally occurring ice crystals. Silver iodide is used to treat the cold clouds. In order to treat the warm clouds hygroscopic salts such as sodium chloride and calcium chloride are used. These giant cloud condensation nuclei attract the water vapour onto themselves and grow larger due to collision and coalescence by a chain reaction known as Langmuir chain reaction and ultimately form into large rain drops that fall on earth. Dry ice (CO2) acts to cool the water to very low temperatures beyond 0o C and thereby cause the  cloud droplets to freeze, thus growing as water freezes on the surface of the ice.

What kinds of clouds and conditions are suitable for seeding operations?

All the clouds are not suitable for the operations. The cloud seeding operations themselves cannot be done if there are no clouds in the atmosphere and hence operators choose Cumulo-Nimbus clouds which are convective in nature with a great deal of vertical mixing. Nimbo stratus clouds and cumulus clouds which are dark grey are also suitable. The cloud must be deep enough with temperatures within a suitable range for seeding. There should be significant levels of super cooled liquid water present in the cloud. The wind also must be below a specified value. Since it takes about half an hour for the artificially injected chemical crystals to grow into raindrops, seeding line must be 30 minutes upwind of the selected target area boundary. If the wind speed is 40 knots the seeding will be done 20 nautical miles or 37km upwind of the target area boundary. If all the criteria are met before launching the operations cloud seeding becomes successful in producing substantial additional rainfall. Cloud seeding operations are conducted during the rainy season in suitable places and the operations take place when it is safe to fly and suitable weather conditions are present, 24 hours a day and 7 days a week. The base of the cloud must be within 1.5km to 2km from the ground. A reasonable degree of local updrafts promote the augmentation of rainfall.

Why are cloud seeding operations conducted?

They are conducted mostly to increase the availability of water in a region by squeezing substantial rainfall in addition to the normal annual rainfall. These operations are used for dissipation of fogs in the airports and for minimizing the adverse impacts of hailstorms. By increasing the annual rainfall from 10% to 20% the additional water will be used for raising the water levels in the rivers and lakes used for drinking, agriculture and hydro-power generation. Cloud seeding is also used to augment the water storage in reservoirs for use in the subsequent years of drought in countries like Indonesia. Cloud seeding is also done for augmenting the snowfall for promoting tourist trade in the Snowy Mountains of Australia and for reducing the summer temperatures in China for reducing the electricity consumption for air conditioning.

Does cloud seeding work economically and effectively?

Cloud seeding experiments were being conducted in about 40 to 50 countries for more than 4 to 5 decades. The additional precipitation obtained from cold clouds by using aeroplanes and ground generators varies from 20% to 30%. In case of warm clouds the sprinkling of hygroscopic particles from the aeroplanes into the warm clouds produced increases of 20% to 25% in the annual rainfalls. On the average the benefits are estimated at about 20 times the cost incurred for the operations. After reviewing the results of cloud seeding operations for several years the world Meteorological Organization (WMO), American Meteorological Society (AMS) and the Weather Modification Association (WMA) issued statements indicating 5% to 20% additional rainfall from clouds over the continents and 30% from the maritime clouds. It was further stated that the hail suppression experiments caused reduction of 20% to 50% in the costs of damage due to hail storms.

How is additional rainfall obtained from the cold clouds?

Cold clouds are those whose tops are at a temperature below the freezing level of 0oC. These clouds precipitate only about 10% to 15% of their water content as rainfall because of lack of adequate number of ice nuclei in them. In order to obtain additional rainfall, extra ice nuclei or their equivalent in the form of silver iodide particles must be introduced in the clouds by using either ground generators located at suitable places or by using aeroplanes. By this method additional annual rainfall upto about 20% can be obtained.

How is additional annual rainfall obtained from warm clouds?

If the top of the clouds are warmer than the freezing level, the clouds are known as warm clouds. The small cloud droplets numbering about a million have to join together to form a big rain drop to reach the earth. It takes a long time for these warm cloud droplets to join and become a raindrop. The lifetime of the cloud itself may be about half an hour to one hour. In order to promote the transformation of the cloud droplets into rain drops within a reasonable time, additional hygroscopic giant cloud condensation nuclei such as sodium chloride and calcium chloride are injected into these clouds with the help of aeroplanes. In the absence of these giant hygroscopic nuclei, the clouds give only 10% to 20% of their water content as rainfall. By cloud seeding operations which inject the above chemicals into the clouds the annual rainfall upto about 20% may be increased.

Are there any regulations to control cloud seeding operations?

In United States about 32 states have rules, regulations and Laws to control cloud seeding operations. Even in India the State Governments have to obtain permits from the Central Government to conduct cloud seeding operations.

Who conducts the cloud seeding operations?

Most of the cloud seeding operations are conducted by a few specialized commercial companies that work under contract to various kinds of sponsors such as state Governments Irrigation Departments, Metro Water Boards and Hydro-Electric Power generation companies.

Do the cloud seeding chemicals cause any damage to public health and environment?

Research investigations on the damaging impacts of chemicals used for cloud seeding indicated no perceptible damage to public health and the environment. Since the seeding materials are used for the operations are very small in quantities their presence in the precipitation has been detected to be very low in concentration. For instance silver in rain water or snow is found to be in the range of 10 to 100 nano-grams/litre. This low concentration of silver is much below its US Public health Standard of 50 micro-grams per litre. In the case of iodine, its presence in the rain water sample collected at the end of cloud seeding operations is found to be far lower in concentration than that found in the common iodised table salt used by the people.

Does cloud seeding in one region prevent the surrounding regions to get their normal rainfall?

This usual question whether cloud seeding “robs Peter to pay Paul” must be answered with an emphatic “no”. The cloud seeding in one place does not cause any perceptible reduction in rainfall that normally occurs in the neighbouring areas. Out of the total atmospheric moisture continuously flowing over any region the normal condensation removes about 20% of the total moisture to form into clouds. Cloud seeding causes additional precipitation although a cloud normally gives 20% of its moisture as rainfall. Further artificial rain making by cloud seeding causes additional precipitation of 20%. So 100 (0.20 x 0.20 x0.20)=0.8% of the water vapour is removed. Hence the total proportion of atmospheric moisture that gets transformed into artificial rain fall of 20% on earth is very small. Moreover the atmospheric moisture gets continuous replenishment due to transpo-evaporation from the forests, evaporation from the natural water courses and updrafts in the mountainous terrain. Some cloud seeding operations indicated slight increase in precipitation in areas upto 100 miles downwind of the target areas. Thus cloud seeding operations do not rob Peter to Paul.

Can cloud seeding operations prevent or control drought conditions?

Droughts normally occur due to deficiency of rainfall consequent to the absence of adequate number of suitable seedable clouds in the required places at the required timings.In the absence of clouds no body can create suitable clouds and conduct cloud seeding operations to obtain additional precipitation

The reasons for the occurence of droughts in different parts of USA can be observed from the following web site

http://geochange.er.usgs.gov/sw/changes/natural/et/

Hence cloud seeding operations cannot create clouds and stop drought conditions. However cloud seeding operations can be used to substantially increase the annual rainfall in a highly favourable year by seeding the clouds and then store this additional water in the reservoirs for use during the subsequent years when drought conditions may occur. Hence one can use cloud seeding operations as a tool for water resources management which can mitigate to some extent the damaging impacts of a crippling drought.

How does Hydro Tasmania conduct the cloud seeding operations?

Hydro-Tasmania has been successfully conducting the cloud seeding operations in Australia during the last 40 years on scientific lines and at an inexpensive cost. It employs an expert who is independent of cloud seeding operations. He identifies the target areas for cloud seeding operations on the basis of water levels in the reservoirs used for hydro-power generation. The organization employs cloud seeding officers who are trained in weather forecasting. The decision to fly for cloud seeding operations is made by the duty officer on the basis of all kinds of weather information provided by the satellite images, radars and the Australian Bureau of Meteorology and their own experience in weather forecasting. The organization employs a twin engine air craft (a cessna conquest) which is taken on contract every year for the period of cloud seeding. This aeroplane is provided with two silver iodide generators or burners containing the silver iodide solution under pressure, one mounted under each wing. Every cloud seeding flight operation is under the control of one pilot and also one cloud seeding meteorologist.  When this aeroplane is conducting the operation the cloud seeding officer advises the pilot on the cloud seeding tract chosen upwind of the target region. Whenever suitable cloud is encountered on the seeding track, the cloud seeding officer ignites the burner by using the concerned switches provided within the aircraft. The independent expert directs the stoppage of the cloud seeding operations in the areas where the storage reservoirs are attaining their full storage capacity.