Prof.T.shivaji Rao,

 Director,centre for environmental studies,

Gitam University.visakhapatnam (best website)

Perhaps a reasonable scientific proposal for rain inducement was for the first time  made by James P.Espy the author of the meteorological classic “Philosophy of Storms”.On 5-4-1839 . James P.Espy proposed in the issue of the “National Gazette and literary Register” of Philadelphia that large fires be built to create good updraft in a humid atmosphere to generate Cumulus clouds that cause rain fall. This suggestion made in 1839 that convective rains can be induced by burning large fires was reexamined in 1938, by Gorog and Rovo of Hungary. By observing the fires on the lakes of oil, Gorog and Rovo made calculations and concluded that the method was practical for producing the rain artificially under certain conditions. This concept was putinto practice by the Frenchman Henri Dessens of University of Toulouse in 1955 when a Belgian plantation company asked him to study the possibilities of artificial stimulation of rain in Congo region of Africa. Dessens and his son Jean designed a controlled heat generator for burning upto 100 tonnes of fuel in 30 minutes. Trials were conducted in 1961 by using a cluster of oil burners consuming 1000 litres (about one tonne) of fuel per minute in South West France. In one of the trial runs the convection generated was so strong that a small tornado was produced.

In 1875 the Frenchman Coulier experimented to prove that particles floating in air served as nuclei around which condensation of water vapour occurs with little or no super saturation. Scottish physicist John Aitken worked for 35 years with his portable expansion chamber known as Aitken nuclei counter (for particles active at 0.01 mm and 0.1mm radius) demonstrated that there were 2 types of nuclei. Those with an affinity for water vapour on which condensation occurs before water saturation occurs, are called hygroscopic particles and others are non-hygroscopic nuclei which require some degree of supersaturation in order to serve as centres of condensation. In 1919 A.Wigan used balloon flights and measured Aitken nuclei upto about 6km height. A.Wagner observed liquid droplets at one station (3106 m) at Sonnblick at sub-freezing temperature.

 A.Wagener in his 1911 book “Thermodynamics of Atmosphere” reported thepresence of liquid droplets at minus 20oC to minus 30oC. In this book he described glaciation and propounded the theory of the initiation of precipitation in mixed liquid and ice clouds. He stated that “the vapour tension will adjust itself to a value in between the saturation values over ice and over water. The effect of this must be that condensation will take place continually on the ice, whereas at the same time liquid water evaporates and this process must go on until the liquid phase is entirely consumed”. Under undisturbed conditions, crystallization requires more special properties of the nuclei than condensation (like solid particles showing the angles of the hexagonal crystal system) and so crystallization will not occur due to lack of such nuclei endowed with the said special properties. In the air there will be some such particles that gradually get into action as sublimation nuclei, as the temperature gets reduced. Further disturbed droplets of certain size crystalise easily. Since the crystallization forces grow with fallingtemperatures the smallest size or disturbance required for crystallization of already formed droplets will get reduced. Within an ascending cold cloud a few scattered elements may spontaneously solidify without any relative velocities of cloud elements at all. By using Wagners diffusion growth theory, Bergeron calculated the time required for the transformation of all liquid into ice. In the case of 4.2 x 10-6 grams of liquid water per cm3 dispersed in 1000 droplets of 20 microns diameter in the presence of one ice crystal and found that complete diffusion of cloud water into crystal would be achieved in 10 to 20 minutes.

 In 1938 W.Findeisen elaborated on Bergeron’s theory and enunciated direct crystallization from the vapour (sublimation) in preference to the freezing of droplets and considered quartz crystals as the possible nuclei and visualised the possibility of initiating nucleation mechanism by introducing suitable nuclei like silver iodide. Although Bergeron accepted that “warm rains” were common in the tropics, Findeisen emphasised in 1939 that most of the rainfall originates as snow or hail. By 1940 even when scientists found that collision coalescence in “warm clouds” occurs in low latitudes or during summer times Bergeron process remained as the dominant feature of precipitation mechanism. In 1938 Henry G. Houghton emphasised on rain formation by the collision-coalescence mechanism but did not deal with the effects of the breaking of large drops suspended in the “updrafts” to create the “chain-reaction” which later became the foundation for the new theory propounded by Irwing Langmuir.

 Langmuir on Warm Clouds : In 1947 Leopold and Halstand of the Pineapple Research Institute told Langmuir that when they dropped dry ice into a cumulus cloud that was everywhere warmer than freezing (warm cloud) rain fell from it and this made Langmuir realise that it was the water which quoted the dry ice pellets that made the cloud rain and then he developed his chain reaction theory of precipitation by the concept of collision-coalescence of raindrops as propounded by Houghton in 1938. Langmuir introduced results on collection efficiencies as calculated by him during the Mount Washington studies. He visualized that as the rain drops grow to an optimum size they would break up and the smaller broken drops get carried upward again by the updrafts to repeat the cycle and this was called (1948) the "chain reaction". Langmuir went to Honduras to work on tropical clouds with Silverthorne who has attempted to stop the blow-downs associated with thunder storms that destroy the fruit gardens. Langmuir advocated use of a single pellet to augment rainfall and stated that large snow flakes will form at all the altitudes above the freezing level, producing cell propagating storms that cause heavy rain. Langmuir advocated introduction of relatively large amounts of dry ice into the clouds to cause over-seeding and thereby dissipate the clouds. He stated that by over seeding the cloud so many nuclei are produced that the snow flakes that form thereby may often outnumber the original droplets and hence they tend not to fall out of the cloud and thereby prevent the cloud from giving even the normal rainfall. Moreover the heat of fusion generated by the conversion to ice-crystal renders the cloud top float away so that it gets separated from the lower part and the cloud gets dissipated. Langmuir stated that one or two pellets of dry ice introduced just above the freezing level are sufficient to transform a cloud into an efficient rain-producer. Langmuir said “the control of a system of cumulus clouds requires knowledge, skill and experience. Failure to consider the importance of the type of seeding, the place and the time, and also the failure to select the best available clouds, explain why the cloud physics project of the US weather Bureau was not able to obtain rainfall of economic importance”.

 Scientific Weather Modification (1946) : Scientific weather modification was carried out not by the meteorologists but by others new to the field of meteorology. The leader of this brilliant development was Dr.Irwing Langmuir, a physical chemist and one of America’s foremost Nobel laureate who was helped by his laboratory assistant Vincent J.Schaefer, a skilled mechanic who was working for about 3 years on cloud physics problems including studies on particle sizes, light scattering, precipitation static and icing mechanisms which led them to make a series of observations in the clouds that sweep the summit of Mount Washington in New Hampshire. Schaefer being a good mechanic worked in the machine shop and produced good apparatus for Army research projects undertaken by the General Electric Company under the leadership of Irwing Langmuir. Since Langmuir and Schaefer were lovers of out-door life and mountain climbing, they frequently used to go to the top of Mount Washington to carry out research on glaciation that occurred as a natural development on this mountain. Being unfamiliar with meteorology and the work of Bergeron (1933) on glaciation, Langmuir was surprised that

anything exposed to the cold clouds on the summit of Mount Washington immediately gets covered with ice and riming occurred all the time in winter days. They made good progresson icing research and measuring liquid water content of the clouds and collection efficiencies of rain drops. They observed that firstly if there are any snow crystals in the cloud, they will be growing and falling and secondly that if no snow is falling from winter clouds with temperatures below freezing it means that there are no ice crystals in the cloud in adequate number. So Langmuir and Schaefer decided to conduct some laboratory experiments to duplicate these conditions and unfold the mysteries of nature.

 Schaefer’s Marvellous Discovery : Vincent John Schaefer, a research assistant to Dr.Langmuir, the Associate Director of the Research laboratories of General Electric Company was working on some research programmes related to critical weather conditions for the Government. Hewas working in a small laboratory on the top of a 6280 foot high mountain known as Mount Washington in the North Eastern state of New Hampshire. Schaefer frequently watched from his windows that the western winds were constantly pushing moist air masses over the sides of the mountain. He noticed that in the colder air masses, at the higher elevation of the mountain, the moisture was getting condensed into cloud droplets and for many days the mountain was covered with super cooled clouds. Because of the low temperatures present over the mountain the super cooled clouds are a normal feature of the region for this latitude and altitude above the mean sea level. Hence, Schaefer concluded that generally the rain and the snow must come from super cooled clouds and that the rain starts as snow. But how this happens is not well understood.

 Schaefer knew about the famous Norwegian meteorologist, Tor Bergeron who postulated

that if ice crystals somehow got into the supercooled clouds from out side or if they were caused to form within the cloud, the moisture in the cloud might freeze on the ice-crystals and this would lead to the formation of snowflakes which give rain as they melt while falling to the ground. Schaefer also read about German meteorologist, Walter Findeison who expanded the theory of Bergeron. Findeison concluded from his observations that the ice crystals in the cloud were created when the super cooled moisture freezes on some specific dust particles present in the atmosphere. He also predicted that such dust particles (or nuclei) would be provided to the clouds one day or the other by artificial methods. At the end of world war Schaefer and Langmuir reproduced supercooled clouds in their own laboratory for conducting experiments under controlled conditions.

Schaefer obtained a home refrigerator unit used for food storage and modified it to suit to his research work. This refrigerator of about 4 cubic ft capacity was lined with black velvet and provided a small powerful search light in it. He directed a beam of light into the freezer box in order to take observations in the chamber. After bringing the temperature in the box below freezing, Schaefer breathed into the box several times until the air was saturated with the moisture from his lungs. The moisture in the box got condensed and formed fog particles which are like cloud particles. Even at –23oC no ice particles were formed in the dust- free freezer box. Schaefer tried patiently for many weeks by sending air through exhalation into the freezer box to change the fog into snow. To produce ice particles in the box he sprinkled particles of salt, clay minerals and natural dust but practically never got any ice particles. Nothing happened. He tried to reduce the temperature of the box to below 0oF (equivalent to minus 20oC) at the bottom of the box and 14oF (equivalent to minus10oC) at the top of the box. Still the ice crystals were not produced. It was still too warm. Natural nuclei do not become fully effective until the temperature is about minus 13oF (equivalent minus 25oC). The experiments continued and still nothing happened. Then the crucial day arrived on 12-7-1946 when the day time temperature was fiercely hot. In order to cool the laboratory to some extent Schaefer left the lid of the refrigerator unit open as is done in the super markets. After returning from the lunch he found that the temperature in the refrigerator box had risen very high and it was only a little below freezing level at the top.

 For a long time Schaefer used to wonder as to what would happen if something extremely

cold was put into the fog of the super cooled cloud in the refrigerator box. The present instant was a good time to put his idea to the test. Schaefer found an 8-inch piece of dry ice (frozen carbon dioxide) with a temperature at minus 109o F (equivalent to minus 79o C) and thrust this dry ice piece into the supercooled box. In an instant the air was just full of ice crystals, millions of them shimmering against the dark velvet background in the search light. Schaefer was very surprised and spell bound. He watched the tiny crystals quickly transformed into snowflakes, growing rapidly larger and larger as they sucked the remaining moisture in the freezer box and the flakes fell to the bottom. Even after removing the dry ice pieces the ice-crystals persisted for some time. Even the tiniest piece of dry ice could fill the super cooled cloud box with crystals.

 This experiment indicated firstly that the dry ice had no direct effect on the super cooled cloud but it was rather its temperature that has played the key role in transforming supercooled water into ice crystals. It was found that instead of cooling the freezer box with dry ice any other substance having temperature less than minus 40oC can be used for this purpose. Several experiments showed that there is a critical temperature of about minus 39oC (the freezing temperature of mercury) where a spontaneous reaction occurs in the supercooled water to produce natural nuclei. But if you keep the freezer box at minus 20oC and let in air from laboratory, often you may notice a few crystals, but if a cover is applied they grow and fall down and there will be no more crystals unless you inject some more new air into the box.

Schaefer sent an article on these experiments to the Science magazine and it was published on 15-11-1946 and it contained the following statement. “It is planned to attempt in the near future a large scale conversion of super cooled clouds in the atmosphere to ice crystal clouds,by scattering small fragments of dry ice into the cloud from a plane. It is believed that such an operation is practical and economically feasible and that extensive cloud systems can be modified in this way”. This statement was very timely. However nature was too slow to provide the appropriate clouds which Schaefer and Langmuir wanted for their experiments under field conditions. The summer season (June, July, August) was over and autumn season (September, October, November) arrived. On 13-11-1946 a mile long Stratiform cloud at 14,000ft elevation with a temperature at minus 23oC was hanging over Mount Greylock on the East of Schenectady, New York. Schaefer took a small aeroplane with Curtis Talbot as pilot and made his historic flight over this cloud and scattered 3 pounds of dry ice(about 1.5 kg of crushed dry ice) into the supercooled Strato-cumulus clouds over Western Massachusetts(3 miles long over the cloud from the cockpit). Within 5 minutes the whole cloud appeared to have been turned into snowcrystals which fell about 2000ft towards the earth before evaporating. So this first field experiment provided the final scientific proof for augmenting precipitation by artificial methods. What was proved in the controlled conditions of the laboratory also seemed to hold true under the field conditions. Once the operation started, the conversion from droplets into ice crystals and then into snow flakes was a fast action. Just like water speeding up as it approaches a narrow path pulling the other water after it, the space vacated by the droplets as they clouded after the growing into flakes, brought other droplets pushing in behind at an accelerating rate. Thus the super cooled water and the water vapour get frozen at a fast rate on the ice nuclei which develop into snow flakes and augment the precipitation either as snowfall or rainfall.


 It was considered that dry ice was bulky, perishable and inconvenient to handle for cloud seeding operations. Scientists began to search if there was some other substance without these drawbacks which would do the job just as well. Bernard Vonnegut another research

worker with the General Electric Company, began to search for the alternate materials for dry ice. He studied the structure of a number of natural crystals and found that the silver iodide particle, a compound of silver and iodine had almost an identical structure of the ice crystal. He applied vapourised silver iodide to the fog in the freezer box in the laboratory of Schaefer and soon ice crystals were immediately formed. Thus silver iodide proved to be an effective freezing nucleus for ice crystals. Thus Vonnegut found that the particles of silver iodide can also generate many number of ice crystals if the cloud is cold enough. Out of the many natural materials tested such as the smoke from forest fires and factory stacks, volcanic dust, salt powder, soil dust, none was effective until the temperature was far below 0oC. Soil dust began to work as very weak nuclei when the temperature reached 18oF (equivalent minus 8oC) But silver iodide was effective much sooner at 25 to 20oF equivalent to minus 4oC where as other particles just began to work slightly at 14oF (equivalent to minus10oC) At that temperature silver iodide was very active and in full operation. Hence silver iodide is practically the most desirable chemical for cloud seeding operations.

In order to vapourise silver iodide, Irwing P.Krick of California Institute of Technology developed a ground generator consisting of a small fire brick oven inside a steel box (the size of a small Television set). In this oven silver iodide was impregnated in coke and burnt at a white heat of 2500oF (equivalent to 1370oC) impelled by a fan blowing through the fire in the fire brick oven. The vapourised silver iodide rose from the furnace and drifted into the clouds along with the rising air currents feeding the clouds at the rate of millions of particles per second. Thus the scientific basis of rain making was at last established.

Immediately many rain makers came into the field but not all of them were as qualified as Vincent Schaefer. Many people wanted to augment precipitation for many purposes. Farmers wanted rain for their crops. Farmers also wanted cloud seeding operations to stop the hail which was causing serious damage to their crops and properties. Cattle farmers wanted rain for their grass lands for rearing cattle for milk and meat. Electrical Power companies wanted more water in their reservoirs to run their hydro-electric power stations to supply inexpensive electricity to the industries and domestic needs. Municipal Corporations needed water in their reservoirs for domestic and industrial consumption. Mountain resort managers wanted cloud seeding to get more snow for the skii slopes to promote tourism industry. Some Government agencies want cloud seeding operations to augment the ground water supplies and to reduce summer temperatures so as to minimise electrical power use. China and Russian states intend to use cloud seeding operations to prevent unwanted rains during national celebrations and international sports festivals.

 Many contractors for rain making came into the field. Some operators used a few buckets of dry ice, dropped from an air craft into the clouds. Many farmers conducted cloud seeding operations by themselves while others used professionals and in the process they covered about one-fourth of the area of the United States. Some charged half to one cent per acre for grasslands and 15 cents per acre for agricultural fields. Some contractors worked on the condition that if there is no rain due to their operations they will not claim for any payment. There were many controversies among the farmers as well as the scientists. The United States Weather Bureau condemned the cloud seeding as "nonsense". Some scientists claimed that it is a new technology that has to be used for the benefit of man-kind.

In order to resolve these controversies and to know the true picture of cloud seeding operations, Senator Francis Case from South Dakota introduced a bill in the US Congress

demanding for a President’s Advisory Committee on Weather Control. President Eisenhower signed the bill into law on 13-8-1953. On 9-12-1953 a committee was appointed under the Chairmanship of Captain Howard T. Orville, Navy’s Chief Weather man and President of American Meteorological Society. The Committee was directed to study and evaluate the cloud seeding experiments carried out by private and Government organizations. On 13-12-1957 this committee concluded that cloud seeding has augmented rainfall by 9% to 17%. If the cloud seeding operations increase rainfall by just one percent during crop growing season, it would pay for the cost of seeding by a good harvest. The National Academy of Sciences was skeptical and it appointed another expert committee which commended cloud seeding as an effective method to improve rainfall and emphasized that rainfall was increased by 10 to 20% rather 9 to 17% as estimated by the Presidents' committee. These findings were upheld by the scientists of Rand Corporation of California and hence the weather bureau finally agreed to uphold cloud seeding and the rain-makers at last earned their credentials!

If silver iodide has become the key to modify these vagaries of weather before one could use this key effectively to modify the weather, one must first know in advance what the weather was going to do?, one must be able to forecast the weather which in turn calls for

knowing more about how the weather operates. But, man with all his studies and research on weather finds that he still knows very little about the weather. Thus weather modification always remains a complex problem which for every answer always appears to raise many more questions. Man is still only at the beginning of what he needs to know to tame the vagaries of weather for the benefit of man kind. But with the recent advances in computers, electronic instrumentation, satellites and space explorations, there are more facilities available to study and understand the weather phenomena. Hence the extensive utilisation of the recent advances in science and technology must help man to unravel the mysteries of the weather phenomena and harness the atmospheric resources for the benefit of man and nature.

Why cloud seeding operations should be promoted?

1. The National Research Council prepared a report on critical issues in weather modification research and it was published on 13th October, 2003. This report concluded that there is still no convincing scientific proof of the efficacy of International Weather Modification efforts and that even though there are strong indications of induced changes yet the evidences have not been subjected to tests of significance and reproducibility.

2. This report however does not challenge the scientific basis of weather modification concepts. The definition of scientific proof perhaps involves randomized experiments, statistical support, physical measurement and understanding and replication. But can any scientist demand for similar scientific proof in dealing with crucial environmental problems of climate change, green house effect and ozone depletion?

3. And yet the National Research council emphasises that there is ample evidence that inadvertent weather and global climate modification is a reality, If a certain criteria adopted for pronouncing a judgement on scientific proof for global climate modification is used to establish as a reality, a similar criteria extended to assess planned weather modification would certainly place it on a higher plane of reality.

4. More convincing scientific evidence on the positive effects of cloud seeding operations conducted in different states in USA and in other countries like China, Thailand, Israel, Australia and other countries are presented to establish the credentials of cloud seeding operations for augmenting precipitation to protect public health and the environment.

5. It is true that the absence of adequate understanding of several critical atmospheric processes have lead to a failure in producing a high degree of precision in ensuring predictable, detectable and verifiable results. But such a situation cannot stop mankind from making use of the best available tools in weather modification technology to fight the recurring damaging impacts of droughts, El Nino impacts and global climate changes. Hence the promotion of further research in critical atmospheric processes can be initiated and continued with vigour if only the cloud seeding operations are also conducted simultaneously.

6. While the water-rich states of USA are taking a tough stand to insist that it is premature to take up cloud seeing operations without conducting more research work on weather modification processes, the water-starved provinces of China are supported by their socially concerned  scientists of the Chinese Academy of Sciences who by their extensive research investigations on cloud seeding operations have endorsed the scientific basis for cloud seeding and are promoting extensive scale of operations in China where more than 35,000 workers are annually engaged in augmenting precipitation from the clouds.

 While many scientists attempted to evaluate the scientific basis and the success rates of cloud seeding operations they have resorted to a statistical approach to make further investigations of this technique which meant that there was no real evaluation of the cause and effect of cloud seeding operations. Essentially the most desirable conditions for seedability must be fixed in the first place to prove statistically that cloud seeding operations are effective but such conditions should be necessarily varied to find the optimum method to carry out the seeding operations. Without following this procedure several cloud seeding projects have been evaluated and most of them provided unsatisfactory results or failed to produce an effect that was physically convincing.

 Hence managers of cloud seeding operations who are convinced that the theory behind cloud seeding has been already established by the pioneering laboratory and field experiments  conducted by Schaefer and Langmuir in 1946 do not expect identical results in terms of economy and success rates for operations conducted under highly varying geographical, topographical and meteorological conditions. Thus the cloud seeding proponents are going ahead with practical methods of cloud seeding as a water management tool that is providing more water for drinking, irrigation, hydro-power generation and environmental improvements for the survival of man and nature.

 The Americans being a nation of people with a scientific bent of mind coupled with patriotic fervour always work towards promotion of the living standards of the people and for the purpose seek new methods of augmenting their water wealth. Consequently the Irrigation Department of US Government known as Bureau of Reclamation planned not only to make maximum use of the conventional ground and surface water resources but also to tap the abundant atmospheric water resources in the form of clouds and started for this purpose a scheme in 1962 under the title of “Project Sky Water”. The main objective of this programme is to investigate how cloud seeding operations can be employed efficiently, economically and on a socially acceptable basis for increasing the annual rainfall and snowfall in water deficit regions of the country; in effect to manage the precipitation from the clouds by making use of the science of weather modification for developing the atmospheric precipitation management technology that has wide general applications in water resources management. The applications of this weather Modification technology include fog dispersal, hail suppression and cyclone modification. The field activities of this project involve cloud seeding operations for seeding.

1. Orographic cloud systems (winter storms ) that form over mountain range as the air is

lifted and cooled by the high mountain barriers.

2. Cumulus clouds that form during the warmer season.

During the winter the cloud seeding programme is used to increase the mountain snow

pack so that additional run-off is received during the spring melt season. The seeding of

cumulus clouds is to provide increased annual rainfall directly on the land.

Principles Of Cloud Seeding : Clouds are made up of millions and millions of water droplets or tiny ice particles or both which form around microscopic particles of dust, smoke, soil, salt crystal and other chemical aerosols, bacteria and spores that are always present in the atmosphere. These particles are classified as “condensation nuclei” (CCN) on which water vapour condenses to form cloud droplets and a few of them are classified as “Ice Nuclei” (IN) on which condensed water freezes or ice crystals form directly from water vapour. In the normal atmosphere there is an abundance of condensation nuclei while there is a scarcity of ice nuclei.

The types of nuclei and their sizes and concentrations present in the air play a significant

role in determining the efficiency with which a cloud system precipitates. Generally tonnes and tonnes of water flow (as rivers of moisture) in the skies over many countries and from these rivers in the sky either little precipitation or not at all falls on the ground because of absence of certain required conditions. Among such important conditions for both initiation of precipitation and the amount of precipitation from a cloud system are;

1. Horizontal and vertical dimensions of cloud

2. Lifetime of the cloud and

3. Sizes and concentrations of cloud droplets and ice crystals.

Under proper conditions one or more of these above 3 factors can be favourably modified

by seeding the clouds with appropriate nuclei, mostly by using either common salt or silver iodide particles.

Warm and Cold Cloud Seeding : Precipitation forms in clouds by two mechanisms namely “warm rain” and the “cold rain” processes. The term warm rain was coined by the scientists who found that the rain in tropical countries often fell from clouds whose temperature throughout the clouds was warmer than the freezing level of 0oC or 32oF. Rain occurs in these clouds when larger droplets collide with the smaller cloud droplets and absorb them in a process known as “coalescence”. The cold rain occurs in clouds whose temperature in all or part is colder than the freezing level of 0oC or 32oF . The regions of the cloud below the freezing level are super cooled and contain both water droplets and ice crystals and sometimes only the former. The ice crystals which form in the super cooled regions of the cloud grow very rapidly by means of drawing the moisture from the surrounding cloud droplets and this growth continues until their weight overcomes the gravity forces and causes them to fall to the ground. While falling from cloud these ice crystals coalesce with other smaller droplets and fall from the cloud as snow or rain. The atmospheric nuclei that play a key role in cloud formation exert a strong influence on the efficiency with which the warm and cold rain processes operate.

For example the giant condensation nuclei (GCN) are prevalent in the oceanic atmosphere that allows for larger cloud droplets to form and the coalescence process to initiate rain within the life time of the cloud. But the continental regions are characterized by much smaller and more number of condensation Nuclei( CCN). Hence medium size clouds formed over the continental areas generally dissipate before the coalescence mechanism has had a chance to initiate rain. Similarly many regions have a shortage of Ice Nuclei (IN) which reduces the efficiency of cold rain process.

Reasons for Injecting Chemicals into Clouds : When a scientist detects by scientific measurements the nuclei in the clouds and realises that mother nature has provided very  few nuclei to initiate precipitation process in the clouds he helps mother nature by injecting appropriate nuclei in sufficient numbers into the clouds by seeding them at the proper time and place so that the moisture in the clouds falls on the ground as snowfall or rainfall. Seeding the cloud with very large or giant condensation nuclei (GCN) (hygroscopic particles such as sodium chloride, calcium chloride etc) can be done to accelerate the warm rain process and seeding with proper ice nuclei such as silver iodide is done to supply the naturally nuclei-deficient cold clouds with an optimum concentration of ice crystals which will substantially increase rainfall through the cold rain process.

Most often a combination of hygroscopic and also ice nuclei seeding can be done since both warm and cold rain mechanisms operate in the mixed phase cold clouds ( practiced in the Thailand by the king). Most important aspect of super cooled cumulus cloud is a dynamic effect caused by ice-phase seeding. It is well known that under favourable natural environmental conditions cumulus clouds can be stimulated to grow larger and also to last longer. The injection of silver iodide particles into the super cooled part of the cloud makes the cloud droplets freeze into ice crystals. This conversion process gets multiplied millions and millions of times within the cloud and releases a large amount of heat, known as the “latent heat of fusion”. This phenomena makes the cloud more buoyant and makes it grow larger in size and thereby makes the cloud process more water longer and more efficiently than would have occurred naturally.

What is Wrong with Natural Clouds? : Chemical seeding of cold clouds which are found as winter Orographic storms is a fairly well established and well understood method. Clouds are formed as the moist air from the oceans, lakes and rivers is lifted and they are cooled as they pass over forests and mountains from West to East during the monsoon. Left to the usual processes and devices of nature most of the clouds are highly inefficient precipitators, content to keep aloft more than 90% of their moisture burden. By seeding some of these cold clouds with silver iodide particles the precipitation efficiency of these clouds can be greatly improved.

As already stated the microscopic particles of silver iodide work as artificial ice-nuclei that form ice crystals which attract moisture from the surrounding cloud droplets and grow large enough to fall to the ground as snowfall or rainfall. These large cumulus clouds form when the warm moist air fairly close to the surface of the earth is lifted into the sky where it gets cooled to the point where the water vapour condenses into cloud droplets. The lifting of the moist air can be induced by the heating of the surface of the earth by the Sun or by the passage of a cold front or a warm front through a given area. The life span of most of the cumulus clouds is generally about 20 to 30 minutes and they give very limited snowfall or rainfall because nature provides too little time for the cloud droplets to coalesce into large drops of sufficient weight to fall on earth.

Critical Temperature for Seeding : The two cloud seeding techniques that are generally used to extract more moisture from the cloud in the form of snowfall and rainfall are the hygroscopic seeding of warm clouds and the ice-phase seeding of cold clouds with silver iodide. The silver iodide seeding is used when the cloud top temperature of the cumulus cloud formation is 23oF to 26oF (-3oC to –5oC) or colder or when the cloud growth indicates that it will shortly reach that temperature level soon in the atmosphere. The main objective of cold cloud seeding is to initiate the dynamic latent heat release which will stimulate growth of the cloud. If the cloud updrafts are slow the objective then is the same as for orographic cumulus clouds (winter storms) that means the objective is to increase the number of ice crystals in the super cooled part of the cloud to the optimum concentration for the given set of cloud conditions. The ice crystals attract moisture and grow rapidly at the expense of cold clouds because of the special properties of water and the ice crystals grow into ice flakes of sufficient weight to fall through the cloud and melt into rain drops before reaching the earth.

In the warm cloud seeding hygroscopic materials like common salt, calcium chloride or a mixture of Ammonium Nitrate and Urea that absorb water vapour from the surrounding air are used. The hygroscopic substances are finely ground into small particles and released into the updraft beneath the growing cloud. The hygroscopic particles will gradually grow in size by condensation and then by coalescence with cloud droplets that grow into heavy drops that produce rainfall. This process is effective particularly in clouds with strong updrafts which might otherwise remain inefficient precipitators. Left to themselves these clouds would blow much of their water content out of the cloud tops where ice crystal clouds (cirrus) form and hold aloft much of the water that has been processed. Hygroscopic seeding of the clouds generally result in formulation of large drops much lower in the cloud and at warmer temperature levels where there is the greatest supply of moisture and hence more of the water content in the clouds reach the ground as rainfall.