Centre for Environmental Studies,
Gitam University, Visakhapatnam,INDIA
http://www-das.uwyo.edu/~geerts/cwx/notes/chap08/moist_cloud.html [cloud properties]
http://commerce.senate.gov/pdf/golden.pdf [Taming Hurricanes,Dr.Golden]
http://www.businessweek.com/magazine/content/05_43/b3956105.htm [see Link on storm control]
The earth’s atmosphere, which works as a giant heat-engine uses the energy from the sun to drive the winds and uplift water vapour from the ocean surfaces, forests and natural water courses to deposit in clouds which give rain. Almost all the solar energy that is absorbed at the earth’s surface, enters the lowest layers of the atmosphere s sensible heat or as latent heat associated with the evaporation of water. Vast solar energy is stored for a time as latent heat of vaporization and it will be released later the vapour condenses into clouds. Hurricanes and thunder-storms are powered by the release of latent heat of vaporization. However, the energy of the winds is mainly derived from the differential heating of the different parts of the surface of the earth. Since clouds give practically all of the atmosphere water deposited on the earth, cloud modification has become a matter of great practical importance.
WHY PEOPLE SHOULD DEMAND THE STATE FOR REDUCING POVERTY, WATER SCARCITY, FOOD FAMINE BY CLOUD SEEDING:
Hybridisation of Roses, mangoes, plaintains and other flower plants and trees have produced new varieties of plants that are giving higher and better yields of flowers and fruits. Hybridisation of domestic cows produced new varieties of cows that are producing far higher yields of milk. Green revolution produced higher yields of paddy that improved the economic prosperity of the farmers and farm workers. Similarly injecting chemicals into clouds convert them into highly developed and productive clouds that are giving higher annual yields of precipitation as snow fall or rainfall. The Chinese intellectuals,scientists and engineers not only recognized these scientific truths but also transformed these scientific principles into useful technologies of cloud seeding to promote agricultural,industrial and economic growth that helps to eliminate poverty and disease. Having recognized these glaring facts many wise people in other countries are also demanding their governments to conduct operations by seeding the clouds with chemicals by using ground generators and aeroplanes to produce additional annual rainfall and snowfall Intellectuals in several countries realised that the conventional water resources like ground water and surface water supplies in the rivers and lakes are insufficient to meet the growing needs of waterfor the escalating population for drinking, irrigation and hydropower generation. They also want cloud seeding to remove fog from airports, suppress hailstorms and hurricanes and to fight the recurring droughts and the damaging impacts of global warming. Unless the clouds are seeding by sprinkling ice particles or silver iodide nuclei into the cold clouds they cannot develop into centres of intense moisture suction from the atmospheric water vapour that is pushed by the updrafts that promote the alround growth of clouds. Such enlargement of clouds caused by cloud seeding envelops the surrounding additional clouds which otherwise may not cause additional precipitation to an extent of 20% to 40%. Moreover some clouds due to their insufficient depth do not give rain by themselves and hence they have to be seeded with chemicals so that they can grow in size and become clouds of sufficient depth for giving additional precipitation. Some clouds even if they are having sufficient depth may not have sufficient ice nuclei and hence naturally give only 10% to 20% of their water content as rainfall. If we inject more ice nuclei so that they reach the optimum level the clouds can convert more atmospheric moisture into higher additional percentage of precipitation. Similarly in the case of warm clouds unless we raise the giant size cloud droplets by injecting adequate dosage of hygroscopic chemicals these warm clouds can not give about 25% additional rainfall over and above the average annual rainfall in a given region. Thus cloud seeding alone provides additional opportunities for the clouds to process more atmospheric moisture which gets transformed into additional rainfall or snowfall. Hence inaction of the State Governments in promoting cloud seeding amounts to their neglect of positive measures to make available substantial additional water resources from the sky water which contains about 10 times more fresh water than that available in all the fresh water rivers on the surface of the earth. Thus any Government which merely uses the primary water resources like rivers and lakes and the secondary water resources like shallow wells and deep bore wells will be merely providing very limited supplies of water to the people. But intellectual Governments make genuine attempts to tap the tertiary water resource in the sky in the form of atmospheric clouds by cloud seeding operations to augment water supply availability to the people for drinking, agriculture, hydropower generation, replenishment of surface and ground water sources on a substantial scale for meeting the needs of escalating population in most of the developing countries. Chinese intellectuals treat non-application of cloud seeding operations by the Meteorologists , bureaucrats and politicians in any tropical country amounts to an indirect promotion of poverty,unemployment,disease,diseconomy and social unrest.
The key role played by a good water supply as an engine of economic growth and as a yard stick of public welfare and national prosperity has been well recognized by the intellectuals of the developed countries like USA who aptly named water as the “Blue Gold”. The more the water wealth of a nation the higher will be the opportunities for achieving high rates of progress in the fields of agriculture production and industrial growth that help in promoting economic wealth, employment opportunities and higher standards of living. Hence the advanced countries are constantly upgrading their water resources by harnessing not only all the ground and surface waters but also by tapping a renewable, virtually unlimited and unexploited sky water resource in the atmosphere in the form of innumerable clouds. Enlightened scientists, bureaucrats, industrialists and statesmen in about 50 countries are frequently using cloud seeding operations for over 40 years for various purposes like
1. Increase of annual rainfall for drinking and agricultural purposes,
2. dispersal of fog in airports and metropolitan city roads
3. Increase of hydro-power generation at the cheapest cost
4. Suppression of hail storms to reduce damage to life, crops and properties
5. mitigation of devastating impacts of recurring droughts
6. mitigation of damaging impacts of global warming and summer temperatures
7. increase of annual rain fall for improving the forests, wildlife and the environment
Several progressive countries like USA, Australia, China, Thailand, European states, former states of USSR, Latin American states, Arab states, Indonesia and Pakistan are getting highly benefited by employing the advanced cloud seeding technologies for the above purposes.
Several Indian states interested in promoting economic growth, agriculture development and public welfare are eager to learn from the successful experiences of other countries like China and USA and adopt those technologies by making necessary modifications to suit the local meteorological, topographical, geographical and other environmental conditions
Cloud droplets form in the atmosphere by condensation on existing particles (heterogeneous nucleation) rather than by aggregating of water molecules from the vapour state to form pure water droplets (homogeneous nucleation). The particles of smoke, dust etc., involved are known as “cloud condensation nuclei” (CCN). The population of CCN fluctuates from time to time and place to place and influences the micro-physical characteristics have an impact in converting cloud-droplets into rain drops. The solid particles placed in a supersaturated solution to promote the precipitation of the dissolved solute (or in a super cooled solution to cause it freeze) are known as “seeds”. Cloud-seeding with artificial chemicals is done to modify the clouds to make the clouds give the rain. Just as cloud droplet formation requires the presence of CCN, the formation of an ice-crystal in a cold cloud (with temperatures below freezing level) generally requires presence of an ice nuclei (IN) and the presence of a giant hygroscopic-nuclei promotes rain formation in a warm-cloud (with temperatures above freezing level and cloud heights limited upto freezing level in the sky).
Injecting the artificial chemical or other substances into suitable clouds to get additional rain-fall in the chosen places for improving the water supplies is known as “artificial rain-making” or “Cloud-seeding” or “Precipitation management”. Experiments and operations in this field of weather modification are conducted in more than 30 countries in the world for increasing the annual rain-fall, dissipation of fog, suppressing hailstorms and taming the cyclones for minimizing the consequential damage to the environment. In some countries like USA these weather modification projects evaluated in this field by the National Academy of Sciences, USA, 13 were successful in more on-target precipitation than expected, with increased rain-fall ranging from 7% to 57% by comparison with nearby untreated controls. For practical person, cloud seeding being both an art and a science, works to achieve the target of improving the health and welfare of the people and their environmental assets. A person who insists on scientific proof through rates of success and reproducibility for all the results from cloud-seeding experiments must realize that there are dozen of critical parameters which are subjected to constant fluctuations and hence it is difficult to expect reproducibility. However, the optimistic scientists interpret theory and experience to present a reasonable guide for action in situations where inaction causes more undesirable risks to the society as can be seen from the experiences from China, Japan, Thailand, Australia, Africa, Israel, Russia, Canada, Texas and USA.
CLOUD AND RAIN FORMATION:
Due to the heat from Sun the surfaces of natural water bodies warm up and the hot air in contact picks up the water vapour to ascend into the sky. As the air parcel ascends it is subjected to adiabatic expansion and cools and the excess water vapour condenses into water droplets on salt particle nuclei at a relative humidity of 78% and on other particles at a relative humidity of 100%. Such cloud condensation nuclei consist of dust, smokes, soil and salt crystals. While the salt particles are one micron (Micron is a millionth part of a meter)in size, others have radius in the range of one hundredth to one tenth of a micron. Ice-nuclei are special particles on which cloud droplets freeze or ice-crystals from directly from the water vapour. While condensation nuclei are abundant in the atmosphere, the ice-nuclei are scanty and without these nuclei there would be no clouds. The water vapour condenses on the nuclei and forms cloud droplets or ice-crystals or both. (Fig-1 and Fig-2)
According to Bergeron per every thousand super cooled cloud droplets one ice crystal that forms on an artificial nucleus is required for enhancing precipitation. Since one cubic meter of cloud contains 100 million cloud droplets one lakh nuclei will be required and such seeding ensures growth of 1000-fold in drop mass and 10-fold in radius. Thus a 10 micron cloud droplet becomes 100 microns ice sphere and its terminal velocity would be sufficiently large to ensure its arrival on earth as rain drop. However the presence of one ice crystal per every super cooled droplet converts the cloud into small ice crystals and such over-seeding results not in the promotion but in the prevention of rain at that particular point of time and place. Since nature usually supplies about 1000 nuclei per cubic meter of cloud, any concentration higher than this leads to increased rainfall. Cloud droplets form on particles of radius 0.5, 0.1 and 0.01 microns at 100.2, 101 and 110 percentages of humidity respectively.
When the fog is lifted above the ground the sheet-like clouds that form are known as stratus clouds. If initial condensation occurs above the ground due to forced updrafts the small heaps of clouds that form are known as Cumulus clouds. In a large area with streets of cumulus clouds, while the smaller clouds are pushed down wards and dissipate the larger ones exposed to winds that move vertically upwards at 5 to 10 meters per second transform the cumulus clouds into a towering cumuli with cloud thickness of 6 kms. Further growth of the cloud leads to freezing of the water droplets, with the tops assuming anvil shape indicating outflow of air and such clouds are known as cumulo-nimbus clouds and extend upto 18kms in height. Depending upon the height of the cloud-base and thickness the clouds may be classified as follows:
NAMES OF CLOUDS
1. High Clouds
5 to 14 kms
2. Medium Clouds
2 to 7 kms
3. Low Clouds
0 to 2 kms
4. Clouds with Vertical growth
0.5 to 9 kms
0.5 to 18 kms
Alto-status clouds give light rain while Nimbo-stratus ones give continuous rain and sometimes with heavy down pour accompanied by hail. Other cloud forms generally do not give rain. (Fig-3)
The cumulonimbus clouds are heavy dense masses with large vertical growth and look like tower and mountains. The upper portion of the cloud develops fibrous structure that looks like an anvil or plume. It has liquid droplets at bottom and ice crystals in the portion above and often produces lightending thunder, showers and hail. The Nimbo-stratus clouds are dark grey, sometimes reaching low and precipitate rainfall. About one percent of the earth’s water vapour is present in the atmosphere and helps the cloud growth. According to Kampa,in a cloud ,the average cloud droplet has a radius [in microns]of 4 in fair weather cumulus and strato-cumulus,7 in stratus-like and 10 in cumulus-congestus.The water content is about 2.5 gm/Cum in large cumulus,0.5 gm/Cum in fair weather cumulus and 0.2 gm/Cum in stratus cloudshttp://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0469(1950)007%3C0054%3AVALWCI%3E2.0.CO%3B2
But how to modify the clouds to get the required water at a given place remained a perplexing issue.(Fig-4)
HISTORY: In 1932 USR established an institute of Artificial Rain to consider the feasibility of weather modification. Un USA, World War II led to the formation of a group at the General Electric Research Laboratories in New York headed by Irving Langmuir to study similar problems. Vincent Schaefer a member of this group, dropped a pellet of dry ice into a cold box in a hurry to cool it fast for his experiments and found to his surprise that a trail of ice crystals immediately appeared along the path of the piece of dry ice. (Fig-5)
Accidentally he discovered a method to glaciate (freeze) supercooled clouds. On 13-11-1946, Schaefer dropped about 1.5kg of dry ice-pellets (Solid Co2) from a light air-craft into a cold cloud near the mountains of Western Massachusetts. Within 5 minutes the cloud turned into snow flakes and the ancient dream of weather modification was realized and in the long run 13-11-1946 is bound to become a fateful as 16-7-1945 when the first nuclear explosion was set off in New Mexico. Subsequently several scientists investigated on the various factors that influenced the formation of clouds and their precipitation. It was found that some clouds give rain when favourable conditions exist.
The water droplets in the clouds are so small (10 micro-meters in radius) that they cannot reach the earth without evaporating. Hence if large rain drops (500 micro-meters in radius) that fall on the earth without evaporating are to be formed, about a million small cloud droplets must coagulate and for this purpose mere adhesion and condensation are not enough. There are 2 theories about the mechanics of rain-drop formation, each of which is valid under different climates.
The first theory of rain-drop formation is based on “Langmuir chain-reaction” that mostly occurs in the hot, humid atmosphere of the tropics. The larger droplets fall much faster than the smaller ones such that bigger drops in a cloud overtake and absorb all the smaller drops found in their downward path. In a rising cloud, the up-currents hold the big-drops in suspension while the smaller ones are caught up and merge with the big ones. If the Large drops become too big, they burst into fragments that again collide and coalesce with small droplets and ultimately fall to earth as rain-drops.
The second theory of rain-drop formation is based on “Bergeron chain-reaction” that mostly occurs in the temperate zones. The clouds that precipitate consist at the higher levels, of the ice crystals. For the formation of big droplets in the cloud an ice-phase is a precondition and for this transition from the liquid to solid phase a freezing nucleus is essential. The freezing nuclei allow supercooling of water droplets upto 15oC before ice-formation occurs. Infact pure water suspended in air does not freeze until it touches about -40oC temperature. The ice-crystals grow rapidly by absorbing all the supercooled droplets on their way down and unwieldy lumps of ice are formed and they melt at about 0oC to become rain-drops that fall on earth.
These basic mechanisms by which precipitation occurs are often called the “Warm-rain” and “Cold-rain” processes. The rain in the tropical regions from clouds whose tops never touch the freezing level is known as “Warm-rain”(Langmuir chian reaction) while the “Cold-rain” occurs when temperatures in all or some parts of the clouds are colder than the freezing levels (Bergeron Chain-reaction).
In warm clouds to be seeded there is a serious shortage of large water droplets over 20 microns in diameter and this reduces the efficiency of the cloud to give rain. In order to induce such clouds to give rain the coalescence mechanisms have to be stimulated by injecting the clouds with the hygroscopic sodium chloride particles from a small aircraft. The Langmuir chain-reaction gets a boost and the clouds which were formerly unproductive become productive. Similarly the dearth of ice-crystals in cold clouds is removed by seeding them with silver iodide to ensure one nucleus per litre of cloud air so that such seeding generates additional ice-crystals and thereby accelerates the rain-forming processes.
CLOUD SEEDING FOR ARTIFICIAL RAINS:
Since salt crystals are abundant in oceanic regions they favour larger cloud droplets that collide and coalescence to initiate rain fall well with in the life time of the clouds. But the atmosphere over continental regions generally contain much smaller and more numerous condensation nuclei and hence the medium sized clouds formed in such regions usually dissipate before coalescence mechanism has had a chance to initiate rain.
Cumulus clouds formed by uplift of warm moist air masses or by the passage of a cold or warm front through any region, can be seeded when their tops touch -5oC or when they are growing with indications that they will reach that temperature level in the atmosphere. When right conditions exist, the cumulus clouds can be stimulated to grow larger and last longer. It must be remembered that the freezing of millions of droplets in the cloud releases enormous latent heat energy and that makes the cloud buoyant, causing it to grow larger, thereby efficiently processing more water for a longer time period than would have occurred without the cloud seeding process.
COLD CLOUD SEEDING:
The optimum nuclei required to convert the available super cooled cloud droplets into ice was answered by the scientists of the first experiment “Project Cirrus”. In 1949 Tor Bergeron estimated that a ratio of one ice crystal (which could be formed on an artificial nucleus) to 1000 super cooled water droplets was adequate for augmenting precipitation. Since there are about 108 cloud droplets/m3 (100 Million cloud droplets per cubic meter) 105 nuclei/m3 (100 thousand nuclei per cubic meter) would be required. This causes a 1000-fold growth in mass and 10-fold in radius. Hence a 10 micron cloud droplet becomes a 100 micron ice sphere and the particle terminal velocities would be sufficiently large to ensure their fall out. But one ice crystal for every super cooled cloud droplet would convert the whole cloud into small ice crystals which would stop the natural precipitation and the result would be over- seeding. But Nature under the favourable precipitation conditions, generally supplies nuclei of about 103/m3 (1000 nuclei per cubic meter) and so theoretically any concentration of nuclei higher than this leads to augmentation of rainfall. http://www.physics.isu.edu/weather/kmdbbd/cloudseed.pdf
It is reported that introduction of 1,00,000 ice particles/litre of cloud air dries up a normal cloud in about 1 or 2 minutes while introduction of 1 to 10 ice particles per litre promotes growth of ice particles for about 20 minutes before the cloud water is completely used up and the ice particles would have grown into snow flakes for precipitation. But the concentration of effective natural nuclei fluctuates with the temperatures. Generally the nuclei are in a concentration of 103/m3 at –21oC. But they tend to increase 10 times with a drop of 4oC in temperature and the nuclei decrease by 10 times with every increase of 4oC in temperature and the crucial temperature lies at the coldest part of the cloud which for augmenting precipitation was found to lie between about –10oC and –23oC An ice crystal grows to an embryo size(about 250 microns) in less than 10 minutes by deposition of water vapour. In a cold cloud the droplets always evaporate to maintain vapour pressure at saturation level for water surfaces.(Fig-6)
Cloud-seeding experiments :
The augmentation of rain-fall from the cold-clouds is based on the assumption that there is a deficiency of ice-crystals and this deficiency has to be rectified by introducing artificial ice-nuclei like silver-iodide into suitable clouds to produce the required ice-crystals for improving the rainfall.
In the case of ice mechanisms it is mainly the artificial ice nuclei that provide more ice crystals that grow by using the cloud droplets and cloud water within the lifetime of the cloud. Ice particles are formed by ice nucleation, that is, conversion of water vapour or super cooled liquid into ice. An ice nucleus is needed for each ice crystal formed in the cloud. Gradually greater numbers of atmospheric aerosols become effective as ice nuclei as the temperatures fall further below 0oC and when the temperature attains –40oC spontaneous nucleation occurs and all the weak electrolytic water becomes frozen.
Sometimes very strong winds do not provide sufficient time for ice crystals to grow to precipitation sizes before being blown over the mountain tops and then sublimating in the sinking sub-saturated air on the leeward side of the mountains. Often moist-laden wind-flow over a mountain barrier makes the orographic lift to produce the clouds or increase the depth of clouds.
Introduction of massive dosages of silver iodide into the clouds at -10oC at the rate of 100 to 1000 nuclei per litre of cloud air is known as dynamic seeding. In this process the conversion of super-cooled water in the cloud into ice-crystals (through Bergeron-Findesen reaction mechanism) release latent heat that increases buoyancy, thereby increasing the size of the cloud with better organization of low level inflows with the probability of cloud-merger and area-wise enhancement of rainfall. Dynamic seeding of individual clouds on a random basis in USA, indicated significant increase in rain-fall by a factor of three.
CLOUD SEEDING EXPERIMENTS:
The augmentation of rainfall from the cold clouds is based on the assumption that there is a deficiency of ice-crystals and this deficiency has to be rectified by introducing artificial ice-nuclei like silver iodide into suitable clouds to produce the required ice crystals for improving the rainfall. One nucleous per litre of cloud air is required for an efficient precipitation under the static approach. But the dynamic cloud seeding approach involves the injection of 100 to 1000 nuclei at -10oC per litre of cloud air. In the process the conversion of super cooled liquid water to ice-crystals releases the latent heat of fusion which in turn increase the cloud buoyancy. Such buoyancy invigorates the cloud and extends its life-time, resulting in increased convergency at the base of cloud. Moreover, such dynamic seeding ultimately results in better collection of the low level inflow, thereby increasing the probability of merger of several nearby clouds and enhancement of rainfall. Cold cloud seeding was done notably in Australia, USA, USSR, Canada and Israel where convincing and significant increases in rain fall were observed on seeded days. Cloud seeding experiments in Australia showed 15% to 20% extra rain fall in the water storage reservoirs of Tasmania. Similar experiment in Israel indicated extra rainfall of about 15% over the catchment of Tiberias lake at a highly statistical significance. Dynamic seeding of convective clouds in South Florida indicated 20% extra rainfall over the target area and 50% more in the area defined as the floating target.
A cloud seeding experiment is very inexpensive and highly successful provided it is undertaken after making the pre-requisite scientific studies including modeling. The rain clouds of Israel area 6 to 9 km high and contain 0.5 million cubic meters of water. By dynamic aerial seeding with silver iodide, the heights of the clouds are raised by 2km and their water content increased to more than 1 million cubic meters. Studies in Ukraine indicate that while seeding a single cloud increases precipitation by about 25% over that of an unseeded cloud, the seeding of multi-cell clouds leads to very high precipitation as compared to single cell clouds. Thus cloud seeding has been successful in several countries including USA, Israel, Russia, Canada, Australia and India.
CLOUD SEEDING BY GROUND GENERATORS:
In this method, silver iodide powder is sprinkled over the white hot coal in the ovens at a temperature of 1200oC, produced through blowers at the ground level. 200gms of silver iodide powder is sprinkled over each hot oven at the rate of 1 tea spoonful or 5 to 7gms every 4 minutes. The hot silver iodide fumes will raise into the clouds and the nuclei thus produced interact with the clouds and it results in good precipitation, if the required conditions are satisfied. For instance, the thickness of the cloud should not be less than 5000ft. Humidity should not be less than 75%. The wind velocity should not be more than 20kms per hour.
90% of the clouds in the tropics are warm clouds whose tops are 3.5km to 5.5kms above the earth. If the base of these clouds is too high, the rain drops evaporate before reaching the earth. Hence the clouds to be selected for seeding must have a good seedability factor, 1.5kms deep with their base at about less than 1.5kms above the ground. Among the favourable factors for seeding are: a relative humidity of about 75% and wind speeds of less than 20kms per hour. As the life span of the usual clouds vary from 30 to 60 minutes the seeding operations have to be planned on scientific lines. Suitable conditions for seeding exist for about 20 days in a year during the South West monsoon in India. Among the Tropical countries are 1) Mexico, 2) Honduras, 3) Kenya, 4) Zimbabwe, 5) South Africa, 6) India, 7) Thailand, 8) South China, 9) Indonesia, 10)Malaysia, 11) Philippines, 12) North Australia and 13) South Arab states . (Fig-7)
FAVOURABLE CONDITIONS FOR INDIA:
Southern states can seed the clouds for some days in the year even during the North East Monsoon period. In addition, they can convert the challenges posed by the frequent depressions and cyclones on the East Coat into opportunities for seeding the clouds that spiral inward towards the eye of the cyclones and thereby not only minimize the cyclonic damage but also augment the annual precipitation in the interior drought-prone regions of the Southern states.
By over seeding the chain of clouds approaching the coromandel coast during the North East Monsoon, the clouds can be made to grow substantially in size and extra precipitation can be obtained by re-seeding them in the rainfall deficit interior areas. If the Tamil Nadu, Karnataka and Andhra Pradesh State Governments and Tirumala Tirupati Devasthanamenter into an agreement to take up cloud seeding work the drinking water supply, agriculture, forestry and environmental problems of the South can be solved. Extensive cloud seeding in the catchments of Pennar, Krishna, Araniyar, Korathalayar, Kaveri and Palar river basins can be taken up and the additional water thus obtained can be stored by improving the capacities of existing reservoirs or constructing new ones at suitable places.
MITIGATING HURRICANE DAMAGE:
The coastal districts of Southern states are constantly facing serious damage due to cyclones. The cyclones are causing thousands of deaths among human and animal populations apart from loss of crops, houses, transport and irrigation systems worth hundreds of crores of rupees per annum. Inspite of the timely warnings about the impending cyclonic hazards, it is difficult to avoid damage to life and properties due to several known and unknown factors and hence the academicians must develop methods to hunt and tame these deadly cyclones. While the impact of a cyclone on a desert may be negligible its impact on coastal villages and fertile fields is highly detrimental. Since the force of he wind in a cyclone varies with the square of the wind speed, a wind of 40 meters per second exerts 4 times as much force as a wind of 20 meters per second. It means a 10% reduction in wind speed causes a 20% reduction in the wind force, with a corresponding reduction by 20% in the damage caused by the winds. In view of the valuable properties and crops in a delta, a 15% reduction in the wind speeds by cloud seeding may result in about 40% reduction in the costs of damage.
Most of the energy needed to create and sustain a cyclone is derived from condensation. The warm moisture rises sharply towards the cyclone band that stretches from 16 to 80km from the centre. As the air in this band rises up, fresh air masses from the ocean, rush into the central zone from distant places.
The air moves towards the centre in the lowest 3kms while the layer between 7 and 12kms height has air masses that flow with maximum wind speed with an outward component. In the middle layer between 3 to7 km height there is little inflow and outflow. Satellite pictures show that a cyclone has spiraling cloud bands that converge to form a ring of clouds around the eye. To curb the cyclone several scientists propose the injection of freezing nuclei in the massive wall clouds around the eye of he cyclone. Silver iodide crystals cause the super cooled water of the ice to freeze rapidly resulting in the release of latent heat of fusion. Such introduction of heat in parts of the hurricane causes reduction in wind speeds and promotes cloud growth when more water vapour in the clouds condenses, making further release of more latent heat of condensation. By redistributing the energy in a narrow band of clouds curving around the centre of the cyclone, the goal of cloud seeding(to drastically reduce the damage due to a cyclone can be achieved)
When the US Navy seeded the clouds in the experiments on Hurricane “Esther” on 16-9-1961, a 10% reduction in wind speed was obtained. Again when Hurricane “Beaulah” was seeded with silver iodide on 24-8-1963 about 14% reduction in the maximum wind speed was obtained. On 18-8-1969 when Hurricane “Debbie” was seeded 5times within 8 hours, a 30% reduction in maximum wind speed was obtained. While the seeded clouds in the rain band grow by dynamic seeding upto the divergent region on the top, the increased inflow into them near the sea surface deflects the air currents from the bottom of the existing eye wall. As the inflow into eye wall clouds is reduced the original eye-wall gets weakened slowly when a new and broader eye is formed with a considerable reduction in the peak wind speeds. Since western countries are not interested seriously in these experiments, tropical countries like India must work in collaboration with the United Nations on “weather modification” projects to minimize the recurring colossal economic losses caused by droughts, floods and cyclones. (Fig-8)
Recent studies in USA and Israel provide enough hope that there are new methods that can be used to drastically mitigate the impact of a hurricane and thereby avoid an economic disaster of any country. It is suggested that by dropping suit from an aeroplane at a great height of about 16kms from the ground by using aeroplanes the fury of the cyclone can be drastically reduced as clearly stated in the illustrative diagram as presented below .
FREEQUENTLY ASKED QUESTIONS BY EXPERTS IN COLLETERAL FIELDS:
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.
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.
Why should we be bothered to take up cloud seeding operations inspite of opposition?
Opposition to cloud seeding is based upon prejudices and some times lack of awareness of the urgent need for cloud seeding as a potent weapon to fight the droughts and the damaging impacts of the emerging global warming. Since one billion people in the world are facing drinking water crisis and since this figure is going to double in the next few decades we do not have any other choice to augment water supplies for the survival needs of the millions of poor people and the environmental resources. Most of the clouds do not give rain because of inadequate depth .Even if the cloud is sufficiently deep it does not give rain because of insufficient nuclei present as ice-nuclei in case of cold clouds and as giant size hygroscopic nuclei in the case of warm clouds. When nature provides suitable nuclei to these clouds they give only 10 to 20% of their water content as precipitation and the remaining water content gets wasted away as moisture in the atmosphere. In order to extract more water from these clouds it is necessary to provide them with the optimum number of chemical nuclei so that additional moisture from the cloud can be harnessed and precipitated as rainfall or snowfall. Thus additional rainfall upto 60 to 65 Billion Cubic Meters from the clouds is planned to extract per year from the clouds by the Chinese at a very inexpensive cost of $0.02 per cubic meter with a cost benefit ratio of 1:15. Since about 40 countries are successfully conducted cloud seeding operations during the last 40 to 50 years for augmenting snowfall, rainfall and for suppressing the hailstorms and dispersal of the fogs in the airports it is essential that all meteorologists in all countries must learn from their counter parts in China to fight the man made and natural weather hazards for the benefit of man kind.
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.
CLOUD SEEDING AND PRECIPITATION EFFICIENCY:
Cloud seeding experiments if planned on scientific lines only become successful. It is necessary to realise that some clouds due to low depth will not give rain. Some clouds due to low updrafts and under atmospheric inversion conditions may dissipate before seeding operations attain a maturity stage to make the clouds precipitate. Most cumulus clouds evaporate without producing rain and some convective clouds also just dissipate at about the time good rain emerges from the base of the cloud and scientists found that some clouds are destroyed often by the precipitation forming within them and such a precipitation break has not been fully understood.
One important concept relating to shower clouds is the precipitation efficiency which is defined as a ratio of rainfall touching the ground to the total quantity of water vapour going up through the cloud base. For very small non-precipitating clouds precipitation efficiency will be zero. It will be 10% for small shower clouds and 50% for very large thunder clouds. In small unseeded thunder clouds about 80% of the cloud droplets generated ultimately get evaporated and hence its precipitation efficiency is about 20%. However giant thunderstorms in squall lines (violent wind storms) are generally 50% to about 100% efficient in giving precipitation. Over small mountain barriers the clouds have precipitation efficiency of about 20% .
Scientists found that clouds with large updrafts above 25metrs per second (5000 ft per minute) are inefficient. A cumulo-nimbus cloud with a water vapour flux of 5ktons per second in South Dakota recorded only 3 percent precipitation efficiency, perhaps due to moisture loss by evaporation from the edges of such clouds in their downdrafts or due to large outflow of condensate including ice particles through the large anvil at the cloud top.
The predominant cumulus clouds can be classified into 3 types for convenience of our study with their base height of 0.5km from the ground level. The small size cloud is 0.5km in diameter and 2.5 km in depth and extends upto +8oC level in the sky. The medium cloud is 1km in diameter and about 5.5 km in depth and extends upto –8oC level. The large cumulus is 4km in diameter and about 12km in height and extends upto –50oC into the sky.
Just like human beings the clouds have a life cycle and they are born, they grow up, they get aged and ultimately die. Some of the small clouds have a life span of 5 to 10 minutes, the medium sized ones last for 30 minutes and larger ones may live for an hour or more. While a small cloud may contain water vapour passing upward through its base of above 1 kilo tonne, a large Cumulo nimbus may process about 10 kilo-tonnes per second of water vapour, amounting to about 50,000 kilo-tonnes during its life span of about 1.5 hours. Just as in human life, there is a balance to be maintained between the forces of buoyant growth of a cloud and those of its destruction.