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Is Artificial Rainmaking a Solution to Drought?

by A M Choudhury
**Bangladesh is facing a terrible drought situation currently and drought is quite a common experience in Bangladesh and in many other countries. The history of artificial rainmaking goes back to 1946 when Vincent Schaefer of USA discovered the general principles. During the last 40 years cloud-seeding activities have stirred the imagination of scientists all over the world. If this could be carried out in an operational manner, mankind could have solved the problems of drought which threatens many parts of the globe with famine.
THE earth's atmosphere always contains some amount of water vapour which doe* not exceed 4% of the total volume of the air . The amount of water substance in a the order of two to three grama foe a square centimetre
capacity for holding water vapour. If the temperature decreases. the capacity of the air for holding the water vapour also decreases. In the case of dry air. the temperature decrease* with height by about 1°C per 100 metre*. In the case of moist air. however, tht* value could be 1/2°C per hundred metre rise. If air containing water vapour Is forced to rise in the atmosphere a* a result of convection, orographic uplift or the large scale motions associated with cyclonic storm*, it will expand and the temperature of the air will therefore fall and eventually may fall to the point where the water vapour contained tn it Is sufficient to saturate the air. Condensation of the water vapour will take place If there la a further cooling of the air and consequently cloud will form Unsaturated air can be brought to saturation either by adding water vapour or by cooling the air.
The condensation of water vapour tn the atmosphere always occurs on small particles called condensation nuclei which exist in air in varying concentrations. These particle* provide a centre around which the water vapour may rapidly condense. Larger parti etc* are more efficient in this respect than the smaller ones and under the same environmental conditions those particles that are soluble tn water produce larger droplets than do Insoluble particles. Over land, there are about 1OOO-100.000 particles tn every cubic centimeter of air and over ocean, their numbers vary from 100-1000 per cc. Their from 10’ cm to 10’ 4 cm tn radius.
If the atmosphere were entirely free of foreign particles and Ions (condensation nuclei!. air would have become supersaturated by a factor of seven (7) without being condensed. However, because of the presence of condensation of nuclei, water vapour may condense when the air Is very slightly supersaturated or even slightly undersaturated. A droplet of radius one ten-miL-Honth of a centimetre (10‘ cm) requires a supersaturation of 223% to persist while droplets of radius greater than 10’° cm require supersaturation of less than 1%. There are far more particles in the air than arc needed as condensation nuclei In clouds and as a result condensation always occurs on the larger particles.
A cloud which 1* not producing rain contains about 100 droplets In every cubic centimetre and the average radius of these droplet* is 10 microns. But the rain producing cloud consists of drops that are one millimetre In radius that is one thousand microns. In order that these droplets develop Into some of the rain drops, they must Increase in radius by one hundred times which correspond to an In-crease In volume by a mllbon fold The main difference between cloud and precipitation la the particle size How the cloud drop particle* grow
sire to form precipitation par ticle* ia the basic problem tn
observed that cloud* at temperature considerably below 0uC consist of liquid drop* rather than ice crystal*. The reason I* that the process of Ice formation whether by freezing of liquid drop* or condensation of vapour directly Into ice crystals. also requires nucleation. The nuclei of ice formation whether tn the liquid or from the vapour take effect constd erably below 0°C For water to freese. molecules in the liquid state, which exist to a large degree In random positions, must align themselves into definite crystallographic positions appropriate to the ice lattice. Thus clouds form ordinarily first as liquid drn^s even at temperatures of -15 C or lower. When the temperature Is sufficiently low. ice crystals occur and once they are present In sufficient concentration* the entire cloud tends to be transformed into ice crystals. The equilibrium vapour pressure over Ice is less than that of liquid water at temperature below 0DC and consequently ice crystals can grow in an air unsaturated with respect to liquid water. For example at -20°C, Ice crystals grow if the humidity exceeds 82%. at 40°C if it exceeds 68%.
Theory of natural rain
Collision and Coalescence Process: Those cloud droplets that grow by condensation to a size which is somewhat larger than the average will fall through a cloud with speeds greater than those of the smaller droplet. As a result the larger droplets may collide and coalesce with some of the smaller droplets lying In their paths. In this way the larger droplets In a cloud tend to increase in size fairly rapidly and on emerging from the base of the cloud may be sufficiently large to reach the ground as raindrops without being evaporated In the air. The productton of rain drops in this way is known as collision and coalescence process of rain formation. The larger drop in the air must exceed some minimum size before this process can proceed at any significant rate. It has been found that this critical size Is about 20 micron In radius. Rain from clouds with temperature greater than 0°C occurs due to this process and this Is frequently referred to as warm rain process. Apart from the condensation nuclei and critical size, electric fields and electric charges also have influence on coalescence. However, critical value* of the parameter* controlling th* rate of development of rain
Bergersoa-Ftadcsea Frathat for the formattorTof rain, a mixed cloud consisting of water drop* and ice crystals are necessary It is known that
I* less than that over water for a given temperature Hence tf both phases are present side by side In a cloud ice crystals grow at the expense of supercooled drops because of the vapour pressure gradient between the drops and ice erv*-tal*. Tht* vapour pressure difference between the Ice cry* tai* and water drop* Is very much greater than the vapour pressure difference between water drops and the environ ment. For example, at a temperature of -10DC. this Is greater by a factor of 1OO and so the ice crystals take up by condensation so much mots ture that the droplets start tr evaporate. Consequently, they begin soon to tall faster than the remaining cloud droplets and begin to collect these all the way taking advantage of the vapour pressure difference.
The latter process may initiate the former and the two acting together can readily lead to the formation of ratn tn subfreezing clouds. In clouds with temperature greater than 0°C. process one could be alone the activating process
The fact that supercooled cloud* are a common occurrence at temperature* above -20°C with the implication that ice forming nuclei are deficient tn the atmosphere, the possibility arose that supply of artificial nuclei could Induce artificial rain.
Artificial rain making
Attempts to increase rain by lighting of fires, the firing of cannons, production of electric discharges by kites, firing of guns and rockets, ringing of church bells and by prayer have long been practised by man from time immemorial. However, in recent times, methods are being practised based on physical processes of rain formation such as the fact that suitable cloud may be made to release their precipl tation by Introducing artificial nuclei. The possibility of ex tending this technique to produce economically increases of rainfall has created a good deal of Interest and lot of controversy. Modem rain making experiments are based on three main assumptions:
I) That either the presence of ice crystals in a supercooled cloud is necessary to release snow and rain by Bergeron Flndesen process or the presence of comparatively large water droplets to Initiate the collision and coalescence process. It) That aome clouds precipitate inefficiently or not at all because these agents are ** wr^aTuSta’defieiency can be remedied by seeding the cloud* artificially with either sohd cartoon dioxide (dry lee) or stiver iodide to produce Ice crystals or by introducing water droplet* or large hygroscopic nuclei
In general, the energy involved in Influencing the amount of precipitation by changing the atmospheric flow pattern* which determine* the amount of moisture being fed into the system and control the cloud forming processes I* so large that It is not economically feasible to introduce similar amounts of energy to alter them artificially. However, there appear to be circumstance* when small amounts of energy applied at the right time and place may produce the desired result
In 1946 Vlneml Sebarfer rr One-n! Company
while working with cloud* in a food freexer dry lee created several million* of Ice crystal embryo* (ice nuclei) In a supercooled cloud of 40 C Schaefer made the first field trial on 13 November 1946 when 31b of crusted dry ice was dropped along a line of 3 miles into an altocumulus deck whose temperature was about 20°C an by an aircraft. This and similar other experiments demonstrated that large areas of supercooled stratiform cloud were converted Into Ice cloud by seeding. Shortly after wards Bernard Vonnegut found that silver iodide was also an, effective nucleating agent with a threshold of -4°C One gram of sUver Iodide can produce 1013 number of nuclei by vapourizing an acetone solution of silver iodide in a hot flame. Then started the history of artificial rain making experiments. Dry ice has to be used for seeding from aeroplanes and hence It is very costly. On the other hand, stiver iodide could be sprayed from the ground by using silver iodide generators. However, stiver Iodide particles are subject to photolytic action In sunlight and Its efficiency Is reduced to one tenth after half an hour of exposure. Silver iodide has arrangements of molecules similar to that of ice tn at least one crystallographic plane and hence it serves as ice nucleus.
Evaluation of seeding experiments
It has been established that artificial seeding of certain types of clouds can modify their structure and tn some cases can initiate and Increase precipitation. The primary goal In seeding experiments is to demonstrate that an Increase in precipitation has taken place as a result of the seeding over the area under Investigation. This must be done using standard statistical techniques for convincing demonstration of success In moot of the early experiments and operations by commercial firms rigorous standards of statistical validity were not maintained. As a result. even though there was demonstration of positive Indication of increase of rain as a result of these experiments and operations, a definite statement could not be made. One of the difficulties which contribute to this uncertainty Is that the quantity of precipitation Is governed not only by the mk-rophyslcs of Individual clouds but also by the dynamic processes Involved in the large scale circulations. Question whether the observed change was the direct and Immediate consequence of the treatment or would have occurred with out 1t Is difficult to answer, he cause the latter quantity cannot be known. Hence It Is nee esaary to turn Io statistical procedure to evaluate the ef-
The estimation of precipitation which would have fallen tn the absence of seeding In much harder than measuring the amount which did fall. Because of the large variability of the amounts of precipitation from year to year, cllmatotogl cal normals tor comparisons cannot be used Target-control area regression is one of the predictive devices used for this purpose. In this procedure an area near the target and otherwise resembling it as much as possible in selected for comparison. It is assumed that natural variation of pre scriptton at the control area are closely similar to those at the target area, so that the amount which would have fallen at the target in the ab sencc of seeding can be estimated from the precipitation which fell at the control area during the period of seeding. It ts further assumed that seeding does not spread Into the control area to con tarn t nate It.
Since the discovery of seeding effect, numerous cloud seeding experiments have been carried out over the last four decades tn different parts of the globe with a view to increase rainfall by seeding clouds with solid ice or silver Iodide. But with the exception of Israel experiment, there is little convincing evidence that seeding clouds with artificial nuclei induce economically significant increases tn pre cipftatton that can be distinguished from natural variation of rainfall However, in recent years, a number of long-term national experiments have been conducted based on statistical designs incorporating randomization between target and control areas and seeded and unseeded periods under the supervision of reputably scientists experienced In the field of cloud physics and cloud seeding. In some cases a consistent decrease of rainfall has been noticed as a resu't of seeding. However, cloud seeding activities have stirred the imagination of scientists all the world over and a great deal of progress has been made in cloud physics.
(The writer is. director. Bangladesh Space Research and Remote Sensing Organisation (SPARSO J