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In this article we will discuss about:- 1. Meaning of Cloud Seeding 2. History of Cloud Seeding 3. Types.
Meaning of Cloud Seeding:
Cloud usually consists of very small droplets of water which cannot fall appreciably below the cloud base without evaporating. In this condition clouds are stable. Rain drops are about a million times heavier than cloud drops and rain develops only if the cloud droplets grow by some mechanism.
Cloud seeding mechanism depends on the difference in vapour pressure over ice and super-cooled water at the same temperature. If ice crystals form in a super-cooled cloud, water evaporates of the droplets and migrates to the ice crystal, which grows and begins to fall increasing their growth rate by coalescence with other droplets and eventually melting and falling as rain drops.
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Presence of ice nuclei determines whether ice crystals form in a super-cooled cloud or not. Ice crystals form on naturally occurring nuclei at temperatures of -10° to -30°C.
At warmer temperatures, either the efficient ice forming nuclei are often deficient in the atmosphere or the cloud may occur with tops super-cooled but not to an extent which activates the ice nuclei available. It should be possible to initiate precipitation by introducing ice crystals or by supplying artificial nuclei.
History of Cloud Seeding:
Vincent Schaefer (1906-1993) discovered the principle of cloud seeding in July 1946. Along with his associate Langmuir, he created a way of experimenting with super-cooled clouds using a deep freeze unit of potential agents (salt, talcum powder, soils, dust and various chemical agents) to stimulate ice crystal growth.
The experiment was easily replicated and he explored the temperature gradient to establish the -40°C limit for liquid water. Within the month, Schaefer’s colleague, atmospheric scientist Dr Bernard Vonnegut is credited with discovering another method for “seeding” super-cooled cloud water.
Dry ice and silver iodide agents are effective in changing the physical chemistry of super-cooled clouds, thus useful in augmentation of winter snowfall over mountains and under certain conditions and lightning and hail suppression.
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In India, cloud seeding operations were conducted during the years 2003 and 2004 through US based Weather Modification Inc. in the state of Maharashtra. Cloud seeding in 12 districts of Andhra Pradesh was initiated in 2008.
Types of Cloud Seeding:
i. Dry Ice Seeding:
It is to change super-cooled cloud droplet to ice crystal in the laboratory and in the atmosphere by dropping pellets of dry ice into the cloud and causing large number of ice crystals to appear spontaneously. Dry ice is solid frozen carbon dioxide at a temperature below -80°C. An air craft drops dry ice pellets of size 0.5 to 1.0 cm at the top of super-cooled clouds.
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They fall through the cloud and result in a sheet of ice crystals. This method is reliable for stimulating showers from cumulus clouds provided their tops have temperatures lower than -5°C and the clouds last for more than 30 minutes.
ii. Silver Iodide Seeding:
Minute crystals of silver iodide produced in the form of smoke acts as efficient ice forming nuclei at temperatures below -5°C to produce enormous number of nuclei (1015 per gram of silver iodide). When this smoke is introduced into super-cooled cloud, some ice crystals appear when temperatures fall below -4°C, but its formation rapidly increases with decreasing temperatures.
Cloud seeding by silver iodide is done either from ground generators or from airborne generators. Substances other than silver iodide as artificial nuclei are lead iodide, cupric sulphide, cupric oxide, ammonium fluoride, cadmium iodide and iodine. However, all these are not as effective as silver iodide.
iii. Warm Cloud Seeding:
It is a process in which nuclei of droplets grow to radii of several microns in coalescence between drops. Once drops have grown to about 40 µ dia, they are capable of colliding with and sweeping up smaller droplets in their path. Once the droplets reach the size of 60 µ dia, coalescence dominates and condensation can be virtually ignored. Coalescence process is mainly responsible for growth of rain drops in warm cloud.
In this case, seeding would be based on the assumption that the presence of comparatively large water droplets is necessary to initiate the coalescence process and some clouds do not precipitate or do so inefficiently due to absence of such large water droplets.
This deficiency can be remedied by introducing in the cloud, water droplets or hygroscopic nuclei. Salt particle of 3 µ becomes 50 µ particle of brine in saturated air of cloud. Water droplets of 20 to 30 µ introduced in the cloud through aircraft at the rate of 30 gallons per minute appears to be effective for obtaining the rain.
Experiments have clearly established that cloud seeding with silver iodide or dry ice in the case of cold clouds and sodium chloride or minute water droplets in the case of warm clouds can stimulate rain from suitable clouds. However, practical experience as of now, may not suggest recommending it for adaption as an option for drought during crop season.