C LOUD S EEDING
C LOUD S EEDING
18.1 CONDITIONS FOR CLOUD SEEDING
Between the vanishing forests and the reluctant clouds, the draught has advanced menac- ingly. This is where cloud seeding will help win the battle.
Cloud seeding was discovered as accidentally as most scientific discoveries. In 1946, when Dr. Vincent J. Schaefer working with GE in New York was trying to create artificial clouds in a chilled chamber, he placed dry ice inside to cool the chamber. Water vapour inside formed a cloud around the dry ice. That is pretty much how in a normal cloud ice crystals form when cold water contacts particles of dust, salt or soot.
In clould seeding, silver iodide or other agents like common salt are introduced, which mimic the ise crystal; the number of these nuclei available is increased and these can take in more moisture in the cloud and form raindrops, which otherwise would not have formed. This increased condensation and freezing releases a large amount of heat that makes clouds more buoyant and extends them sidewards and upwards. As clouds grow taller, their updraft in- BLOG creases, they draw in more moist air from the near surface and their size increases further.
Enlarged clouds then encroach over several smaller clouds nearby and grow further, and hence the duration and quantity of rainfall will increase.
Rain clouds 6–9 km high and containing 0.5 mcm (million cubic meter) of water seeded with silver iodide can be raised by 2 km and their water content increased to >1 mcm. Seeding of multi-cell clouds leads to very high precipitation as compared to single cell clouds. Cloud seeding has been carried out quite successfully in several countries including USA, Israel, Russia, Canada, Australia and India. Clouds have been seeded at a height of around 2 km above sea level.
However, it should be known that cloud seeding can only accelerate and increase the amount of rainfall and not create rainfall, when the conditions are not favourable, as what hap- pened when seeding was done in 1975 in Linganamakki area of Sagar (Shimoga Dist., Karnataka); the seeded clouds had drifted away do to wind; also the clouds were floating below 1.2 km level.
Cloud seeding can not be done in the areas where there are no chouds as it can only make the bad clouds to yield more.
The conditions favourable for cloud seeding are: (i) the lower surface of the clouds should be within 1 km from the ground surface.
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(ii) the relative humidity must be high, >75% and wind velocity < 15–20 km/hr. (iii) the temperature inside the cloud should be less than the freezing point of water. Cloud seeding can be done either from above or from below, as illustrated in the following. Seeding from above is resorted when a large area is concerned. This requires special
seeding material. The four major requirements are: (i) equipment of clould-condensation-nuclei measurement, (ii) facility for photographing of cloud growth, (iii) temperature measurement, and (iv) 2-D Doppler Echo measurement of precipitation particles.
The weather radars are setup at land surface at air bases for locating the movements of rain potential clouds and will cover nearly 400 km 2 . A Doppler weather radar with a range of 200–400 km was setup at air bases of Bidar, Hubli and Jakkur (in Karnataka).
The operation of monitoring the rain efficient clouds are done through the computer- aided system on board the cloud seeding aircraft with the support of the communication devices from the base station. The turbo-prop aircraft titled Piper-PA 31 with special equipment and extra fittings arrived from the US-based Weather Modifications Inc. (WMI) for enhancing the rainfall by 25% in the drought-hit districts of Karnataka following the failure of rainfall for the third consecutive year at a cost of Rs. 5.6 crore, in August 2003. The tender proposal was prepared by experts from IITM and IMD.
The WMI, a cloud seeding company based in Fargo, north Dakota in the USA, has carried out cloud seeding operations in fifteen countries and most of their operations were successful. WMI had quoted Rs. 8 crore for the cloud seeding operations in Karnataka in August 2003. WMI’s competitor was Atmospheric INC. (AI), which is also a US-based company.
had the radar and control room at Agni Aerosports in Jakkur Airfield at Bangalore. The radar BLOG
Experts of WMI started the 90-day ‘‘Project Varuna’’ from August 18, 2003. The WMI
to scan for rain-bearing clouds in the range of 200 km and give informulation to the pilot and crew of the aircraft, who in turn fly to the region and carry out the cloud seeding operations.
While the popular form of cloud seeding is through spraying from aircraft, approach from below also has been used, as in Hyderabad in July 1993. Using ground generators consisting of hot ovens and a blower, coal is heated to 1200 °C and silver iodide sprinkled. The rising fumes went straight into the sagging clouds. In minutes-there was a downpour for half an hour resulting in increased waterlevel in Osman Sagar Lake (Hyderabad). The samples showed traces of silver iodide proving the rains were due to the seeding.
18.2 CLOUD SEEDING TECHNIQUE
When nature is reluctant to produce ice in super-cooled clouds, it is possible to lend a hand by providing the ice nuclei that nature is lacking. This is commonly called cloud seeding and it could be done with a variety of ice-inducing agents like silver iodide or sodium chloride.
When silver iodide is used, small amounts are burned in flares or solution at the cloud top or in the updrafts at the cloud base with the help of aircraft flying, the rapid development of large number of small ice droplets soon follows.
When sodium chloride is sprayed on to the clouds with the help of aircraft, it precipi- tates the cloud particles, which are usually of 100 microns, µ, (1000 µ = 1 mm), into larger
CLOUD SEEDING
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particles upto 6-7 mm. When the particles increase in size, they can not hold together and come down to the earth resulting in rain. This precipitation also happens naturally during normal rains.
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Fig. 18.1 A hygroscopic cartridge being burnt to demonstrate the process of cloud seeding, which was inaugurated at the Jakkur airfield in Bangalore on Monday-Aug. 2003
Fig. 18.2 An aircraft fitted with flares containing nuclei used for cloud
seeding operation can be seen in this file photo
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Cloud seeding however, artificially presses the cloud particles to precipitate and result in rain. The chances of rains increase by about 30% due to cloud seeding.
Though sodium chloride is normally used in the tropical region which gets Warm rains (where small cloud particles become bigger resulting in rain), silver iodide is used in cold
result in rain. Silver iodide enhances the chances of water particles freezing. Both the chemicals were used for cloud seeding in Karnataka during August 2003.
18.3 CLOUD SEEDING OPERATION
To begin with, one has to identify the clouds, which can not yield rains due to shortage of (ice/ water) nuclei; after that, the exact number of nuclei to be added to such clouds to make them yield rains, has to be found out.
Then, the technical team will calculate the quantity of silver iodide that has to be added to the clouds to get the required increase in nuclei. This could be done through computer-aided system on board the aircraft with the input as well as communication from the base-station. The addition of nuclei would be facilitated by burning silver iodide flares. The crew may have of six members—two pilots, two meteorologists and two technicians for carrying out the cloud seeding operations. The aircraft should be capable of 6 hr endurance at operational altitudes, having computer-aided control for scientific management of cloud sceding operations.
Cloud seeding is not so costly and the benefit-cost ratio may be around 60: 1. Several scientists are still sceptical as to how effective the cloud seeding is ! Is the rain in most cases due to the seeding or was it anyway poised to rain, they ask. But one thing they all agree, is that there have been no negative effects from cloud seeding so far with regard to polluting the environment or the chemicals used. The criticisms are about the possibility of creating bad weather or floods; and of course increasing rainfall in one region is at the expense of rainfall in
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a neighbouring local region.
18.4 RECENT CASE HISTORY
Russian air force’s special squad air craft was pressed into service on Monday (9th May 2005) to clear cloud-cover over MOSCOW, so that no rain clouds dampen the Victory Day celebrations (marking the 60th anniversary of the Allied Victory over Nazi Germany in World War II) attended by leaders of 53 nations.
At 5.30 A.M. (local time), 12 Antonov An-12 and Ilyushin IL-18 aircraft took position in the sky over Moscow, divided in 10 zones, as a cyclone from the west advanced towards the capital.
Flying at 3-8 km altitude, the planes sprinkled various chemicals including silver iodide, liquid nitrogen, dry ice (frozen carbon dioxide) and even ordinary cement to seed the clouds at
a distance of 50-150 km away. By 10 A.M. sharp at the start of the military parade, rain had stopped in Red Square and the sky was clear for the impressive flypast by Sukhoi and MIG fighters. For the first time in Moscow, computer-designed fireworks were used on Monday night, and the efforts were to keep the sky clear till date in the night.
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