CONDITIONS FOR CLOUD SEEDING
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- 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.
HYDROLOGY
(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
aircraft that can travel to heights where precipitation occurs, and can carry large loads of 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.
Experts of WMI started the 90-day ‘‘Project Varuna’’ from August 18, 2003. The WMI had the radar and control room at Agni Aerosports in Jakkur Airfield at Bangalore. The radar 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.