How is artificial rain produced?
R.Narasimhan, Chennai
ANSWER 1: The need to develop and improve rain-making techniques in terms of design, operation, monitoring and evaluation by giving them a more scientific character is today's need.
This includes using computers to study cloud formations and help the rain-making operations achieve the goals of the project. The role of weather modification, or rain-making, is an important component in water resource management.
The process involved in artificial rain-making involves three easy-to-understand stages. The first stage is agitation. That is using chemicals to stimulate the air mass upwind of the target area to rise and form rain clouds.
The chemicals used during this stage are
calcium chloride calcium carbide,
calcium oxide, a
compound of salt and urea, or a compound of urea and ammonium nitrate. These compounds are capable of absorbing water vapour from the air mass, thus stimulating the condensation process.
The second stage is called building-up stage. Here the cloud mass is built up using chemicals such as kitchen salt, the T.1 formula, urea, ammonium nitrate, dry ice, and occasionally also calcium chloride to increase nuclei which also increase the density of the clouds. In the third stage of bombardment chemicals such as
super-cool agents:
silver iodide and dry ice are used to reach the most unbalanced status which builds up large beads of water (Nuclei) and makes them fall down as raindrops.
In planning every stage a high degree of expertise and experience is required, in selecting the types and amounts of chemicals to be used, while taking into consideration weather conditions, topographical conditions, wind direction and velocity as well as the location or delimitation of the area for chemical seeding. Several other ideas are also involved in rain making. Rockets containing rain-making chemicals can be fired into the clouds either from the ground or from aircraft.
A jet of rain-making chemicals is shot from a highly pressurised cannister directly into the cloud base, so as to coerce clouds which normally hang above mountain tops to cluster up and rain on the mountain or their slopes.
Rain-making chemicals are added to super-cooled clouds, i.e., those at altitudes above 18,000 metres, to stimulate the formation of ice crystals in the cloud or cloud cluster.
The Hindu S&T desk
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ANSWER 2: Artificial rain is produced by spraying clouds with substances like Silver Iodide (costly) or cheaper ones like solid carbon dioxide (dry ice) or even finely powdered Sodium Chloride. The process is called seeding.
Often there are clouds, but no rain. This is because of a phenomenon called supercooling. The temperature of the cloud might be close to zero and there might even be crystals of ice in it.
The water vapour in the cloud does not condense to liquid water. The super cooling gets disturbed by spraying the cloud with the chemicals mentioned above, using a small aeroplane for the purpose.
The `super' phenomena (cooling, heating, saturation etc.) are perverse in a sense. Very pure water when heated in a clean vessel, often does not start boiling when expected. Crystals of the photographer's hypo (Sodium thiosulphate) easily dissolve in a little water when heated. But on cooling, crystals do not separate out.
If the vessel is shaken vigorously, or if a small crystal of hypo is freshly added, then crystallization starts immediately.
Making artificial rain is a similar way of intervening in the super cooling phenomenon.
N.B.Nair, Banglore
http://www.hinduonnet.com/seta/2002/...3000190300.htm
Dù AgI đắt hơn CO2 rắn nhưng để làm mưa nhân tạo (hạ nhiệt độ xuống gần -30 đến -70oC) thì dùng AgI kinh tế hơn.
Thân!
Mưa nhân tạo
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