It’s a practice of artificially inducing or increasing precipitation through clouds by adding external agents.
The foreign particles which are drenched over these clouds can be Dry Ice(solid carbon dioxide), Silver Iodide, Salt powder etc. This process is known as Cloud Seeding. This stimulation is done by aeroplanes or rockets. And then finally "rain on".
Why Artificial Rain?
We know how the process of rain works. It starts from evaporating of water from the sea or any other water body. Then it travels upstream as the wind rises high in the troposphere(the bottom layer of atmosphere) causing some drops to merge and then this merging process goes on and when the thickness of the drop come in the vicinity of 0.1mm they cannot float up there anymore and finally we get bestowed by the almighty rain.
But then why do we have to tackle drought? Often there are clouds but no rain. Well, one could try to make it rain. And that’s where ARTIFICIAL RAINFALL comes along. When the temperature of clouds is close to "0" and there might be crystals of ice in it. Then water vapors do not condense to its liquid form. This is metalized by the cloud seeding and once again it rain, but this time artificially
How it is done?
Agitation : The first stage includes the use of chemicals to stimulate the air mass upwind of target area to rise and form clouds. These chemicals absorbs the water vapour and helps in the process of condensation. Chemicals like - Calcium Oxide, compound of Urea and Ammonium Nitrate or Chloride Calcium Carbonate.
Building Up Stage : In the second stage the mass of the cloud is build up by urea, dry ice, kitchen salt etc to increase the density of the clouds.
Seeding : In the final stage of persuing the artificial rain, super cool chemicals (dry ice or iodide) are bombarded with the presurized canister onto the base of the clouds to build the beads of water and make them fall as rain.
Its a simple phenomenon. Let me explain you with an example, think of a water tank with many impurities and dust particles floating over it, when you add alum to the water tank, the impurities settles down. What happens then? The particles collide with each other after adding alum as a catalyst, becomes heavy and settles down. That's how it goes up there in the clouds. The charged particles are suspended and are very small to fall down as rain. That's why an agent is sprayed over there which makes them collide and thererby it becomes heavy and falls down. kudos! you got the whole concept of artificial rain.
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