The ability to form large cloud droplets from existing water vapour molecules requires an artificial agent, Cloud Condensation Nuclei (CCN) that has hygroscopic properties aiding in attracting, absorbing and retaining water vapour in the atmosphere. Natural table salt or sodium chloride is an example of such a hygroscopic agent.
The general aim of cloud seeding is to improve the effectiveness of cloud microphysical processes involved in forming water droplets from existing water vapour molecules. After cloud droplets are formed, they must increase in size by about a million times in order to become rain droplets. In order for this to occur, the cloud droplets will have to undergo multiple collision-coalescence processes (collisions of droplets of different sizes result in bigger droplets). The process of cloud droplets colliding and coalescing results in the release of latent heat, which encourages further collision - coalescence processes to occur. Cloud droplets can only coalesce or combine if they have opposite electrical charges; otherwise, they will simply bounce off one another. As the cloud droplets grow larger and heavier, they begin to fall through the cloud. Larger drops fall faster and simultaneously collect the smaller drops as they continue to fall. This causes the droplets to grow even larger and therefore increases the intensity of the resulting precipitation.
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