I've heard that dry ice, iodides have been used as seeding agents to induce rainfall. Is this technique really being used widely? What are the prerequisites for initiating seeding? Is it as bounteous as natural rainfall is?
Cloud seeding has been attempted for well over a century, with mixed success. There is no doubt at all that, locally, rain can be increased by seeding the cloud base with such nucleating agents as dry ice or silver iodide smoke. There is a great deal of doubt as to whether there is any overall gain in rainfall - or is it just an expensive way of 'robbing Peter to pay Paul', of increasing rain in one location at the expense of rainfall further downwind. Is there any nett gain in regional rainfall? Probably not. Of course, there are many cases where locally increased rainfall may be worth the effort.
Not all cases of cloud seeding work. There are several requirements. First, the process involves vigorous convection within the cloud towers in order to loft the nucleating agent into a sufficiently super-cooled part of the cloud. If the cloud is too warm it won't work - which rules out many tropical applications. Second, the 'seed' has to be injected into the cloud base at about -10 to -20 deg C, which allows 'graupel' to form, i.e. a kind of granular ice that flash freezes onto snow or ice crystals direct from super-cooled water vapour. Third, the timing has to be right, just as the cumulus towers are beginning to form, usually in the early afternoon. Fourthy, the economics are very moot. Wikipedia ('Cloud Seeding') claims that the subject is no longer 'fringe science', which is true in that the physics have been studied seriously, but mostly untrue in the sense that it is now considered economically attractive. The great majority of studies have shown that cloud seeding is unreliable, expensive, and of dubious value, with other ways of delivering water (such as desalination) being a better bet.