Rainfall occurs when supercooled droplets of water – those that are still liquid but are at a temperature below the usual freezing point of zero centigrade – form ice crystals. Now too heavy to remain suspend in the air, these then fall, often melting on their way down to form rain.

Even in dry areas the air usually contains some water. This can be made to come together and form ice crystals by seeding the atmosphere with chemicals such as silver iodide or dry ice.

They work to promote rainfall by inducing nucleation – what little water is in the air condenses around the newly introduced particles and crystallises to form ice.

The ‘seeds’ can be delivered by plane or simply by spraying from the ground.

The process of condensation of the droplets is very important. It takes place as the air temperature drops; with that, the capacity of air to hold water vapour is reduced. If cooling continues, a point comes where the amount of water that the air can hold reaches its maximum capacity. This temperature is called the dew point. At this point, water vapour begins to condense into tiny droplets.

The condensation process can be of two types — ‘warm rain’ and ‘cold rain’. The latter happens in tropical regions; the process is so called because precipitation falls from clouds where the temperature is more than 0c. Rain is formed in the ‘warm clouds’ when larger water droplets collide with and absorb smaller ones. Substances capable of absorbing moisture from the atmosphere — such as common salt — are used for inducing ‘warm rain’.

The ‘cold rain’ process occurs when the temperature, in all or some parts of a cloud, is less than 0c. These clouds are usually composed of both ice crystals and water droplets. The crystals grow rapidly, drawing moisture from the surrounding water droplets, until their weight causes them to fall. While falling, the crystals may melt and join with small liquid water droplets; they may then form into raindrops in a manner similar to the ‘warm rain’ process. If the crystals do not melt, they may grow into large snowflakes by agglomeration and reach the ground as snow. Silver iodide or dry ice (solid carbon dioxide) is used to supply naturally deficient clouds with the proper concentration of ice crystals to increase rainfall through the ‘cold rain’ process.

Source: NCERT