Precipitation in clouds may be initiated by two different processes. One of those is the coalescence process which is favoured in clouds that are relatively warm with high water content. In this process once the precipitation particles are formed they grow primarily by sweeping out cloud droplets on its trajectory or by combining with one another. This process depends upon various factors such as water content and droplet size.
The second process is known as ice crystal process. Ice crystals appear to form in clouds when temperature drops down to -15 degrees Centigrade. In general, water in contact with most materials freeze at temperatures warmer than -40 degrees Centigrade, and sublimation will occur on most materials at super cooling’s. So the ice crystals present in clouds serve as ice nuclei (around which a droplet may form). So precipitation may be encouraged by exploiting one of these two mechanisms.
Meteorological conditions essential for artificial production of rain are similar to those leading to natural rainfall. In natural rain process in convective clouds (warm clouds) one droplet in 10 6 grows to become a raindrop. This is about one droplet in five litres.
One approach to stimulate rain in warm clouds is to increase this concentration of large droplets by spraying water from airplanes flying at cloud base. These droplets (radii of 20-30 microns) should be large enough to be in a favoured position for growth. Salt particles may also be injected in cloud base to provide centres on which cloud droplets can form. For super cooled (below freezing) clouds, the most effective seeding agent is dry ice.
Nucleation occurs most readily on surfaces having a lattice structure geometrically similar to that of ice. The material that most closely approximates ice in lattice structure, known so far, is silver iodide (AgI).
In case of stratiform clouds (clouds with no vertical extent but cover a large area) whose top is super-cooled, the natural precipitation process may proceed slowly due to scarcity of ice nuclei.
In such cases the introduction of ice crystals near the cloud top by seeding AgI or dry ice may cause precipitation that would not occur otherwise. The introduction of these agents artificially into the clouds in concentration of about one per litre is expected to promote precipitation.
Some of the clouds which cause rain are the stratus and stratocumulus clouds. Cumulonimbus clouds are the main rain clouds.