Under normal conditions, when a mass of air is saturated, further cooling will lead to condensation. As water vapour does not contain solid impurities, the tiny liquid droplets formed as a result of condensation do not freeze and can remain in a super-cooled liquid state even down to minus 40 degrees Celsius. The super-cooled liquid droplets are in a state of partial instability described as “Metastable”, wherein the introduction of nuclei will trigger the freezing process. The technique of cloud seeding in such a situation involves spraying of Glaciogenic (ice forming) nucleating agent in the form of dry ice (frozen CO2) or silver iodide in the layer of cloud mass where super-cooled liquid droplets predominate. When in contact with the nuclei, the super-cooled liquid droplets freeze and continuously grow in size due to contact with adjoining liquid droplets till they are large enough to descend to the ground as snow flakes if the atmospheric and surface temperatures are below freezing. Formation of snow flakes is described as the “Bergeron Process” and the method called “Cold Cloud Seeding” is effective in winter but is also employed in summer months in vertical development clouds such as large cumulus and cumulonimbus clouds where super-cooled droplets are available.

Dry ice has a very brief life span as it evaporates quickly and hence to be effective, it must be injected accurately in terms of both time and space. Silver iodide is injected in vapour form obtained by burning silver iodide candles carried on the flare racks behind the trailing edge of the wings.  On dispensing, the vapour condenses to form a cloud containing billions of silver iodide crystals of the size of 0.01 to 0.1 microns. Unlike dry ice, silver iodide cloud has considerable staying power and hence has better chance of inducing precipitation. In a vertically developing cold cloud, seeding is done from on top of the cloud by ejecting ignited silver iodide candles vertically downwards into the cloud from underbelly ejector racks fitted on the aircraft.