<P> Cloud seeding also occurs due to ice nucleators in nature, most of which are bacterial in origin . </P> <P> The most common chemicals used for cloud seeding include silver iodide, potassium iodide and dry ice (solid carbon dioxide). Liquid propane, which expands into a gas, has also been used . This can produce ice crystals at higher temperatures than silver iodide . After promising research, the use of hygroscopic materials, such as table salt, is becoming more popular . </P> <P> When cloud seeding, increased snowfall takes place when temperatures within the clouds are between 19 and − 4 ° F (− 7 and − 20 ° C). Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing nucleation . </P> <P> In mid-altitude clouds, the usual seeding strategy has been based on the fact that the equilibrium vapor pressure is lower over ice than over water . The formation of ice particles in supercooled clouds allows those particles to grow at the expense of liquid droplets . If sufficient growth takes place, the particles become heavy enough to fall as precipitation from clouds that otherwise would produce no precipitation . This process is known as "static" seeding . </P>

Cloud seeding using silver iodide only works in​