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supercooling?
http://en.wikipedia.org/wiki/Supercooling
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Supercooling is quite imprtant in weather. Supercooled water droplets commonly occur in clouds, and sometimes fog. When you have supercooled fog, you get rime ice formation on trees and bushes. Freezing rain and freezing drizzle occur when super-cooled rain drops fall, and rapidly freeze when they hit the ground, roads, trees or power lines.
Precipitation happens largely because some, but not all, of the droplets in supercooled clouds nucleate on dust particles. Because ice is a lower energy state than super-cooled water, the vapor pressure over water is higher than that over ice, so the ice crYstals grow (and become snowflakes) at the expense of the super-cooled water droplets, eventuaolly becoming heavy enough to fall out of the cloud. A lot of rain starts as snow, and melts as it falls into warmer air at lower altitudes.
Cloud seeding attempts to increase precipitation by providing more particles (usually very small silver iodide particles) for nucleation.