In air, pure water will not freeze until far below 0 degrees C. Crystallization occurs much more easily around a particle of dust, or even a bacterium. Similarly, water vapor joins together much more easily around a tiny nucleus. Cloud seeders try to cause rain by inserting these nuclei into clouds, but their effectiveness is debatable. At any rate, once the congregation process starts, it cascades. Just as water coalesces much faster on existing droplets, ice crystals in clouds readily gather onto existing ice crystals.
Deposition occurs when water in the gas phase deposits onto an ice crystal.
Aggregation is the union of existing ice crystals. Snowflakes may join together after leaving a cloud. If the air beneath the cloud is warm, the falling snowflakes may melt, forming raindrops. Snow more Crystals are molecular structures with a specific bond geometry. Because water molecules only bond at 120 degrees, ice crystals and snow flakes are all based on hexagons, which have 120-degree intersections.