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Similar to rain we need condensing humidity for snow development. The condensation can build on condensation cores, which are small particles (dust, sand) in the air where small humid drops can attach and freeze. Gravitational force and upwind makes the crystals move into the cloud and they can grow together with other crystals or attach further humidity. Due to different temperatures inside a cloud several kinds of crystals develop. If the winds inside the cloud cannot hold the crystals they move towards the earth: it snows. Snow contains of many small ice crystals bounded together. It is white because the small ice particles break the light very often and spread it diffuse.