Water vapor condenses into tiny droplets in the atmosphere. As more and more water vapor condenses onto its surface, the droplet grows. If the air is cold enough, the water droplet will freeze to become an ice crystal. As increasing water vapor collects on the ice crystal and freezes, the ice crystal becomes heavier, and eventually falls from the sky. As the crystal falls, it continues to grow by accreting more water vapor. As it is falling, the ice crystal can interact with warmer air that causes it to melt somewhat. The melting causes the crystals to bond together into larger flakes, forming a fluffy snowflake. If the crystals melt too much but then refreeze as they reach the Earth’s surface, the ice crystals precipitate as sleet.
Snowflakes, the basic unit of snow, originate as tiny ice crystals within “cold clouds.” Cold clouds are clouds that exist within air that is at, or below, the freezing point. As an ice crystal is blown back and forth between the top and bottom of the cloud, it grows in two ways: by coalescence and by deposition. In coalescence, the ice crystal collides, and sticks to the cold water droplets it encounters in the cloud. In deposition, water vapor molecules (particles made by the combination of two or more atoms) within the cloud freeze directly onto the ice crystal.
As the ice crystal grows, it bonds with other ice crystals and takes on the six-sided shape of a snowflake. When the snowflake becomes heavy enough, it falls to the ground.
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