Ice crystals form in the atmosphere when water freezes onto tiny particles. Common sources of these particles are dust, volcanoes, factory smoke, forest fires, and salt from ocean spray. The ice crystals begin to grow around the particles, called "cloud condensation nuclei" (CCN). The CCN may be as small as one millionth of a meter in diameter and one trillionth of a gram in mass!

Question: If you have a pencil that uses lead that is .5 mm thick, how many of these particles (CCN) could be lined up side by side across the width of the lead?

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Snowflakes begin when water molecules freeze onto a particle. Particles provide a place for the water molecules to "organize" and form a crystal. Crystallized water is known as "ice."

Snowflakes grow when several ice crystals stick together. They fall slowly through a cloud and grow by several processes as they fall. Water vapor can freeze directly onto the surface of an ice crystal. Or, flakes can grow by "riming," where liquid cloud drops collide with an ice particle and freeze onto the crystal. Also, multiple individual ice crystals can collide and stick together to form a larger snowflake.

You have probably heard every snowflake is unique. The exact form of a snowflake is determined by its path though a cloud. Each snowflake's path is unique and its final form tells the story of its journey.

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