摘要：Snow is a form of frozen water.It contains many groups of tiny ice particles called snow crystals.These crystals grow from water particles in cold clouds.They usually grow around a piece of dust.All snow crystals have six sides.but they grow in different shapes.The shape depends mainly on the temperature and water levels in the air. Snow crystals grow in one of two designs-plate­like and columnar.Plate­nar snow crystals look like sticks of ice.They form when the temperature is about five degrees below zero. The shape of a snow crystal may change from one form to another as the crystal passes through levels of air with different temperatures.When melting snow crystals or raindrops fall through very cold air.they freeze to form small particles of ice.called sleet. Groups of frozen water droplets are called snow pellets.Under some conditions.these particles may grow larger and form solid pieces of ice.or hail. When snow crystals stick together.they produce snowflakes.Snowflakes come in different sizes.As many as one hundred crystals may join together to form a snowflake larger than two and one­half centimeters.Under some conditions.snowflakes can form that are five centimeters long.Usually.this requires near freezing temperatures.light winds and changing conditions in Earth’s atmosphere. Snow contains much less water than rain.About fifteen centimeters of wet snow has as much water as two and one­half centimeters of rain.About seventy­six centimeters of dry snow equals the water in two and one­half centimeters of rain. Snowfall helps to protect plants and some wild animals from cold.winter weather.Fresh snow is made largely of air trapped among the snow crystals.Because the air has trouble moving.the movement of heat is greatly reduced. Snow also is known to influence the movement of sound waves.When there is fresh snow on the ground.the surface of the snow takes in.or absorbs.sound waves.However.snow can become hard and flat as it becomes older or if there have been strong winds.Then the snow’s surface will help to send back sound waves.Under these conditions.sounds may seem clearer and travel farther. Generally.the color of snow and ice appears white.This is because the light we see from the sun is white.Most natural materials take in some sunlight.This gives them their color.However.when light travels from air to snow.some light is sent back.or reflected.Snow crystals have many surfaces to reflect sunlight.Yet the snow does take in a little sunlight.It is this light that gives snow its white appearance.1．What plays a major role in determining the shape of snow crystals?

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