[译林湖南版创新设计]2011高考英语一轮复习提能训练

Ⅱ.简答题

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—plate-like and columnar.Plate-like crystals are flat.They form when the air temperature is about fifteen degrees below zero Celsius.Columnar 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 one-half centimeters.Under some conditions，snowflakes can form that are five centimeters ually，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 one-half centimeters of rain.About seventy-six centimeters of dry snow equals the water in two and one-half 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.