填空题

Snow is a subject of great interest to weather experts. Experts sometimes have difficulty in (36) where, when or how much snow will fall. One reason is that heavy amounts of snow fall in (37) small areas. Another reason is that a small change in temperature can mean the difference between snow and rain. Snow is a form of (38) water. It contains many groups of (39) ice particles called snow crystals. These crystals grow from water particles in cold clouds. They usually grow around a piece of (40) . All snow crystals have six sides, but they grow in different shapes. It (41) mainly on the temperature and water levels in the air snow crystals grow in one of two (42) --platelike and columnar. Platelike crystals are (43) 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 Celsius. (44) . 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 long. (45) . 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. (46) .