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.
BOB DOUGHTY: 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. Hail can be dangerous to people, animals and property.
(MUSIC)
FAITH LAPIDUS: When snow crystals stick together, they produce snowflakes. Snowflakes come in different sizes. As many as one hundred crystals may join to form a snowflake larger than two and one-half centimeters. Under some conditions, snowflakes can form that are five centimeters across. Usually, this requires near-freezing temperatures, light winds and changing conditions in Earth’s atmosphere.
Snow contains much less water than rain. About two and one-half centimeters of rain has as much water as fifteen centimeters of wet snow. About seventy-six centimeters of dry snow equals the water in two and one-half centimeters of rain.
BOB DOUGHTY: Much of the water the world uses comes from snow. Melting snow provides water for rivers, electric power stations and agricultural crops. Mountain snow provides up to seventy-five percent of all surface water supplies in the western United States.
最新
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25