When snow crystals stick together, they produce snowflakes. Snowflakes come in different sizes. As many as 100 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 15 centimeters of wet snow. About 76 centimeters of dry snow equals the water in two and one-half centimeters of rain.
Much of the water the world uses comes from snow. Melting snow provides water for rivers, power stations and crops. Mountain snow provides up to 75 percent of all surface water supplies in the western United States.
Snowfall helps to protect plants and some wild animals from 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 limited.
Snow also influences the movement of sound waves. When there is fresh snow on the ground, the surface of the snow absorbs, or takes in, sound waves. However, snow can become hard and flat as it grows 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 may travel farther.
Generally, snow and ice appear 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.
最新
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25