VOA Special English 2014 - Science - All About Snow

From VOA Learning English, this is Science in the News. I'm Caty Weaver. Snow is a subject of great interest to weather experts. It is difficult to predict where, when or how much snow will fall. One reason is that heavy amounts of snow fall in surprisingly small areas. Another reason is that a small change in temperature can mean the difference between snow and rain. Just what is snow, anyway? Snow is a form of frozen water. It contains groups of ice particles called snow crystals. These crystals grow from water droplets in cold clouds. They usually grow around dust particles. 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 15 degrees below zero Celsius. Columnar snow crystals look like little 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. Hail can be dangerous to people, animals and property. 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 can also influence 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. Snow or ice may sometimes appear to be blue. The blue light is the product of a long path through the snow or ice. Think of snow or ice as a filter. A filter is designed to reject some substances, while permitting others to pass through. In the case of snow, light makes it through if the snow is only a centimeter thick. If it is a meter or more thick, however, blue light often can be seen. Snow falls in the Earth's extreme North and South throughout the year. However, the heaviest snowfalls have been reported in the mountains of other areas during winter. These areas include the Alps in Italy and Switzerland, the coastal mountains of western Canada, and the Sierra Nevada and Rocky Mountains in the United States. Snow is even known to fall near the Equator -- but only on the highest mountains. The continental United States has an average of 105 snowstorms each year. An average storm produces snow for between two and five days. Almost every part of the U.S. has received snowfall at one time or another. Even parts of southern Florida have reported a few snowflakes.