VOA慢速英語:今冬美國大部分地區(qū)都被雪覆蓋

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From VOA Learning English, this is Science in theNews. I’m Caty Weaver.

And I’m Mario Ritter. Today, we will tell you aboutsnow. People in many parts of the United States havehad the chance to learn a lot about snow this winter. Snow fell in almost every state.

Large amounts of snow fell on the northeastern statesof New York and New Jersey in January and earlyFebruary. Millions of Americans live in, or travelthrough, these states. The Northeast is home to manymajor airports. The bad weather forced officials tocancel thousands of flight.

Earlier, in December, there were several big rain andsnow storms. At one point, snow fell on almost every part of the country and in almost every state. The National Oceanic and Atmospheric Administrationsaid it was the eighth largest December snow cover on record, and thelargest since December of 2009.

Weather experts say snow may continue to fall in many areas through March.

Snow is a subject of great interest to weather experts. It is difficult to predictwhere, when or how much snow will fall. One reason is that heavy amountsof snow fall in surprisingly small areas. Another reason is that a small changein temperature can mean the difference between snow and rain.

Just what is snow, anyway? Snow is a form of frozen water. It containsgroups of ice particles called snow crystals. These crystals grow from waterdroplets in cold clouds. They usually grow around dust particles.

All snow crystals have six sides, but they grow in different shapes. The shapedepends mainly on the temperature and water levels in the air.

Snow crystals grow in one of two designs: plate-like and columnar. Plate-likecrystals are flat. They form when the air temperature is about 15 degreesbelow zero Celsius. Columnar snow crystals look like little sticks of ice. Theyform when the temperature is about five degrees below zero.

The shape of a snow crystal may change from one form to another as thecrystal passes through levels of air with different temperatures. When meltingsnow-crystals or raindrops fall through very cold air, they freeze to form smallparticles of ice, called sleet. Groups of frozen water-droplets are called snowpellets. Under some conditions, these particles may grow larger and formsolid pieces of ice, or hail. Hail can be dangerous to people, animals andproperty.

When snow crystals stick together, they produce snowflakes. Snowflakescome in different sizes. As many as 100 crystals may join to form asnowflake larger than two and one-half centimeters. Under some conditions,snowflakes can form that are five centimeters across. Usually, this requiresnear-freezing temperatures, light winds and changing conditions in Earth’satmosphere.

Snow contains much less water than rain. About two and one-halfcentimeters of rain has as much water as 15 centimeters of wet snow. About76 centimeters of dry snow equals the water in two and one-half centimetersof rain.

Much of the water the world uses comes from snow. Melting snow provideswater for rivers, power stations and crops. Mountain snow provides up to 75percent 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. Becausethe air has trouble moving, the movement of heat is limited.

Snow can also influence the movement of sound waves. When there is freshsnow on the ground, the surface of the snow absorbs, or takes in, soundwaves. However, snow can become hard and flat as it grows older or if therehave been strong winds. Then, the snow’s surface will help to send backsound waves. Under these conditions, sounds may seem clearer and maytravel 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 givesthem their color. However, when light travels from air to snow, some light issent back, or reflected. Snow crystals have many surfaces to reflectsunlight. Yet the snow does take in a little sunlight. It is this light that givessnow its white appearance.

Snow or ice may sometimes appear to be blue. The blue light is the productof a long path through the snow or ice. Think of snow or ice as a filter. A filteris designed to reject some substances, while permitting others to passthrough. In the case of snow, light makes it through if the snow is only acentimeter thick. If it is a meter or more thick, however, blue light often can beseen.

Snow falls in the Earth’s extreme North and South throughout the year. However, the heaviest snowfalls have been reported in the mountains of otherareas during winter. These areas include the Alps in Italy and Switzerland, thecoastal mountains of western Canada, and the Sierra Nevada and RockyMountains 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.

Many countries have little or no natural snowfall, but people who live in thoseareas still want to see snow. So scientists have come up with a solution. In 1936, a physicist from Japan produced the first man-made snow in alaboratory. Then, during the 1940s, American scientists developed methodsfor making snow in other areas. Clouds with extremely cool water are mixedwith man-made ice crystals, such as silver iodide and metaldehyde crystals. Sometimes, dry ice particles or liquid propane are used. Today, machinesare used to produce limited amounts of snow for ski areas when not enoughnatural snow has fallen.

Snow can be fun and also dangerous.

Snow may be beautiful, but it can be deadly. It isresponsible for the deaths of hundreds of people in theUnited States every year. Many people die in trafficaccidents on roads that are covered with snow or ice. Others die from being out in the cold, or from heartattacks caused by too much physical activity.

You may not be able to avoid living in areas where itsnows often. However, you can avoid becoming avictim of a snowstorm. Most people are told to stay intheir homes until the storm has passed. When removing large amounts ofsnow, they should stop and rest often. Difficult physical activity during snowremoval can cause a heart attack, especially among older adults.

It is always a good idea to keep a lot of supplies in the home, even beforewinter begins. These supplies include food, medicine, clean water and extrapower supplies.

Some drivers have become trapped in their vehicles during a snowstorm. Ifthis happens, people should remain in or near their car unless they see somekind of help. They should get out and clear space around the vehicle’sexhaust pipe. This will lower the chance that poisonous carbon monoxide gasenters the car.

Drivers should tie a bright-colored object to the top of their car to increase thechance of rescue. Inside the car, they should open a window a little for freshair and turn on the engine for ten or fifteen minutes every hour for heat.

People living in areas where winter storms are likely should carry emergencysupplies in their vehicle. These include food, emergency medical suppliesand extra clothing to stay warm and dry. People in these areas should alwaysbe prepared for winter emergencies. Snow can be beautiful, but it can also bedangerous.

This Science in the News was written by Christopher Cruise. Our producerwas June Simms. I’m Mario Ritter.

And I’m Caty Weaver. Join us again next week at this time for more newsabout science on the Voice of America.