As snow accumulates on a car windshield. It sticks to the cold surface of the window. When the driver turns the ignition and starts the car, the windshield begins to warm from the increased temperature in the car\'s interior. As the temperature rises, the snow on the windshield begins to slide off, sometimes in large chunks. Although the driver might not recognize it, an avalanche has just occurred on the windshield.
A windshield avalanche imitates what is known as a slab, or large scale, avalanche. However, it is not the deadly force of nature that occurs during an avalanche in a large area. A real avalanche is one of the most powerful natural events. Its formation depends upon three factors: snow, slopped surface, and a trigger. A snowy slope with an angle between 30 degrees and 45 degrees is considered a potential avalanche site. The first day following a heavy, sudden snowfall over 12 inches is prime time for an avalanche to form.
Several natural circumstances can cause an avalanche. An environment with forests is much safer than open terrain. The trees create a natural buffer to anchor now and prevent it from causing an avalanche. Valleys or sloped areas are possible danger zones, as they can quickly accumulate large amounts of snow.
The condition of snow layers in a snowpack, or accumulation of snow, also influences the likelihood of an avalanche. In areas that are quite snowy, snowpack consists of several layers of snow that have developed over time. The types of bonds throughout the snowpack\'s layers cand determine whether an avalanche might occur. If the snow crystals do not create a strong bond, they create a weak layer in the snowpack. IF the weak layer exists near the surface, it may cause a sluff, or small slide of dry powdery snow. If the weakened layer is at the base of a snowpack, it can result in a deadly slab avalanche. Dramatic changes in temperature also can cause melting in the snowpack, causing weak bonds.
Many natural events can trigger an avalanche. However, it is estimated that about 90% of avalanches are caused by human activity. Skiers, snowboarders, and snowmobilers all depend upon snowpack to enjoy their sport. If they are not careful, their activities can result in a deadly avalanche.
Areas with smooth, steep slopes and few obstacles create an ideal setting for an avalanche. Winter sports enthusiasts seek out these exact conditions. They must use extreme caution in order to assure their safety. Usually, these types of areas will provide signs to warn people of dangerous conditions. Most parks also offer web pages or hotlines with avalanche forecasts.
Of course, skiers or snowboarders who are in avalanche-prone areas should be aware of their surroundings at all times. A partner should always be present. If travel in an area that is potentially dangerous is required, it should occur above the avalanche zone. not through the center. Basic equipment, such as shovels, rescue beacons, and avalanche probes, is essential and should be present during any such activity.
If areas where many people gather to enjoy snow sports, such as ski resorts, there are methods to control the likelihood of avalanches. One example is avalanche control teams, whose job is to trigger controlled avalanches. They do this with explosives or by using a cannon. These methods clear the area of new snow, which can overwhelm a sturdy snowpack. This, in turn, prevents an avalanche from occurring when people are using the area for recreation. An avalanche can be a powerful and deadly force, but with caution and a little luck, most winter athletes will never encounter one.
单词:
avalanche- n. 雪崩
ignition- n. 点火
prime time-n. 黄金时间
buffer-n. 缓冲
bond- n. 化学键
问题:
1. In the selection, the author uses an analogy to compare two things. What are they?
2. How does the analogy add to your understanding of avalanches?
3. What are the three elements necessary for avalanche formation?
4. What is snowpack?
5. Where is an avalanche more likely to occur--in a wooded area or an open terrain?
6. What can create a weak layer in snowpack?