单选题
Tornadoes
Tornadoes, or twisters, are nature"s most violent storms. 1) The United Kingdom experiences an average of 32 tornadoes each year, which is the highest frequency of reported tornadoes per unit area in the world. But the deadliest and most violent tornadoes occur in the United States. Here they are most common in Oklahoma and Texas, an area otherwise known as Tornado Alley due to the frequency and severity of these phenomena. Tornado Alley stretches from central Nebraska southward to Central Texas.
Each tornado is unique, and as such none is of the same size as the other, but on average each is between 130-170 meters wide and can travel up to 90 kilometers per hour. Tornadoes can last several minutes at a time and leave behind a path of destruction a couple of kilometers long. Some tornadoes can last up to three hours causing damage as far as 300 kilometers away, with winds of more than 400 kilometers an hour. Thankfully, though, these types of tornados are very rare.
The word tornado comes from the Spanish "tronada" meaning thunderstorm and "tornar" meaning to turn. Virtually all tornadoes develop out of thunderstorms. 2) They form where warm, moist air and cold, dry air meet and create upward currents of air that develop into massive rotating cumulonimbus clouds. The center of the tornado"s vortex is a low-pressure area. When air rushes into the vortex, its pressure lowers and cools the air, condensing water vapor in the air into the funnel-shaped cloud. Depending on the type of debris it accumulates along the way, the color of the funnel varies between white, red, black and other shades. A funnel cloud refers to any rotating column of air growing from the base of a cloud. It will become a tornado if its circulation reaches the ground, so most of the visible funnel clouds are tornadoes.
Tornadoes usually occur in the afternoon and early evening and they wreak destruction when they touch the ground. Most tornado deaths are related to building collapse, collisions by flying objects or failure to escape the tornado in a car. The average annual mortality rate in the U.S. due to tornadoes is about 42, but 1998 will be remembered as one of the worst years for tornado damage across the USA since around 100 people lost their lives in tornado incidents across many states. In May 1999, the Oklahoma City tornado was the deadliest in over 20 years, with more than 30 direct fatalities. But the number of tornado deaths per year has generally been much less in the last quarter of the 20th century than it was in previous ones. 3) Improved observation systems, communications and networks have all likely provided a significant decline in the life-threatening consequences of tornadoes.
4) Still the most accurate observation method for tornadoes is direct tracking. Thus, geologists are likely to be on standby should a tornado prove imminent. They would send a couple of scientists out into the open field for live tornado surveillance and direct their actions by microphone from a nearby computerized control area. The tornado surveyor stays locked in their car and records the tornado"s speed and movement as it happens onto a special log. Measurement is then later processed in the lab to form accurate prediction patterns for future tornados in the area.
Glossary
thunderstorm:
a transient, sometimes violent storm of thunder and lightning, often accompanied by rain and sometimes hail
cumulonimbus:
a dark cloud of great vertical extent charged with
electricity:
associated with thunderstorms
funnel:
a conical shape with a wider and a narrower opening at the two ends
mortality rate:
the ratio of deaths in an area referred to the population of that area