【推荐1】Tornadoes

Tornadoes are essentially funnels of air rotating as fast as 300 miles per hour. Of course, not all tornadoes are so dangerous.     1     Such small tornadoes occur in many places around the world, even in southern Taiwan, where several are usually reported every spring.

It is, however, the big tornadoes that receive the most attention.     2     There can be flocks of dead birds dropping out of the sky, chickens still alive but with all of their feathers gone, pieces of straw stuck in trees. In a 1999 US storm, violent winds from one tornado lifted a church into the air and dropped it onto a house. In the same storm, a baby was whipped from its mother’s arms and, miraculously, found alive in a nearby tree.

    3     Although scientists now know a great deal about tornadoes, they still cannot determine exactly when and where one will form. In addition, the path of a tornado can also be impossible to predict. It is unlikely to move in a straight line, and will often turn quickly. A tornado might blow down a line of houses, then suddenly lift up and leave one home unhurt, then touch back down and destroy the rest of the line.

To better understand tornadoes, scientists have developed special heavy-duty vehicles and weather-monitoring equipment that allow them to make close-up observations of these dangerous storms. Radar equipment is put on trucks and driven near a tornado, where it records wind speeds and other weather data.     4    

In addition, better communication and warning systems are also helping to reduce deaths and injuries in many tornado-prone areas. Radio and television warnings as well as sirens help to alert people to potential danger. Public awareness is also important. When tornadoes do occur, people must act quickly. They should move immediately to a safe place such as a basement or under a strong stairway and cover themselves with blankets or heavy coats.     5    

【推荐2】A flood occurs when water overflows. This can happen in many ways. Most common is when rivers or streams overflow their banks. Excessive rain, a ruptured (破裂的) dam or levee (防洪堤), rapid ice melting in the mountains can overwhelm a river and send it spreading over the nearby land, called a floodplain. Coastal flooding occurs when a large storm or tsunami causes the sea to surge inland.

Most floods take hours or even days to develop, giving residents enough time to prepare or leave. Others generate quickly and with little warning. These flash floods can be extremely dangerous, instantly turning a babbling brook into a thundering wall of water and sweeping everything in its path downstream.

Disaster experts classify floods according to their likelihood of occurring in a given time period. A hundred-year flood, for example, is an extremely large, destructive event that would theoretically be expected to happen only once every century. But this is a theoretical number. In reality, this classification means there is a one-percent chance that such a flood could happen in any given year. Over recent decades, possibly due to global climate change, hundred-year floods have been occurring worldwide with frightening regularity.

Moving water has awesome destructive power. When a river overflows its banks or the sea drives inland, structures poorly equipped to withstand the water's strength are no match. Bridges, houses, trees, and cars can be picked up and carried off. The erosive force of moving water can drag dirt from under a building's foundation, causing it to crack and tumble.

When floodwaters recede (退去), affected areas are often blanketed in silt and mud. The water and landscape can be contaminated (污染) with hazardous materials, such as sharp debris, pesticides, fuel, and untreated sewage. Potentially dangerous mold blooms can quickly overwhelm water-soaked structures. Residents of flooded areas can be left without power and clean drinking water, leading to outbreaks of deadly waterborne diseases like typhoid, hepatitis A, and cholera.

But flooding, particularly in river floodplains, is as natural as rain and has been occurring for millions of years.