Popular Science Reading： Causes and Effects of Tornadoes

　　Tornadoes are a kind of disastrous weather.

　　Tornado is a kind of vortex with strong wind and not too large range, which is like a funnel, with extremely fast wind speed and great destructive power. The air pressure in the center can be 10% lower than the surrounding air pressure. The appearance and disappearance of tornadoes are very sudden, so it is difficult to make an effective forecast.

　　The upper end of the tornado is connected with the thunderstorm cloud, and the lower end is suspended in mid-air, and some extend directly to the ground or water surface, while rotating and moving forward. From a distance, it looks not only like a giant python dangling in the air, but also like an elephant’s nose that keeps swinging. It happens at sea, just like the phenomenon of "dragon sucking water", which is called "waterspout" (or "waterspout"); A tornado that appears on land, blows up dust and takes away houses, trees, etc., is called "landspout" (dust-tube tornado is called by the National Weather Service). Waterspouts may appear in oceans and lakes all over the world. In the United States, waterspouts usually occur on the southeast coast of the United States, especially in southern Florida and the Gulf of Mexico. Although waterspouts are a kind of tornadoes by definition, they are less destructive than the most powerful prairie tornadoes, but they are still quite dangerous.

　　The survival time of tornadoes is generally only a few minutes, and the longest is no more than a few hours. Tornadoes often pull up trees, overturn vehicles, destroy buildings and other phenomena, sometimes sucking people away, which is very harmful. The deadliest tornado on record in the United States occurred on March 18, 1925, crossing the "Three-continent Tornado" in southeastern Missouri, southern Illinois and northern Indiana, killing 695 people.　　

　　

　　Tornadoes are very powerful. According to reports, at 4 pm on May 30, 1879, two dark and thick clouds merged over the northern part of Kansas, USA. After 15 minutes, a vortex was generated at the lower end of the cloud. The vortex grew rapidly and became a huge pillar of indomitable spirit. In three hours, it ran amok all over the state like a wicked dragon, and no one was spared wherever it went. However, the strangest thing happened at the beginning, when the tornado whirled across a small river and met a cliff. Obviously, it was impossible to surpass this obstacle, so the vortex folded westward, and there happened to be a newly built 75-meter-long railway bridge over there. The tornado vortex actually "pulled" it from the stone pier, twisted it a few times and threw it into the water. In China, there was a tornado in Shanghai on September 24, 1956. It easily lifted a large oil storage barrel weighing 220,000 Jin to an altitude of 15 meters and then threw it 120 meters away.　　

　　In the United States, the number of deaths caused by tornadoes is second only to lightning every year. Its damage to buildings is also quite serious, often devastating. Under the attack of a strong tornado, the roof of the house will fly like a glider. Once the roof is swept away, the rest of the house will also collapse. In 1995, a tornado occurred in Adelmore, Oklahoma, USA, and heavy objects such as roofs were blown dozens of miles away. Most of the debris falls on the left side of the tornado passage, and there is often a clear landing zone according to the weight. Lighter debris flew more than 300 kilometers before landing.

Causes of tornado

　　How on earth is a tornado formed? We can get enlightenment from a phenomenon that we can often see in the playground in summer: a gust of wind blows, and suddenly an airflow vortex appears in the middle of the playground. It rolls up sand and leaves and rotates with the airflow, and moves faster and faster. After a while, it slows down quickly and suddenly disappears. This is caused by small-scale airflow instability.

　　Tornadoes are caused by mesoscale airflow instability, with an average diameter of 200-300 meters, the smallest diameter is only a few tens of meters, and only a few large ones reach more than 1000 meters. Its life is also very short, often only a few minutes to dozens of minutes, not more than a few hours at most. Its moving speed is 15 meters per second on average, and the fastest can reach 70 meters; The length of the moving path is mostly about 10 kilometers, which is only tens of meters short and hundreds of kilometers long. The width of the ground that it causes damage is generally l-2 kilometers.

　　It is generally believed that tornado is the product of thunderstorm in clouds, and it is a form in which a small part of the huge energy of thunderstorm is released in a small area.

　　The formation process of tornado can be roughly divided into four stages:

　　(1) The instability of the atmosphere produces a strong updraft, which is further strengthened due to the influence of the maximum transit airflow in the rapids.

　　(2) Due to the interaction with the wind with shear speed and direction in the vertical direction, the updraft starts to rotate in the middle of the troposphere, forming a mesoscale cyclone.

　　(3) With the development and upward extension of the mesoscale cyclone to the ground, it becomes thinner and stronger. At the same time, a small area of enhanced cooperation, that is, the primary tornado formed inside the cyclone, forming the tornado core.

　　(4) The rotation in the tornado core is different from that in cyclone, and its intensity is enough to make the tornado extend all the way to the ground. When the developing vortex reaches the ground level, the ground air pressure drops sharply and the ground wind speed rises sharply, forming a tornado.

　　

　　It can be seen from the above that the formation and disappearance of tornadoes are both processes of unstable airflow movement.

　　Why are there so many tornadoes in America?

　　Tornadoes in the United States are not only numerous, but also strong, so they are called "the land of tornadoes". This is mainly related to the geographical location, climatic conditions and atmospheric circulation characteristics of the United States. The United States is bordered by the Atlantic Ocean in the east, the Pacific Ocean in the west, and the Gulf of Mexico in the south. A large amount of water vapor continuously flows from the east, west and south to the mainland of the United States. The United States is mainly located in the mid-latitude, which is often controlled by the subtropical high in spring and summer, and even in autumn and winter, it is often affected by its edge. The south and west of the subtropical high are the most active places in the east and southeast, which continuously transport a large amount of warm and humid air from the Atlantic Ocean and the Gulf of Mexico to the mainland of the United States, which is a sufficient condition for the generation of thunderstorm clouds. Thunderstorm clouds continue to develop strongly, and tornadoes follow.

　　Tornadoes in the United States occur mostly in the vast areas of the midwest and south. Some people call this area where tornadoes often occur in the United States "tornado alley". About 54% tornadoes in the United States occur in spring. In May, the subtropical high pressure controls the United States, and its western edge just stays in the midwest. At this time, the southeast airflow transports a large amount of warm and humid air from south to north in the Gulf of Mexico. When there is enough water vapor in the air and there is a strong vertical upward movement, thunderstorm clouds will be strongly generated, which often develop into tornado clouds and produce tornadoes. In June, a large amount of warm and humid air moved northward to Kansas, Nebraska and Iowa, and in July, it moved to Canada. Since then, the number of tornadoes in the United States has been greatly reduced.

　　

　　According to statistics in the past 50 years, the number of tornadoes over the United States has increased by at least 35 times. Sometimes there are no tornadoes, but there will be tornadoes. This special tornado is called "no cloud dragon tornado", which accounts for about half of American tornadoes. How did this strange phenomenon come into being?

　　Some people think that this is closely related to the increase in the number of Mercedes-Benz cars on the road. At present, there are no fewer than 2 million cars and more than 600,000 trucks running on the main highways in the United States. The traffic in the United States drives on the right side. Whenever two cars running at high speed cross the bus, an air vortex in the counterclockwise direction will be formed. When the air vortices generated by many cars are superimposed, a powerful vortex will be formed. This powerful vortex will also induce tornadoes once it meets the right weather system and high temperature and humidity conditions.

　　In North America, although the tornado season usually lasts from March to October, tornadoes may occur at any time of the year, especially in the afternoon and evening, and more than 80% tornadoes occur between noon and midnight.

　　Tornadoes are found on every continent except Antarctica, but the United States suffers more tornadoes than any other country or region. In addition, tornadoes often appear in southern Canada, south-central and eastern Asia, central and eastern South America, southern Africa, northwestern and southeastern Europe, western and southeastern Australia and New Zealand.

　　

　　(Author: Professor of Tianjin University, Wang Zhendong)