You can make tornadoes at home.-Can you believe it?

On the afternoon of June 23rd, Funing County, Yancheng City, Jiangsu Province was suddenly hit by a double disaster of heavy hail and tornado, with heavy casualties and heavy losses. At 22: 00 on the 23rd, the Ministry of Civil Affairs released a message. According to preliminary statistics, 78 people were killed and nearly 500 people were injured in severe convective weather, including nearly 200 seriously injured. A large number of houses were damaged, power communication facilities were destroyed, power was cut off in some areas, and communication base stations had no signal.

The word "tornado" is probably the most frightening thing in this disaster.

How did this scene, which can only be seen in European and American disaster films, come into being? How good is it? In fact, we can see the principle of "tornado" at home every day, but we just don't know it.

What is a tornado?

In physics, a tornado is actually a whirlpool.

The so-called "vortex", translated as vortex in English, is the rotating motion of fluid mass, which can be commonly understood as "vortex".

Aerodynamicists vividly described it as "the tendon of fluid movement", and Professor Lu Shijia of Beihang University explained it more succinctly: "The essence of fluid is vortex, because fluid can't stand friction, and it will rub out when it is rubbed."

"Friction" here refers to the shearing force acting on the fluid. For example, planes, ships, cars, trains and other objects are in air movement. The air attached to the surface of an object will be taken away by the object, because it is attached to the surface of the object, while the fluid in the distance is static, generating a shear force and rubbing the fluid.

The vortex of airflow brushed off the tail of the plane.

Tornado is one of many whirlpools in nature. Common eddy currents include:

Spiral galaxy: A spiral galaxy in space, the largest vortex in nature, with a scale of "light years".

20 13 spiral galaxy NGC6984 photographed by NASA with Hubble telescope.

Typhoon, hurricane, tropical storm, tropical cyclone

On the satellite cloud image of weather forecast, we can often see the large-scale rotating fluid vortex shown by large white clouds, and its radius can reach hundreds of kilometers.

In 2003, the photos of Hurricane Kate captured by Terra satellite in the United States.

At the time of shooting, the sustained wind speed of Hurricane Kate was 1 15 mph, and it moved westward at the speed of 12 mph.

tornado

According to the definition of meteorology, tornado is a strong, small-scale air vortex, which is produced by strong convective movement of the air under strong unstable weather conditions, usually a strong whirlwind produced by funnel clouds extending from thunderstorm clouds to the ground. The air pressure in the center of the tornado is very low, and the central wind force can reach more than 100-200 meters per second, which is extremely destructive.

In 2008, a tornado over the prairie in Kansas, USA.

In 2009, a waterspout appeared on the St. John River in the United States.

Others, such as sandstorms and tsunamis, are also common forms of eddies in nature.

Dust storm vortex photographed by NASA satellite in Asia in 200 1 year

20 1 1 After the tsunami and earthquake in Japan, a huge whirlpool was formed at sea.

In daily life, we often see this phenomenon: in a sink or bathtub filled with water, pulling out the plug will form a vortex, which is scientifically called "bathtub vortex", which is a typical vortex phenomenon.

The bathtub whirlpool in the sink

The American Science and Technology Museum simulates the "bathtub vortex" of a tornado.

So it can be said that in the kitchen or bathroom at home, we can easily simulate the physical mechanism of tornadoes.

Formation mechanism of tornado

Now we know that tornado is a kind of vortex, so how is tornado formed?

In April this year, Yolanda Primary School in Guazhou County, Jiuquan City, Gansu Province held a spring sports meeting. The weather was fine, but suddenly a whirlwind blew, and a 9-year-old boy was instantly rolled up several meters high. Is this a tornado?

Live picture

In fact, this is only caused by small-scale airflow instability. Generally, we are called "sandstorm". Similarly, there was a sandstorm in Beijing Beihai Park in early April, when several ships capsized.

Tornadoes must be caused by the instability of mesoscale airflow, with an average diameter of 200-300m, a minimum diameter of several tens of meters and a maximum of 1000m. Generally, the duration is several minutes to more than ten minutes, and the maximum duration is no more than several hours. The moving speed is 15m/s to 70m/s. ..

The formation of tornadoes is related to strong thunderstorm clouds, and a small part of the huge energy of thunderstorms is released in a small area, bringing energy to tornadoes. At the same time, some cloud columns extend downward, rotate downward, and converge horizontally, which is similar to a skater grasping his arm when he rotates, making the rotation speed faster and faster. This extended vortex tube rotates very fast, forming a funnel-shaped cloud and forming the shape of a tornado.

The skater's arm will increase the rotation speed.

Specifically, the formation of tornadoes is 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 begins to rotate in the middle of the troposphere, forming a mesoscale cyclone.

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

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

The mechanism of tornado formation (picture from mechanics and practice)

The direction of airflow in a tornado

Simply put, the formation and disappearance of tornadoes are both processes of unstable airflow movement.

How to save yourself when encountering a tornado?

Leave the dangerous house, activity place or other simple temporary residence immediately and take refuge in a relatively strong house nearby.

If you are in a car, you should leave in time and reach a low-lying place to escape, because the car itself has no ability to defend against tornadoes. Once the car and people are caught up in a tornado at the same time, the harm is even greater.

Quickly run along the vertical direction of the tornado's moving direction and lie on low-lying land, ditches, etc. Keep away from trees, telephone poles, billboards, fences, etc. , in order to avoid being bruised, crushed or get an electric shock.

The safest place to avoid a tornado is an underground space or place. At this time, all buildings on the ground are not the safest places to avoid.

Producer: Computer Network Information Center

Produced by: Popular Science China

Production: Third Institute of Mechanics, China Academy of Sciences

"Popular Science China" is an authoritative brand of science that China Association for Science and Technology cooperates with all sectors of society and carries out scientific communication by means of information technology.

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