How do tornadoes form?

How do tornadoes form? We can get inspiration from a phenomenon that we can often see on the playground in summer: a gust of wind blows, and a vortex of air suddenly appears in the middle of the playground. It rolls up sand and leaves, and rotates with the airflow, and the speed is getting faster and faster. After a while, it slowed down and suddenly disappeared. This is caused by the instability of small-scale airflow.

Tornadoes are caused by unstable mesoscale airflow, with an average diameter of 200-300 meters, the smallest is only tens of meters, and only a few large ones reach 1000 meters or more. Its life is also very short, often only a few minutes to dozens of minutes, no more than a few hours. Its average moving speed is15m per second, and the fastest speed can reach 70m. The length of the moving path is mostly around 10 km, the short path is only tens of meters, and the long path can reach hundreds of kilometers or more. The ground width caused by it is generally L-2km.

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

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 mesoscale cyclone to the ground, it becomes thinner and stronger. At the same time, the strengthening of cooperation in a small area, that is, the formation of a primary tornado inside the cyclone, forming a 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 to the ground. When the developing vortex reaches the ground, the ground air pressure drops sharply and the ground wind speed rises sharply, forming a tornado.

The formation and disappearance of tornadoes are unstable processes of airflow movement.