Tornadoes are cause by the thunderstorms being extremely large, unstable and with wind shear in the lower region of the atmosphere. Instability refers to the hot and humid conditions in the lower atmosphere and cooler conditions in the upper atmosphere. Due to the variation in the vapor pressure of hot and cool air, the hot air rises above and the cool air drops below. Wind shear refers to the changing of wind direction and wind speed increasing with height.
The formation of tornadoes is followed in a set pattern of simple steps. Firstly, when the thunderstorm approaches, the wind direction, speed changes in the upper area of the atmosphere and causes an invisible horizontal spinning effect in the lower region. Secondly, the horizontal air is tilted to vertical because of the rising hot air’s updraft. It eventually forms into a spiral with diameter ranging from 2-6 miles of rotating air. Thirdly, a rotating wall cloud originating from the center of the storm (eye of the storm) is formed as the base cloud. This region is rain-free. Just moments later, a tornado is formed and it starts revolving around violently at high speeds and cause havoc.
Even weak tornadoes can occur when the wind shear conditions are strong, but the atmosphere is not very unstable as both instability and wind shear are necessary for tornado formation. For instance, in California in the winter when a strong low pressure system comes along. Similarly, weak tornadoes can occur when the air-mass is very unstable, but has little wind shear.
Tornadoes are very dangerous and if you are around one, you should probably take cover and wait. Tornadoes are very exciting to watch too, but always be safe while watching a tornado as everybody cant be a storm chaser with all lucky stars!
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