Some violent thunderstorms, develop into tornadoes, when there is enough amount of- instability and wind shear in atmosphere (lower).
Instability and wind shear- major reasons of tornadoes
Instability means a situation in which extremely humid and warm air is present in the atmosphere (lower) and at the same time exactly opposite conditions are present in upper atmosphere.
Wind shear is a situation when wind changes its speed and direction with the height. Example of this situation is- southerly wind flowing at the surface with the speed of 15 mph changing into westerly or southwesterly wind at the height of 5,000 feet now flowing at the speed of 50 mph.
This kind of instability and wind shear exists only before a low pressure and cold front system. The intensified spinning of some tornado is because of the downdrafts and updrafts in the thunderstorm, (and it is caused by presence of unstable air). These downdrafts and updrafts of the thunderstorm interact with the wind shear, this leads to the wind shear tilting, which finally creates tornado vortex.
Now, the air that flows cyclonically around the cyclone (which is already spinning in anti- clockwise direction) participate in spinning of the cyclone as a result spinning speed increases. To understand this processes one can consider the analogy of the case of some ice skater. An ice skater pulls his arms, toward his body in order to increase the spinning speed. Same happens in the case of a tornado.
Some other processes can also increase the chances of the formation of tornado. For example- rapid cooling of the dry air of the atmosphere (middle) by rains of the thunderstorm, further causes strengthening of the downdrafts, which are extremely essential for the formation of tornadoes. If somebody observes some pictures of tornadoes then one can easily note that tornadoes always appear in between the boundary of bright clouds (region of downdraft) and dark clouds (region of updraft). This fact clears the importance of downdraft and updraft regions, for the formation of tornadoes. Moreover, some strong thunderstorm, which is present near a squall line, when merges with that squall line, it becomes tornadic. In fact, isolated storms are very prominent formers of tornadoes, because an isolated storm will face less competition for the “fuel” of unstable air as compared to the storms of some “line of storms”.
Both conditions of instability and wind shear are necessary requirement for the tornado formation. But, if only one of these conditions occur then even weak tornadoes can appears. This happens in some rare cases, especially in the southern California (during winter season) and Florida. But most violent forms of tornadoes can occur only, if both of these violent conditions are present.