The exact mechanism by which the tornadoes form is still not clearly understood. Meteorologists agree on one point at least that violent convection in conditionally and convectively unstable air column is the main cause of their development.
Tornadoes associated with cold fronts are easy to explain. When the cold and dry polar air mass pushes the warm and humid tropical air mass (mT) aloft, an element of instability is introduced in it.
The latent heat of condensation retards the adiabatic rate in the rising warm and moist air to one-half or one-third of that in the dry and cold air. Thus, the rising air currents reach great heights. Rising air moves spirally in a counterclockwise manner.
This causes a very low pressure in the centre of the air column. Lower pressure initiates stronger winds. As the winds increase their velocities the gyratory motion in the updraft increases. This results in lowering the pressure at the centre further.
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The lowering pressure cools in the air rapidly to below the dew point. In conformity with this chimney of low pressure the characteristic funnel shaped storm cloud develops.
The tornadoes have their beginnings at the cloud level with a whirling cyclonic motion around a vertical axis. The whirl is initiated by the upward movement, and after it has begun, contributes to the updraft.
However, there are conflicting views regarding the origin of these storms. According to some meteorologists, the cold and dry air aloft, usually polar continental air, overrides tropical maritime air.
Thus, a lid is formed in the atmosphere. Whenever this lid is penetrated by the tropical maritime air, it causes a violent updraft as in a chimney, which results in cyclonic or whirling motion. Other meteorologists suggest that it is the polar continental air which penetrates the lid forcing the tropical maritime air violently upward.