While no two tornadoes are the same, there are certain conditions that are required for their formation. A tornado's development can be described by a sequence of distinct phases.
There are two important aspects needed in the formation of a tornado, geography and rotation. The formation and life cycle of tornadoes can be explained in a series of stages:
Stage 1 - Storm development
Sunshine heats the ground which in turn heats the air near ground level. Localised pockets of air become warmer than their surroundings and begin to rise.
Where these warm bubbles of air (thermals) rise to sufficient height, shallow cumulus clouds develop.
If the temperature in the surrounding atmosphere decreases rapidly with height (an unstable atmosphere), the warm bubbles may rise to much greater heights, resulting in the development of much deeper, stronger currents of ascending air (updraughts) and associated deep cumulus and cumulonimbus clouds (i.e. thunder clouds).
Stage 2 - Storm organisation
When the above process occurs in an environment where winds increase strongly with height (strong vertical wind shear), the thunderstorm updraught may begin to rotate.
This happens because the strong wind shear creates a horizontal spin in the atmosphere.
The strong updraught tilts this rolling motion into the vertical, so that the spin occurs about a vertical axis, in the same sense as the rotation of a merry-go-round.
Thunderstorms that exhibit persistent and deep rotation are called 'supercells'.
Stage 3 - Tornado formation
Downdraughts within the supercell storm (descending currents of relatively cold, dense air) help to concentrate the rotation and to bring it down to lower levels.
Eventually the rotation may become so strongly-focused that a narrow column of violently rotating air forms. If this violently-rotating column of air reaches the ground a tornado is born.
The tornado is often visible because of the presence of a condensation funnel - a funnel-shaped cloud which forms due to the much-reduced pressure within the tornado vortex.
Dust and other debris lofted by the intense winds can also help to make the tornado visible.
Stage 4 - Tornado dissipation
Cold downdraughts eventually wrap around the tornado, cutting off the supply of warm air. The tornado typically narrows during this stage, and eventually the vortex dissipates.