How does a tornado start and end?

Answer 1

There are aspects of both that are not fully understood.

For formation:

First, a condition called wind shear, in which the speed or direction of the wind changes with altitude. If the shear is strong enough it can essentially tilt a thunderstorm, this separates the updraft and downdraft of the thunderstorm, preventing them from interfering with one another. This allows the storm to become stronger and last longer.

Additionally, if the wind shear is strong enough it can start the air rolling in what is called horizontal vorticity. This horizontal vorticity can then be turned vertical by a thunderstorm's updraft. When this happens, the thunderstorm may start rotating. The rotation is especially strong in an updraft called a mesocyclone. If the storm intensifies rapidly enough, a relatively warm downdraft called a rear-flank downdraft or RFD can wrap around the bottom part of the mesocyclone. This can then tighten and intensify its rotation and bring it down to the ground to produce a tornado.

For how tornadoes end, it is thought that cold air coming out of a thunderstorm (called outflow) undercuts the mesocyclone, the rotating updraft that drives the tornado. This chokes off the supply of warm air that the updraft feeds the updraft.

Answer 2

A tornado forms when warm, moist air meets cool, dry air, creating a rotating column of air that extends from a thunderstorm to the ground. The tornado ends when the rotation weakens and dissipates, which can happen when the storm loses energy or encounters terrain that disrupts its structure.