What causes the formation of tornadoes?

Tornados are formed when hot, moist air meets cold, dry air. When these opposite air properties combine they make a supercell thunderstorm. The part of the supercell thunderstorm that a tornado forms in is called a mesocyclone. Another requirement for tornados to form is an unstable atmosphere. When the wam air rises it creates an updraft which mixes with the heavier cold air. This finally makes the wind spin in a circular or cyclonic motion making a tornado happen.
The directional change in wind is known as the wind sheer. When winds develop more the faster they rotate. When they rotate faster they also become more compact which contributes to making the air around the tornado have very low pressure. It also contributes to the air's instability and helps the likelihood that the tornado will be devastating.

All of these factors help tornados form, and some of them enhance the power of the storm. But you always have to have a preexisting thunderstorm to start a tornado. One sign of a tornado forming is an anvil-looking cloud that is located in the eye of the storm. And even if a tornado forms there still is a chance that it won't touch ground, which means that everyone has paniced over nothing! That, is how tornados form. ( http://www.nssl.noaa.gov/edu/safety/tornadoguide.html)