Although the the full details of how tornadoes form are not well know, they generally form in a huge, rotating thunderstorm called a supercell. Supercell thunderstorms form where cold dry polar air meets warm moist tropical air. When this happens the warm updraft punches through the overlying, stable layer and continues upward into a zone of cool, dry air. The resulting instabilities produce powerful vortex motions, the lifeblood of tornadoes.
Tornadoes generally form in a part of the supercell called a mesocyclone.
A mesocyclone draws engergy into the storm, helping the storm to last for hours. The funnel clouds that are the most easily recognizable parts of a tornado usually seem to descend from the cloud base. This does not mean that the vortex is descending from the cloud base, rather the vortex already extends from inside the clouds all the way down to the ground. Moisture in the atmosphere condenses at a certain temperature and pressure combination, called the dewpoint. In general, in the vortex of a tornado, the pressure is fairly constant along the entire height, but the temperature is usually higher near the ground than neat the top of the vortex.