How a  tornado works

1.  A large, thermally stratified situation develops in the atmosphere, with  plenty of hot, humid air trapped beneath cold, dry air.

2.  For some reason, the "cap," (the stable layer of air between the hot and  cold air) is disturbed. The disturbance can be caused by an upper-level air  disturbance, or the arrival of a front

3.  As the lower-level air rises, it expands in the reduced air pressure aloft  (air pressure drops as altitude increases), and it cools. Eventually, the  cooling causes the moisture to condense

4.   Condensation releases latent heat, warming the air, making it bouyant, and  causing it to rise quickly (at speeds up to 150 mph). By now, the cloud has  formed into a thunderstorm. Upper-level winds tilt the thunderhead to create the anvil at the top.

5.  The thunderstorm may die out in intense rain and/or hail. Or it may spawn a  tornado.

6.   Interactions between air at various altitudes, humidities and temperatures  causes rain, lightning, air circulation and an intensification of the rotating  updraft, called a "mesocyclone." Low-level wind helps cause this  rotation, which is almost always counter-clockwise (seen from above) in the  Northern Hemisphere.

7  A tornado may form below the mesocyclone. As the spinning column of air  narrows, it rotates faster and extends higher into the storm.