The Cause of Tornadoes
Vortexes are created by a fluid rushing in a horizontal direction. This effect is readily observable with whirlpools in a river. Such a horizontal rush of air is consistent with the characteristics of tornadoes.
Tornadoes are caused when a cloud of the right size precipitates rapidly releasing heat, which causes it to rise, and creates a vacuum under it. Air rushing under it creates the vortex.
It is known that a sudden drop in air pressure precedes tornadoes. The pressure drop is caused by a cloud near the ground rising rapidly creating a partial vacuum below it. The vacuum seems to be quite noticeable, as persons who were near tornadoes often mention it.
Precipitation releases as much heat as evaporation absorbs. But precipitation tends to be much faster than evaporation. So a very large amount of heat is released when a cloud precipitates.
Heat of course causes air to rise. When a cloud near the ground rises, it creates a partial vacuum under it.
The cloud must be the right size for a tornado to occur. A very large cloud would not precipitate uniformly, so the whole cloud would not rise at once. A very small cloud would not produce enough precipitation or heat to create a large enough vacuum for a tornado to form.
Also, the height from the ground would be important, because the speed at which the air moves in rushing under it will depend upon the amount of space below the cloud.
These dynamics only exist during the first few minutes of the formation of a heavy cloud. Older clouds precipitate gradually and higher in the air, so no vacuum is created.
Significantly sized rain drops do not form by the time a tornado occurs. Tornadoes always occur before rain, which may not occur at all. Creating large drops is a separate process from precipitation.
Modern doppler radar substantiates this point. When a tornado is reported, doppler radar shows that a new cloud formed out of nowhere where the tornado was said to be.
Therefore, if people are to be warned in advanced, it has to be for an area where clouds are expected to form but have not yet appeared. Sometimes this is occurs on the edge of a developed storm cloud.
Cumulous clouds will not create tornadoes, because they dissipate energy continuously, and they precipitate too high in the air. A tornado cloud has to form rapidly and dissipate its energy all at once. This occurs when hot, humid air hits colder air. A typical example is gulf air turning north and colliding with other air over Arkansas. In the northern plains, clouds usually form more gradually and dissipate energy through cumulous formations.
NOAA Web Page on Tornadoes