Meteorologists still do not know exactly how tornadoes form, or why. But many facts are known.
Tornadoes almost always form in strong thunderstorms which develop along what is called a "dry line." A dry line is a boundary between warm, moist air and cooler, drier air which tries to slip over it. This causes a tremendous instability in the atmosphere, as warm air rises, and cooler air wants to sink. As the warmer, more humid air surges upward through the cooler air mass, water vapor condenses, causing huge cumulonimbus clouds.
The condensation of the water vapor also generates more (latent) heat, which causes the warm air, already rising, to rise even faster. In large cumulonimbus clouds, often over five miles high, there can be winds of over 70 miles per hour -- going straight up. But, as these streams of air go up, other streams of cooler air must come down. As these downward winds reach the earth, straight-line winds can go just as fast as the upward streams of air. You may have noticed in a thunderstorm that the first winds you feel are quite cool.
Meteorologists believe that where the upward and downward flow of winds meet, a horizontal (flat) area of spinning winds form, or a vortex lying sideways, so to speak. This is called horizontal vorticity. This process is greatly enhanced by strong wind shear, in which the speed or direction of the wind changes with altitude.Below this horizontal vortex, in certain places of the storm, the upward push of the warmer air is stronger, tending to "tilt" the vortex. As this process continues, they believe, the vortex is gradually changed from horizontal to vertical (up-and-down), and the storm may start rotating. .
As the vortex changes its orientation, it begins to act as a special kind of conduit for warm and cold air. Cold air descends, in the rotating winds that can be seen. But within the ring of rotating winds is an area of extreme low pressure; once again, caused by rising warm air, in an upward flow of tremendous speed. 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
The general principle can sometimes be seen in a draining sink. If you drain a sink with very still water, sometimes it just drains; nothing special happens. But if you give the water a little extra motion, you will see a spinning "funnel" form, directly above the sink drain. The mechanics are pretty much the same. Water is heavier than air. The air in the drain pipes wants to rise out, and the water wants to sink. The vortex that forms lets this happen faster.
What is not known about how tornadoes form is why some giant thunderstorms produce tornadoes, and others, seemingly having the same characteristics, don't. Weather scientists are still studying what the differences may be, and trying to find a more exact understanding of how tornadoes form.
It is known that a dry line moves, generally, from the northwest toward the southeast. The warm air is usually moving from the southwest toward the northeast. It would be like two arrows crossing each other at right angles, with the "cold air" arrow on top and the "warm air" arrow on bottom. Meteorologists theorize that this crossing pattern adds to the development of a tornado, determining the direction of rotation (counterclockwise in the Northern Hemisphere), and determining where in a storm a tornado is most likely to form (the southeastern quadrant). Doppler radar has been very successful at predicting areas of likely tornadoes in storms, as a tornado will generally have a "signature" pattern on radar, resembling a sort of curlicue.
Scientists look at what does and doesn't happen in a storm, to try to understand more about why tornadoes form. It has been observed that there is not usually any rain near a tornado, although there may be heavy hail near one. It's also been noticed that there is heavier lightning around tornadoes. Some theorize that electrical charges may aid in the formation of a tornado, while others believe a tornado is the reason for the increased electrical charge. Some have theorized that even truck traffic on a fast highway can contribute to tornado formation, when the conditions are right, as a truck leaves a vortex of air behind it as it passes.
While no one yet knows exactly why a tornado forms, knowledge is growing every year. It is known what conditions have to be present, and it's likely that most of the current theories are pretty close to the truth. But dedicated scientists want to know more, and there are numerous scientific projects underway to try to understand tornadoes better, to aid in predicting them and saving lives.
Meteorologists and other scientists, as of 2009, still do not know exactly how tornadoes form, or why. But some facts are known.