Tornados are a part of life for many people, especially in the Midwest where as many as a thousand tornados occur each year. What causes tornados? What are the processes that assist in tornado genesis? Nobody knows exactly why some thunderstorms produce tornados and other storms do not, but we have some clues based on years of observation and more recently, on the preliminary findings of the VORTEX project. At the core of a thunderstorm, warm moist air is being sucked upward in the updraft which is usually near the rear (typically southeast) of the storm. This is called the inflow. Strong storms must have a significant inflow to feed themselves, and as a storm strengthens its updraft, and hence inflow, becomes more powerful and violent. Usually this process does not spawn tornados, but in some cases it does. What the VORTEX project discovered, other than the disheartening fact that there are many yet little understood mesoscale and microscale processes that participate in tornado genesis, is that quite often tornado formation occurs along or following the passing of the storm over an existing outflow boundary (or any other kind of boundary for that matter). Usually these boundaries are left from earlier storms, and are characterized by an mesoscale area of cool, dry air and high pressure which are the remains of the outflow of an earlier storm. When a new storm passes over this boundary, or passes along it, helicity develops. Helicity is the horizontal "rolling" rotation of air as the different air masses separated by the boundary tumble over each other along the surface. This rotation can be tilted up into the updraft of the storm and become vertical rotation, or vorticity. When vorticity is present in the updraft, an area of low pressure develops around the immediate area of the updraft and this is called a mesocylone. Mesocylonic rotation is common in severe thunderstorms, perhaps more common than we ever imagined, as new doppler radar (NEXRAD) is helping to show. As the updraft strengthens, rotation increases and the pressure further drops. In order to get more air, the updraft begins to extend downward and the resulting low pressure causes moisture to condense in the rotating column of air which becomes visible as a "funnel cloud" extending from the cloud base (known as the "shelf cloud") and usually from a further lowered shelf known as a "wall cloud". Most people mistake funnel clouds for tornados, but a funnel cloud is not a tornado, though it is almost always found prior to tornado formation. A tornado isn't classified as such until the rotating air reaches the ground, and funnel clouds are usually visible prior to reaching the ground. If it does reach the ground, then it becomes a tornado, and will usually kick up debris into the updraft which is visible as a dark swirling mass at the surface known as the debris cloud - in a strong tornado, the debris cloud and funnel cloud are mixed together and extend all the way from the base of the wall cloud to the ground.
Tornados are classified according the Fujita scale based on its level of damage (although with new doppler we are beginning to use wind speeds as the basis for classification) or the width of the damage path. F1 is the weakest, all the way to F5 which is the strongest (which can have winds of over 300mph), popularized in the fictional movie Twister as being "The Finger of God" because of its immense destructive path. Some F5s have been known to produce a 2 mile wide damage path in places! Tornados can be easily confused with gustnados, which are tornado-like phenomena causes by chance vorticity along a gust front, but bear little threat. A tornado that is over water is called a water spout (there is even such a thing as a land spout!).