Published: 2007-11-24 11:32:00
Updated: 2007-11-24 11:34:20
Posted November 24, 2007
MIKE MOSS SAYS: Sarah, That's a great question and one that there is no single clear answer to in the same way that there is for hurricanes. The formation and life cycle of tornadoes is much more difficult to study in depth because of their sporadic and short-lived nature. Field measurement programs, theoretical studies and simulations with computer models have all given some insight, but the details of why and how tornadoes develop and dissipate remain poorly understood in many respects.
All that said, there are factors that fuel tornadoes that have some overlap with tropical systems. For example, relatively warm, moist air in the lower atmosphere (in comparison with the temperature of air aloft) is a key ingredient in that it provides instability, meaning that air that is lifted above its original level becaomes warmer than its surroundings and continues to rise. When that air also happens to be moist, it will become saturated and condensation will occur, releasing latent heat and sustaining the upward buoyant motion. This is a basic requirement for thunderstorms to sustain themselves or to become more intense. In addition, for supercell thunderstorms (the ones most likely to both persist for some time and to develop significant tornadoes) to form requires significant vertical wind shear, usually involving winds that both increase with height and also winds that shift direction in the lowest levels of the atmosphere. The vertical shear is thought to assist in producing air that has a degree of pre-existing rotation that can be concentrated and intensified by tilting it within a tornado updraft and stretching it vertically. In addition, there are some storms in which a downdraft that develops toward the left rear flank of the storm helps to lower and concentrate rotational energy associated with the supercell storm's more boradly rotating mesocyclone.
Once a tornado vortex forms, sustaining it requires a continuing feed of converging air from outside the storm that can be accelerated by conservation of angular momentum and that can be lifted into and exhausted by the parant thunderstorm. If this inflow is interrupted, for example by a drier airmass being entrained into the storm, or by a cool, stable downdraft/outflow airmass that may originate from another storm nearby or even from another part of the parent storm, the required moisture, heat and instability may be lost, or the air flowing into the tornado region may simply be disrupted or diverted in such a way as to cause the tornado to dissipate.
Most of the above pertains to supercell thunderstorms, but there are also tornadoes that form from non-supercell storms. These usually fall into the category of "gustnadoes," in which a thunderstorm outflow forms a boundary that creates localized areas of horizontal wind shear and in which this shear becomes concentrated into a vortex that gets stretched vertically as the cool outflow air expands. These are usually very brief and fairly weak. Likewise, there are times when an existing weak vortex develops due to a shear zone associated with a frontal boundary, seabreeze front or terrain feature, for example, and a non-supercell thunderstorm or even just a vigorously growing towering cumulus cloud happens to develop over or move into position over the pre-existing vortex. If this happens, the vortex can be stretched vertically and concentrated, with air drawn in from the sides and accelerated, again through conservation of angular momentum. These tornadoes are sometimes called "landspouts," because they share many similarities in structure and environment with run-of-the-mill waterspouts. Landspouts also tend to run toward the weaker end of the tornado intensity spectrum and tend to have rather short lifetimes.
For more info on tornadoes, you might also like to look over the information at these addresses: