Funnel cloud: a rotating column of air that usually develops within a cumulonimbus cloud and subsequently extends toward the ground. We see this stage as a rotating funnel cloud that decends from the cloud base and approaches the ground.







Tornado: When the rotating column of air reaches the ground, by definition the disturbance becomes a tornado. We can see the funnel cloud reach the ground: sometimes we can also see dust and debris whirling on the ground before the funnel actually touches down. In weak tornadoes, particularly in dry climates, the ground level dust whirl may be visible before the funnel cloud.







Mature Tornado: During the tornadoes mature stage, the funnel reaches it's greatist width. It is always almost verticle and most of the time is touching the ground, though skipping may occur along a lengthy path. At this time the tornado causes severe damage to whatever it encounters.







Shrinking Stage: During the shrinking stage, the funnel narrows and tilts away from it's verticle position. At this time the path of damage becomes smaller.







Decaying Tornado: As the tornado decays, the funnel stretches into a rope shape and the visible portion becomes contorted and finally dissipates. This stage is often called the rope stage because of its appearance.



(National Audubon Society: Feild Guide to North American Weather; pg.109)

Origins of a Tornado & optimal weather conditions

The distinguishing feature of the tornado is the funnel. The visible funnel is formed by the condensation of water vapor resulting from the lower pressure within the whirl and from debris drawn into the spinning mass as the size and strength of the funnel increase



Several funnels may develope in a mature tornado system, with small vortices continually forming and dissipating while whirling around the central core of the main tornado circulation. A tornado funnel can assume various forms, from a thin, writhing, ropelike column of grayish white to a thick, amorphous mass of menacing black.



In the Northern Hemisphere, tornadoes almost always spin counterclockwise, although verified instances of clockwise circulation have been reported. Films of tornadoes haverevealed that the visible flow on and just outside the funnel surface is nearly always spiraling upward.



In recent years, the exact mechanism that causes a tornado to form, called 'tornadogenisis,' has been the subject of increasingly fruitful research. Nevertheless, some mystery still surrounds tornadoes, and their formation cannot be predicted with absolute accuracy, even when conditions for their occurance seem just right.



Tornadoes are usually associated with thunderstorm conditions. They require a moist airstream that is warm for the season and usually comes from a southerly direction. A 1986 survey has shown that most tornadoes occur when air temperatures are above 65 degrees F and the dewpoint (temperature at which condensation begins) is 50 degrees F or higher.



Sometimes a tornado is formed by a small cyclonic circulation, called a mesocyclone or tornado cyclone, embedded in a larger storm circulation. Mesocyclones are about ten miles in diameter and contain miniature cold and warm currents that interact to form a tornado funnel.



Certain atmospheric conditions are conductive to the formation of tornadoes. One such condition is a teperature structure of the atmosphere in which warm air overlies cold air (called an inversion). Inversions inhibit verticle currents and help maintain the concentration of low-level moisture.



The early pesence of the inversion also prevents deep convection from dissipating heat energy upward. At intermediate levels of the atmosphere, a warm, dry layer usually has an air movement from the southwest: at high levels, a strong westerly flow with jet stream characteristics prevails. A graphic cross section of the entire mass of air would show deep convective instability, with large drop-offs of temperature and moisture through a great depth.

Dynamic lifting in rising air currents soon destroys the inversion. Strong low-level vertical wind shear and a low-level jet flow assist in tornado formation



The contraction of the tornado cyclone into a tighter circle causes increasing wind speeds, which leads to decreased air pressure inside the cyclone and more violent updrafts. This process, known as the conservation of angular momentum, can cause winds of 50mph to 250mph: the process works in much the same way ice skaters preforming spins increase speed by drawing their arms in tight to their bodies.



Direction of Movement: Generally speaking, 87 percent of all tornadoes have preferred directional movement, which is from southwest to northeast. Rarely do tornadoes move toward the west. Some tornadoes, however, have been reported to move from any quadrant, change directions abruptly, follow zig-zag paths, become stationary, or perform loops or complete circles.

(National Audubon Society: Field Guide to North American Weather; pg.111)