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IX. THE TORNADO

Life Cycle

Figure 31 illustrates the life cycle of a tornado. Although not all tornadoes form from mesocyclones, most of the larger and stronger tornadoes are spawned from supercell storms with mesocyclones. Recall that a supercell's environment usually contains strong, veering winds in the lowest mile or so of the atmosphere. These strong, veering winds produce horizontal vorticity ("rolls") in the lower few thousand feet of the atmosphere. The thunderstorm's updraft then tilts these horizontal "rolls" into vertically-oriented rotation and allows the mesocyclone to form.

The tornado circulation develops at mid levels (about 20,000 feet) in the storm where the storm's updraft and mesocyclone are strongest. The circulation gradually builds down (and up) within the storm. At about the same time, a downdraft develops at mid levels near the back edge of the storm. This downdraft, called a rear flank downdraft (RFD), descends to the ground along with the tornado circulation. Rapidly lowering barometric pressure near the ground is believed to be the primary means of drawing the tornado circulation and RFD down toward the ground. The RFD may reveal itself as a "clear slot" or "bright slot" just to the rear (southwest) of the wall cloud. Sometimes, a small shelf cloud will form along this clear slot. Eventually, the tornado and RFD will reach the ground within a few minutes of each other (figure 31a).

After the tornado touches down, an ample inflow of warm, moist air continues into the tornado/mesocyclone. The RFD, though, will begin to wrap around the tornado/mesocyclone after the RFD impacts the ground. The RFD will actually cut off the inflow to the tornado as it wraps around the tornado/mesocyclone. Wind damage may result from the RFD's gust front as it progresses around the mesocyclone (figure 31b).

When the RFD completely wraps around the tornado/mesocyclone, the inflow to the tornado/mesocyclone will be completely cut off. The tornado will gradually lose intensity. The condensation funnel will decrease in size, the tornado will tilt with height, and the tornado will eventually take on a contorted, ropelike appearance before it completely dissipates (figure 31c).

Figure 31(a): Funnel cloud extending toward ground from wall cloud.

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Figure 31(b): Mature tornado (note clear slot in front wall cloud).

Figure 31 (c): Dissipating (rope) stage of tornado. Photos - NSSL.

Tornado Variations

Not all tornadoes go through the life cycle outlined above. Some tornadoes proceed from the developing stage directly to the dissipating: stage, with little time spent in the mature stage. As can be seen in the figures accompanying the above section, tornadoes take on quite different appearances as they develop, mature, and decay. Additional tornado examples are shown in figure 32.

Figure 32 (a): Tornado examples. Thin tornado.

Figure 33 illustrates a multiple-vortex tornado. As their name suggests, multiple-vortex tornadoes have two or more circulations (vortices) orbiting about each other or about a common center. The public often describes multiple-vortex tornadoes as "several tornadoes which join together to form one large tornado." Most of the deadly, destructive tornadoes the United States has experienced in the past (Oelwein, lowa, 1968; Xenia, Ohio, 1974; Wichita Falls, Texas, 1979; Albion, Pennsylvania, 1985, to name a few) were multiple-vortex tornadoes. If you observe a multiple-vortex tornado, relay that fact to your dispatcher/controller, and stay clear!

Figure 32 (b): Large violent tornado.

Figure 32 (c): Dust-tube tornado. Photos - Tim Marshall, Institute for Disaster Research, George Kuykendall.

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Figure 33: Multiple-vortex tornado. Photo - Howard Bluestein