The tornado’s shape can also differ, looking like a long, thin rope or an upside-down bell. The length from the base of the funnel to the tip of its “trunk” can be from 800 to 2,000 feet. Faster upper winds usually blow the top of the funnel more quickly than the lower portion, making the tail seem to drag behind. Earth friction also adds to the lag.
Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/tornadoes/tornadobehavior.shtml
Tornado behavior is never precisely predictable. The funnel may or may not touch the ground, touch and vanish, touch, rise, and touch again, move in circles, spin in one spot for 20 minutes, or cut zigzags through the countryside. When a tornado does touch the earth, its neck can measure from 50 yards to as wide as one mile. It usually moves about 40 miles an hour, cutting a path 10 to 40 miles long. Some tornadoes, however, have traveled nearly 300 miles, cutting a path of destruction 2 miles wide.