| MadSci Network: Earth Sciences |
Good Question! A meteorologist friend and I have even come up with some imagery that is intuitive (we hope).
If you picture a tornado as a cone, there is a low pressure area in the center of the cone (like a straw, extending from the top to the bottom of the cone). This low pressure area draws air into the tornado, mostly at the bottom, and this influx of air can 'pinch' the bottom of the tornado down to a narrow tube. Larger tornados with higher wind speeds resist 'pinching' better than smaller ones, and can appear as large barrels (no pinching or coning).
Radar returns (and visual clues) tell us that the bottom of a tornado spins faster than the top of the tornado, caused by the greater influx at the bottom of the tornado. The faster spin at the bottom of the tornado is analygous to an ice skater. In a spin, the skater can pull in her arms and spin faster, or allow her arms to extend and slow her spin (angular momentum is conserved). The tornado, also, spins faster as its diameter gets smaller (pinched) by the influx, while the upper part of the tornado spins slower, with a larger diameter.
Again, good question. Additional information on tornados is available as follows:
The last address (www.tornadoproject.com) has a 'Top Ten' list of tornados, a must for those folks who aren't properly respectful of how destructive big tornados can be.
Try the links in the MadSci Library for more information on Earth Sciences.