A tornado captures the force of a very large mass of rotating air in a supercell thunderstorm as much as 10 miles in diameter and concentrates its momentum onto a much smaller spot of ground.
The result is a vortex of incredible speed and power.
Meteorologists often use the image of a twirling figure skater to illustrate the effect — how she rotates slowly when her arms are fully extended and then spins dramatically faster as she pulls her arms inward. The physical principle involved is known as the conservation of angular momentum.
Call it what you will, the director of the national Storm Prediction Center in Norman, Ok., observes that it hit with a vengeance Wednesday.
"What we just experienced will go down as an outbreak of historic proportions," Russell Schneider told Discovery News. "These were very large and strong tornadoes over very long paths. The details of the statistics will emerge, but the toll already is very large."
The tornadoes get their spin from their parent storms, as Schneider put it, "from the organizing effect of very strong winds that change with height, which we call wind shear. There is natural spin within the air, and it's greatest on days when winds are very strong and change rapidly with height. Yesterday was one of those days over Alabama, Mississippi, Georgia and Tennessee."
Wednesday's was the second largest single-day outbreak in terms of fatalities — following the April 3, 1974 outbreak that killed 315 people.
For all of the technology and scientific effort that has poured into the subject from the Storm center and other agencies of the National Oceanic and Atmospheric Administration, still there are mysteries about how tornadoes form and how they are able to reach wind speeds as high as 300 miles per hour.
"It is an area of very active research within NOAA and the academic community," said Schneider. "The details of the phemonenal wind speeds will be unlocked by that research."
IMAGE: A supercell thunderstorm and the features that produce a tornado. CREDIT: NOAA National Severe Storms Laboratory