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."