Tornadoes are unpredictable and deadly--witness the set of twisters that tore through Bangladesh on May 13, killing at least 500 and injuring up to 50,000 people (and look at recent tornado statistics.
Tornadoes are also irresistible, it seems. Stout-hearted storm chasers race to put themselves right in the path of danger, both for the sheer thrill of it and to gather information that will clarify how tornadoes form, gather strength and dissipate. The obsession seems to be catching: thrill-seekers are flocking to see real and imagined tornadoes blast across movie screens throughout the U.S.
The basic atmospheric physics that gives rise to tornadoes is well understood by now. Thunderstorms usually contain updrafts, large rising swells of warm, moist air. As the updraft moves, it rotates; if the rotation grows sufficiently intense, the storm can evolve into a tornado or funnel cloud (a tornado whose bottom does not touch the ground). Supercomputer simulations depict this process quite dramatically.
Most tornadoes form within an especially intense weather system known as a supercell. Supercell thunderstorms occur when the warm updraft punches through an overlying, stable layer and continues upward into a zone of cool, dry air. The resulting instabilities produce powerful vortex motions, the lifeblood of tornadoes (a pair of computer-generated images depicts the difference between supercell and non-supercell storms). Within the fiercest tornadoes, wind speeds can approach 300 miles per hour. Air rushing in to fill the low-pressure void left by the tornado creates additional fierce, potentially damaging winds. Staying out of danger is no easy task when a tornado is anywhere near.
One of the most dangerous aspects of tornadoes is their capriciousness. Sometimes an updraft gives rise to a tornado; sometimes it does not. Scientists are still hard pressed to predict exactly when and where a tornado will appear; that uncertainty makes it difficult to raise the alarm in time tosave lives.
In order to nail down some of the tricky details of tornado formation, Erik Rasmussen of the National Severe Storms Laboratory in Norman, Okla., banded together with several of his colleagues to undertake storm studies of unprecedented accuracy. The researchers called their project Verification of the Origins of Rotation in Tornadoes Experiment, usually shortened to its handy acronym of VORTEX. During tornado season (April 1 to June 15) of 1994 and 1995, the VORTEX scientists intercepted and monitored 10 severe twisters.
Those studies were greatly aided by a tool known as Doppler radar, which can measure local wind speeds at very high resolution. During the Dimmitt, Tex., tornado of June 2, 1995, the researchers used a single, portable radar device (the "Doppler on Wheels") that produced sharp images showing wind features just 200 feet across--good enough to show clearly the tornado, its evacuated core and the surrounding cloud of debris. This year Joshua Wurman of the University of Oklahoma and his collaborators plan to use a pair of Doppler radars, which will enable them to assemble more complete, three-dimensional maps of wind patterns
At present, the VORTEX researchers are still digesting their massive files of data. A preliminary report from the project participants describes some of the remarkably intricate observations of the Dimmitt tornado, generally considered the most thoroughly studied twister in history.
One frustration voiced in that paper is the extreme speed of these weather outbursts. The time from formation of a tornado to touch down is no more than a few tens of minutes, "providing little time for operational observation, identification and warning," the VORTEX team notes.