After decades of research, scientists know that tornadoes form in "supercells" of thunderstorms and unstable air. And they know the best place to hide from one.
But as funnel clouds swept through the Midwest last week, killing at least 42 and causing hundreds of millions in property damage, they continued to look for the answer that eludes them: Why does one storm cell turn into a deadly twister while another disappears in a puff of wind?
"There's a lot of things about them that we just don't understand," said Howard Brooks, a meteorologist with the National Severe Storms Lab in Norman, Okla., the epicenter of tornado research.
Tornadoes move with such destructive force that it's hard to get close enough to study them. And unlike hurricanes, which are tracked and filmed for days, tornadoes are ephemeral.
"It can last only a couple of minutes, and can be there one minute and gone the next," says Roger Wakimoto, a meteorology professor at UCLA.
Even the exact force of their terrible winds is unknown. Officially, tornadoes are ranked on the Fujita damage scale. As the name implies, it is based on after-the-storm estimates of destruction, not on real wind measurements. "It's still an open question as to how powerful they really are," Wakimoto said.
The tornadoes that hit Kansas, Missouri and other Midwestern states last week were rated F-4, with winds estimated at more than 200 mph. The scale ranges from F0, light damage, to F5, incredible damage.
Researchers are trying to figure out why some storm systems create tornadoes while similar systems do not. "We don't understand what goes on in the last few minutes before a tornado forms," said Brooks, who served as a technical adviser on the 1996 movie Twister.
Scientists do know that tornadoes form in supercells - powerful storm systems spawned by temperature shifts that come with changing seasons. Supercells contain the key ingredient for a tornado: unstable winds.
Each spring, masses of warm, moist air move north from the Gulf of Mexico and mix with cooler, drier eastbound air higher up in the jet stream. Oklahoma is the heart of "tornado alley" because it is where the two air masses often collide.
The mixing creates updrafts that can rotate for hours. In most systems, there's enough moisture to create rainfall, which quiets the movement before tornadoes can form. But when there's no rain, the updrafts can spawn a tornado inside the supercell, essentially creating a storm within a storm.
Less than 10 percent of supercells spawn tornadoes. Why some do remains a mystery. But researchers have made progress on warning systems and building methods to minimize damage.
"We've come a long way, but there's still a lot of work that should be done," said Howard Bluestein, a University of Oklahoma meteorologist.
A study of a 1999 Oklahoma storm that damaged 8,000 buildings led to a federal report that helped communities prepare for tornadoes and pass tornado-resistant building codes. Other studies have confirmed the long-held belief that the safest haven is a small and low interior room.
"We found the advice we were giving people really worked. If nothing else, it was reassuring," said Brooks.