May 15, 2013: A tornado is seen in North Texas.myfoxdfw.com
One of several tornadoes observed on May 3, 1999, in central Oklahoma.NOAA
A screenshot from a video of a tornado over the town of Millsap, Texas, taken by storm chaser Greg Kourounis on May 15, 2013.Screenshot, gkourounis @YouTube
Born from thunderstorms, tornadoes can rip apart homes and toss cars around like toys. The country was reminded of that Monday when a massive storm killed at least 24 people in Oklahoma, crushing homes into piles of broken wood and crumpling cars and trucks on the roadside.
Tornadoes are the most violent storms on Earth. Globally, latitudes between 30 and 50 degrees have the most favorable environments for tornadoes. But under the right conditions one can form anytime, anywhere.
Forecasters struggle to learn more about these deadly storms so they can more accurately predict when and where they will strike next.
How tornadoes form
The U.S. experiences the most of any country. They occur on average 1,000 times and kill 60 people per year, mostly in two regions -- Florida and the so-called "Tornado Alley," the area between the Rocky and Appalachian Mountains. But they've been recorded in all 50 states, usually between late winter and mid-summer.
No one knows what forms a tornado, but scientists agree on a few general points. The most common tornadoes come from rotating thunderstorms, called supercells.
In a supercell, the updraft of warm air rotates counter-clockwise in the northern hemisphere, reaching speeds of up to 175 mph and forcing cool air to descend. The updraft can pull cool moist air -- which would ordinarily fall from clouds -- back up into the sky. The saturated air then condenses to form a rotating wall of clouds, typically toward the back of a cloud in rain-free zones.
With enough strength, the spinning effect creates a funnel, hits the ground and creates a tornado that can last several seconds to more than an hour. Damage can range from superficial, such as roof damage from a falling tree, to completely sweeping away a structure, leaving only the foundation.
Tornadoes are measured on the Fujita scale, created in 1971 by Dr. T. Theodore Fujita of the University of Chicago. It rates the twisters by intensity and area, grading them from weakest to strongest:
* F0 (Gale) * F1 (Weak) * F2 (Strong) * F3 (Severe) * F4 (Devastating) * F5 (Incredible)
The damage caused by tornadoes grows exponentially. An F1 storm can cause moderate damage, peeling tiles from roofs and pushing parked cars; an F3 storm will tear walls from houses and uproot trees. An F5 storm, the most dangerous and deadly, will carry cars through the air and damage even concrete buildings.
Nature's destructive power
The deadliest storm in the U.S. was called the Tri-State Tornado; it swept through Missouri, Illinois and Indiana, killing 695 in March 1925. The biggest outbreak of tornadoes happened in April of 1974, when 148 tornadoes ravaged 13 states. In the end, 310 people lost their lives.
Objects can be thrown long distances by tornadoes. Items as large as cars and trucks can be tossed about or pushed along the ground for dozens of yards. But smaller, lightweight items -- or bits of them -- can stay aloft for many miles.
Some examples from the University of Oklahoma's Tornado Debris Project: A flag from a golf course was found 43 miles away, and a cancelled check drifted 125 miles from the site of a 1995 tornado.
The 1996 movie "Twister" depicted storm chasers trying to place small instrument pods in the path of a tornado. The instruments would be carried aloft by the tornado and provide valuable information for forecasters. The biggest obstacle to such efforts is also the most basic.
With limited lead time, getting to the right place at the right time has so far proven to be an insurmountable task. But researchers continue to try more sophisticated methods in an effort to find out what happens inside a funnel cloud.
Much remains unknown but researchers continue to figure out more accurate ways to predict when and where they'll happen next.
The Associated Press contributed to this report