Communities across a wide swath of the country are cleaning up from violent storms that have been sweeping through, one after another, over the past few days. More than 200 people have been killed. This year has been unusually active for tornadoes---more than 300 have touched down so far, and we haven't yet reached May, typically the worst month. Decades of research have made these storms more predictable, giving people more time to find shelter, but we're sadly still vulnerable.

1 ) In order for a vortex to be classified as a tornado, the violently rotating column of air must be in contact with both the cloud above and the ground below.

2 ) Though tornadoes do occur on other continents, North America's geography makes it more vulnerable to them. Bradley Smull, an atmospheric scientist at the National Science Foundation, explained yesterday in a Washington Post online chat: "In particular, the proximity of a major north-south mountain range...and the Gulf of Mexico...all in a latitude range frequented by strong upper-level jetstreams amounts to something of a "perfect storm" for severe (supercell-type) thunderstorm formation."

3 ) Tornadoes are rated on the Enhanced F (EF) Scale (the old scale was called the Fujita (F) Scale), which assigns a number (0 to 5) based on estimates of 3-second wind gusts and damage. There have been more than 50 F5/EF5 tornadoes recorded in the United States since 1950.

4 ) Rain, wind, lightning and/or hail may accompany a tornado, but none of them is a reliable predictor of an oncoming tornado.

5 ) A tornado can last from a few seconds to more than an hour. On average, they persist for about 10 minutes.

6 ) It is a myth that a tornado cannot pass over features like valleys, mountains, lakes and rivers. When it passes over a lake or river, a tornado becomes a waterspout.

7 ) Tornado alley is the region in the middle of the United States where tornadoes are most frequent. However, every U.S. state and every continent (except Antarctica) has experienced a tornado.

8 ) A tornado watch means that conditions are ripe for a tornado; a warning means that a storm has been spotted on the ground or via radar (and you should take cover immediately).

9 ) Since the first tornado forecast was made in 1948, tornado warning lead times have been increasing and now average 13 minutes. However, they have a 70 percent false alarm rate, which may lead some people to take them less seriously than they should.

10 ) Mobile homes aren't more likely to get hit by a tornado than any other type of building, but their flimsy structure provides little protection against strong winds and flying debris.

11 ) It's also a bad idea to take shelter in a car---which can be easily tossed about---or under a bridge, where a person would be vulnerable to flying debris or a bridge collapse.

12 ) The worst tornado outbreak was on April 3 and 4, 1974, which saw 147 tornadoes across 13 states. The single deadliest tornado killed 695 people in Missouri, Illinois and Indiana on March 18, 1925.

13 ) A tornado that struck Washington, D.C. on August 25, 1814, is credited with driving the British invaders out of the city and preventing them from carrying out further destruction. They had burned the White House and much of the city the day before.

14 ) The city of Greensburg, Kansas was flattened by a tornado in 2007, but instead of abandoning the town, the people are rebuilding with an emphasis on green technology.

15 ) In 2009 and 2010, more than 100 scientists participated in VORTEX2 (funded by the National Science Foundation and National Oceanic and Atmospheric Administration), which set out to track tornadoes as they formed and moved across the landscape. The V2 researchers are trying to answer many basic questions about tornadoes, such as how, when and why they form, how strong the winds get near the ground, how they do damage, and how predictions can be improved. During the two years, they collected data from dozens of storms and tornadoes.

V2 scientists are the focus of a new IMAX movie Tornado Alley, which I watched this morning, marveling at the 50-foot-high twisters. Tornadoes are incredibly ephemeral and dangerous phenomena, and the movie drove that home, leaving little question why scientists are still struggling to answer these questions. With all of the data collected recently, warning times are sure to improve, and scientists now are able to debate how much time might be too much (that is, given enough notice, would people would fail to take shelter and instead get in their cars to drive away, potentially exposing themselves to more danger).

The destruction this week is a reminder that we need to take these storms seriously, and we might want to start by reviewing this tornado safety advice from NOAA.