How much do you know about nature's deadly tornadoes?

Tornadoes are swirling columns of air that touch the ground. Most tornadoes are fairly minor and don't cause too much damage, but once in a while they strike populated areas and cause destruction and death reminiscent of a war zone.

We mostly call them tornadoes, but they're also called twisters and cyclones. Nobody calls a tornado Ethel -- and it might hunt you down if you tried.

By a wide margin, the U.S. experiences the most tornadoes each year. The country sees roughly 1,000 tornadoes; Canada has the second most, with just 100.

When conditions are at their most violent, tornadoes can gather wind speeds of up to 300 mph. Few human structures can withstand monster storms of this strength.

Scientists have been studying tornadoes for decades but they still don't fully understand how they form. But they generally occur during severe thunderstorms. Researchers are constantly looking for better ways to understand twister dynamics.

Tornado Alley is a name for the swath of the U.S. where tornadoes most often strike. The boundaries aren't strictly defined, but Texas, Oklahoma, Kansas and Nebraska are four states where twisters frequently form.

Most weather pundits refer to tornado strength using the Enhanced Fujita scale, which ranges from EF0 (smallest) to EF5 (largest). Small F0 tornados have relatively weak and non-destructive winds. F5 tornadoes, on the other hand, can scour the Earth and are terrifying to behold.

The U.S. has some geographical quirks, like the north-south alignment of the Rocky Mountains and the location of the Gulf of Mexico, that contribute to strong thunderstorms … and thus, tornadoes.

A tornado watch means that a tornado is possible in your area. A tornado warning, on the other hand, means that radar has detected an actual tornado, and you should take appropriate action.

In a tornado warning, take cover. Hide in the lowest level of a building or an interior area with no windows, which turn into deadly shrapnel during a tornado.

Tornadoes often create a deep, thunderous roar that's something like a freight train. People who survive tornadoes say the sound is something they'll never forget.

Like something out of sci-fi movie, the clouds boil and swirl and they may take on a bluish or greenish tint. These strange hues are a major clue that Mother Nature is in a violent mood.

As the heat of a day increases so too does the possibility of severe thunderstorms and tornadoes. Most cyclones happen in the late afternoon or early evening, but they can strike at any time.

Some tornadoes are just brief wisps; others can ravage the Earth for hours on end. The average twister lasts for about 10 minutes.

Wall clouds are low, isolated clouds that often show signs of high-speed updrafts. Sometimes you can witness wall clouds transforming into funnel clouds right before your eyes.

The 2011 tornado that hit Joplin, Missouri, caused around $2.8 billion in damage, in part because it destroyed an incredible 7,000 homes. Worse, it killed 158 people.

Many tornadoes travel at less than highway speed -- you can outrun them if you're careful. In a populated area, though, take cover. The twister might catch up to you, and equally bad, driving like a maniac on city streets is likely to end in a car crash. In many cases, tornadoes are obscured by rain or clouds, meaning you're often better off just taking cover when possible.

Supercells are thunderstorms with intense rotation characteristics. These thunderstorms are gigantic events and often notable for their anvil-like shaped clouds.

It's a common misconception that tornadoes can't climb or cross mountains. They are rare in mountainous areas, but they can occur in very rough terrain at more than 10,000 feet.

Tornadoes are most common in North America but they've been spotted on every continent except Antarctica, which is good, because the penguins would have nowhere to hide.

Violent thunderstorms may churn the air and cause clouds to ominously rotate. Rotating clouds are a sign of peril -- funnel clouds can form in these situations.

The 1925 Tri-State Tornado swept through Missouri, Illinois and Indiana, killing at least 747 people. At least nine schools were destroyed. It was easily the deadliest tornado in American history.

Bridges and overpasses can actually cause tornado winds to surge. And they offer no protection from flying debris, which is one of the most dangerous aspects of twisters. Take cover in low areas instead.

A waterspout is a tornado that forms over water. These funnels sometimes appear during hurricanes.

It's dust and debris that give tornadoes their dark color. Some twisters, particularly those that have just formed, may be almost transparent until they begin sucking up dirt, buildings and cows.

Greensburg's residents took the disaster as an opportunity to rebuild every structure to green standards laid out by the Leadership in Energy and Environmental Design (LEED). It's sometimes called the greenest town in America.

In May 2013, an F5 tornado with winds of nearly 300 mph developed in Oklahoma. Its path was 2.6 miles wide, a swirling mass of destruction that killed at least 18 people.

Dust devils look just like tornadoes but they occur during fair weather, often with blue skies. They are generally fairly weak, but they're not to be trifled with. They've caused major damage and death.

The Great Tornado hit during a peak time of river traffic. The twister killed at least 269 people who were on boats, and then dozens more on land. With roughly 320 victims, it's the second-deadliest U.S. tornado on record.

Since 1950, there have been about 60 EF5 tornadoes recorded in the U.S. That's about 1 EF5 twister for every 1,000 or so funnel clouds that touch down.