Tornadoes, nearly three-quarters of which occur within the U.S., are unpredictable and can cause massive damage. New tools and data are helping scientists learn more about when they might form and what paths they might take. But what exactly is a tornado and how does it form? Exactly how and why tornadoes occur is still a bit of a mystery, but an indication that one is forming is a wall cloud. Fast-moving winds roll air below into a horizontal vortex—a spinning tube—above opposing surface winds.Warmed by the sun, buoyant air near the ground begins to lift a section of the horizontal vortex into a vertical position. The vortex extends toward the ground. As it picks up speed, anything in its path can be ripped apart or thrown in the air. Upper-level winds tilt the rotating updraft, called a mesocyclone. This allows the storm to keep growing, as warm air is sucked into the storm away from the cool downdraft. The vortex spins like a top, sometimes up to 300 miles per hour. And while some tornadoes move slowly, others can travel over the ground at speeds of over 110 miles per hour. Some tornadoes are small, but some can be huge. The widest tornado ever recorded, the May 2013 El Reno Tornado in Oklahoma, was 2.6 miles across. That’s wider across than Manhattan. During this tornado National Geographic Explorer Tim Samaras, a storm chaser and scientist with over 20 years of experience lost his life when a tornado picked up and tossed his car up in the air. Research has allowed us to identify the essential ingredients needed for twisters to form in the U.S.: warm moist air, typically from the Gulf of Mexico, cool air typically from Canada, and dry air typically from the Rockies. When these air masses collide they create the perfect condition for a tornado. Tornadoes can spend minutes or just a few seconds on the ground, and the damage can cost billions of dollars to repair. And they can cause mass casualties. The deadliest tornado in history was not in the United States, but a tornado that ripped through Bangladesh in 1989 killing an estimated 13-hundred people. We measure the amount of damage to land and property using the E-F Scale, which ranks tornadoes on six different levels. From zero, a heavy breeze that can uproot crops, to level five, enough wind to pick up a house. Nearly three quarters of the world’s tornadoes occur in the US, and the peak season is between May and June. A majority touch down in Tornado Alley - a section of the central US that runs from southern Texas to the Canadian border. But tornadoes have been recorded in all 50 states and on every continent except Antarctica. Tornadoes are very dangerous and you should never underestimate their power. The average time between a tornado warning and a strike is estimated at a mere 13 minutes, giving residents little time to seek shelter. On average, tornados kill 60 people every year in the United States. It’s the desire to better understand them that sends scientists like Tim Samaras into the field year after year in search of answers that can hopefully save lives, while putting theirs at risk.