More than 1100 tornadoes tear through the U.S. every year, causing serious damage to lives and property—such as the one in North Dakota, whose 120 mile-per-hour winds injured nine people and destroyed 13 homes on Monday night. Or the April 2014 outbreak where, over a period of just three days, 75 tornadoes battered the swath of land between Nebraska and North Carolina.
The timing of these severe storms isn’t a coincidence. Tornado activity peaks in April, May, and June, especially in “Tornado Alley”—the area around Texas, Oklahoma, Kansas and Nebraska, where conditions are just right for brewing up a twister.
Unfortunately, weather systems are extremely complicated and scientists don’t fully understand how or why tornadoes form. What they do know is that there are three basic ingredients: moisture, atmospheric instability (which tends to lift air upward rapidly), and clashing air fronts.
The process starts when warm, low-lying air from the Gulf of Mexico crashes into cooler, higher-elevation air coming off of the Rocky Mountains. The clashing fronts create a swirling thunderstorm called a supercell.
What comes next isn’t exactly clear. The leading hypothesis suggests that as the cool air flows over the top of the warm air mass, it creates a horizontal spinning vortex. The horizontal vortex gets tipped into a vertical orientation as the warm air mass rises, pushing one end upwards, and as rain or hail drags the tail downwards.
Another hypothesis posits that the twister actually starts out in a vertical orientation. As the air masses crash into each other, the warm air rises, hollowing out a column in the supercell where winds spiral upwards in a clockwise direction. The rotating column is called a mesocyclone, and it occurs a few miles up in the atmosphere. Here, again, the vortex is pulled down to the ground as warm air moves upward and cool air drags the tail downwards.
When the vortex touches ground, that’s when it is officially designated as a tornado. The National Weather Service issues a tornado warning, and local residents run for cover.
The twister begins to die down when the cold downdraft chokes off the supply of warm air that feeds it. The funnel contracts in size but—like an ice skater who pulls her arms in during a spin—the winds speed up and the tail may still whip around violently. Eventually, the storm fizzles.
Scientists have measured tornado wind speeds of up 318 mph. To date, the U.S.’s deadliest twister was the Tri-State tornado of 1925, which rampaged through Missouri, Illinois and Indiana, killing 700 people and destroying 15,000 homes.