Every year around this time, tornadoes materialize out of the ominous slate-grey skies of the Midwest, hurtling across the plains with unpredictable ferocity. Large ones, like the behemoth that obliterated the town of Moore, OK, earlier this month, leave behind a path of gravely familiar damage. The physics of tornado formation, however, remains somewhat mysterious. Even so, meteorologists agree on a certain combination of factors necessary for breeding tornadoes factors that help explain why most of the tornadoes on Earth occur during the spring in a swath of the central United States known as Tornado Alley.
First of all, tornadoes nearly always spin off of intense thunderstorms. These storms form when air at the surface is heated until it becomes less dense than the air around it, and rises. This process, known as convection, actually serves an important role in cooling the surfacewarm air cools as it rises, effectively redistributing heat throughout the atmosphere.
However, if the air near the ground also contains moisture, and the cooler air above is relatively dry, vertical convection can get carried away. Moist air cools at a slower rate than dry aira consequence of the high latent heat of water — meaning that even as the air rises and cools, it remains warmer and less dense than the surrounding air at any given altitude. This runaway effect builds the tall, billowing thunderclouds that meteorologists recognize as clear signs of atmospheric instability.
Not all storms produce tornadoes, though. Only massive, other-wordly supercell thunderstorms possess this destructive power, one they derive from their extraordinarily long durations. Normally, thunderstorms spell their own demise as rising air removes from the surface the heat that drives convection in the first place while the evaporation of falling rain cools it further. In supercells, the vast vortex of the updraft, which looks like a spaceship getting ready to suck Earthlings into its sinister embrace, operates on one side of the storm cell while the downdraft occurs in another. This prevents the storm from exhausting its vital supply of hot air, and allows it to sustain its power over many hours, growing ever more severe.
But the secret ingredient to forming supercells over other kinds of thunderstorms lies in a factor atmospheric scientists refer to as wind shear. Wind shear occurs when air at different levels of the atmosphere flows in different directions. Over the Great Plains, this happens when warm, low-level air flows north from the Gulf of Mexico onto the central states while cold air aloft swoops down from Canada and the Rocky Mountains. This situation occurs commonly in the spring — peak tornado season — because of shifts in atmospheric pressure patterns associated with the changing of the seasons and the strong temperature gradient between latitudes.
As summer approaches, a region of high pressure over the Atlantic Ocean migrates westward toward Bermuda, spinning clockwise and driving moist air up over the plains. At the same time, the strong Westerly jet stream moves north from its southerly winter position, pushing cold air from the still-snowy mountains on top of the Gulf air and creating wind shear. This difference in direction rolls the air into a vortex in the same way that you can roll a ball between your palms by moving your hands in different directions. This creates a rotating mesocyclone that defines and drives supercell storms and which, if the funnel tightens and touches down on the ground, leads to the formation of tornadoes. Check out this video by Scientific American for a visual depiction of this process.
The prevailing wind patterns also account for why most tornadoes in the northern hemisphere rotate counterclockwise (or cyclonically). They are always flung in the same direction by consistent patterns of wind shear over the Midwest, like a top spun by a left-handed spinner. This pattern is reversed in the southern hemisphere where air rotates clockwise around centers of low atmospheric pressure.
Lastly, the Westerlies also play a role in steering tornadoes on their chaotic, short-lived visits to Earth, accounting for the uncanny similarity between tornado paths shown above. The town of Moore experienced other devastating tornados in 1999 and 2003 that tore along the very same stretch of real estate as the 2013 twister, careening from southwest to northeast. The reason for this consistent trajectory is that tornados and supercells live within larger climate features called mid-latitude cyclones that also rotate counterclockwise around centers of low pressure. These atmospheric eddies are the defining climatic feature of life at temperate latitudes, as seen in the global climate model results shown below which shows their long, swooping swirls bringing air from the tropics toward the poles. As it turns out, some of the largest, most everyday features of the climate system help drive some of the most compact, but violent episodes of weather experienced on Earth.
Learn more about temperature and pressure in Earth’s atmosphere in our module.
Explore other kinds of extreme weather at NOAA’s Severe Storm Laboratory.
Check out the mothership of supercell storms and other amazing photos on NASA’s Astronomy Picture of the Day.
Written by Julia Rosen
Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelors degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her Master's in backpacking around the world and skiing. Julia is a periodic contributor to Oregon States research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.