What Causes a Tornado?

Nothing short of a hurricane can fascinate like a tornado. Every year, hundreds of people – from meteorologists and nature photographers to the merely curious – go storm-chasing across the countryside to measure, document, or simply witness tornadoes’ awesome power. Often, the sight of a funnel cloud in the sky will send folks running for their cameras before running for the lives.

But how do these meteorological monsters form? What force can make a violently rotating column of rising air, with winds swirling up as fast as 318 miles per hour (512 km/h)? Scientists still aren’t sure of every detail, but they do understand some of the basics.

Tornado Maker #1: Updraft

Tornadoes typically form when a mass of cold, dry air blows in over a mass of warm, wet air near the ground (a fairly common occurrence on the American plains as cold, dry air moves in over the Rockies and warm, wet air comes up from the Gulf). Sometimes an intervening layer of hot, dry air keeps the two air masses separate – with one over the other, like a big air sandwich, but separate all the same. Yet if this “cap” gets disturbed or weakens, sunny days are over.

The air near the ground, because it’s warmer than the air around it, can then rise in an “updraft,” like a hot-air balloon. And as the warm air rises and cools, the moisture in it can condense into a storm. In fact, condensation releases latent heat, which can make all that cooling warm air warm again and cause it to rise even higher and more quickly. A particularly powerful updraft can give birth to a towering 50,000-foot thunderstorm in just minutes.

Tornado Maker #2: Wind Shear

Many storms pretty much live by updraft alone. But the worst and longest lived of the lot also manage to hitch their wagons to “wind shear.” Wind shear happens when winds change speed or direction over a short space. For example, the wind at ground level might be a lot slower than the wind just a few thousand feet up. Or, it might blow from the southeast, while the winds above blow from the southwest.

These differences can create a horizontal vortex in the air beneath a storm cloud, with winds that spiral like corkscrews as they advance. And if these horizontal corkscrew winds enter an updraft, they can get tilted up into it. Pretty soon, the updraft takes on their rotation and becomes a vortex, too – a vertical corkscrew of rising air starting a few thousand feet up and spanning across miles. Meteorologists call this rotating updraft a “mesocyclone” and a storm that has one a “supercell.” Supercells are maternity wards for tornadoes.

Tornado Maker #3: ???

Not every supercell spawns tornadoes, but those that do typically spawn them below their mesocyclone – often near a cool downdraft full of rain or hail. Scientists know that these downdrafts play a role in bringing the upward spinning mesocyclone closer to the ground.

But, unfortunately, that’s where sure and certain tornado knowledge ends. Why a huge vertical vortex swirling overhead across miles of sky sometimes gives rise to the pinpoint power of a tight twister in contact with the ground is still shrouded in the dust and debris of the tornado, waiting for science and stormchasers to find.

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