Did you know? The strongest winds are at the edge of the tornado.

Tornadoes aren’t just funnels. In fact, they sometimes contain funnels. Many of the most intense twisters start out as “multi-vortex tornadoes,” a larger rotating drum enveloping several smaller whirls that orbit a common center.

These churning “subvortices” can add to the rotational speed of the main funnel. If a subvortex is spinning at 100 mph but revolving in the main funnel at 80 mph, that can leave a narrow swath of 180-mph damage. That’s what makes tornado damage so irregular and variable over short distances: If a subvortex strikes your home, it could be swept away in a 180-mph gust — while your neighbors remain relatively unscathed at “just” 80 mph.

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Did you know? Tornadoes have calm centers.

Just like the eye of a hurricane, large tornadoes have calm centers. Proof came May 3, 1999, when a vicious F5 tornado tore apart Moore, Okla. A nearby Doppler On Wheels mobile radar probed the vortex at extremely high resolution, revealing a void at the funnel’s center. For years, survivor testimony had indicated a zone of tranquil winds at the core of the most massive wedge tornadoes. Nowadays, we can see that eerily silent hole on radar.

Did you know? Tornadoes can bring friends.

Particularly large tornadoes might be flanked by smaller, weaker tornadoes at a considerable distance from the main funnel. These are not the same thing as subvortices, but are considered entirely different tornadoes. “Satellite tornadoes” have greater odds of spinning anticyclonically, or clockwise, in the Northern Hemisphere. Nearly 99 percent of well-developed tornadoes spin counterclockwise.

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The cause of satellite tornadoes isn’t known. It may be the tendency of some larger tornadoes to shed some of their spin into the surrounding environment, fostering the growth of new funnels. In some cases, an oppositely spinning tornado might dance around an ordinary one. Other times, one tornado may weaken as a storm drops a new, mature tornado and the decaying circulation is drawn into the growing tornado and ultimately absorbed. In any case, the results are spectacular.

Did you know? Most tornadoes in the Northern Hemisphere spin counterclockwise.

It’s extremely rare for a tornado north of the equator to spin clockwise. That’s a result of the local environment, which gives rise to supercell storms.

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Supercells form in association with low-pressure systems, which derive their counterclockwise rotation from the Coriolis effect. But these sprawling “synoptic-scale” lows stretch hundreds or thousands of miles across. Tornadoes are too tiny and short-lived to “feel” Earth’s sense of rotation.

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On tornado days, strong wind shear — or an increase in wind speed with height — gives rise to horizontally rotating tubes of air near the ground. We don’t normally see these rolls until they’re stretched by a thunderstorm updraft, which then begins to rotate. As loops of spin are drawn into an updraft, a pair of vertical rotations develops — one clockwise and one counterclockwise. The dynamic processes unique to developing supercells favor the intensification of the counterclockwise updraft only.

On occasion, a supercell thunderstorm can split into two cells. The left-split spins clockwise. These left-split storms can be prolific hail producers, and, in the rare event one of them drops a tornado, it would be anticyclonic. Again, because of supercell dynamics, clockwise-spinning updrafts are seldom sustainable.

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Did you know? Mountains can play a role in strengthening tornadoes.

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The myth that a tornado will never strike you if you live in a mountainous area is false. In fact, mountains can sometimes encourage tornadogenesis on their downslope side — a frequent catalyst for twister formation in the lee of the Appalachians.

When an area of spin rides down a mountain, it becomes stretched as the ground becomes farther away. When a vortex is stretched, it grows stronger. That’s thanks to the conservation of angular momentum.