Why Do Tornadoes Spin?

Wind usually just blows past you. So how does it end up whirling in a giant circle?

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The quick answer

A tornado's spin starts long before the funnel shows up. Winds blowing at different speeds and directions set the air rolling, a thunderstorm tips that roll upright, and then the spin squeezes tighter and faster until it reaches the ground.

How it works

Wind shear gets things rolling
Near the ground the wind might be a gentle breeze. Higher up it can be racing along from a different direction. That mismatch is called wind shear, and it makes the air between the two layers roll like an invisible pencil between your hands.
The storm tips it upright
A big thunderstorm has an updraft: a powerful stream of warm air rushing upward. When the updraft lifts the middle of that rolling tube, the roll gets tilted onto its end. Now the storm itself is slowly rotating. Storms like this are called supercells.
Stretch it and it speeds up
As the rotating air gets stretched taller and squeezed narrower, it spins faster. It is the same trick a figure skater uses when they pull their arms in.
Touchdown
If that tight, fast rotation reaches all the way to the ground, it is officially a tornado. Most supercells never make one, and scientists are still working out exactly why some do and some don't.

Thunder Tidbits

Tornadoes do not all look like skinny funnels. Some are fat wedges, some are thin ropes, and some hide inside rain where you cannot see them.
A single tornado can have several smaller whirls circling inside it. Those are called multiple-vortex tornadoes.
North of the equator most tornadoes spin counterclockwise, but a few spin the other way.

Words to know

Wind shear: Wind that changes speed or direction as you go higher.
Updraft: Air rising quickly inside a storm.
Supercell: A long-lasting thunderstorm with a rotating updraft.
Mesocyclone: The rotating part of a supercell, usually a few miles wide.
Vortex: Any spinning column of air or water.

Try it: Tornado in a bottle

Fill a clear plastic bottle about three quarters full of water and add a drop of dish soap and a pinch of glitter. Screw the lid on tight, turn it upside down, and swirl it in fast circles for a few seconds. Stop and watch: a mini vortex forms in the middle. Notice how the narrow bottom spins fastest.

Stay safe

If a tornado warning is issued, go to the lowest floor of a sturdy building, get into a small room in the middle with no windows, and cover your head.