A spin tipped upright
How Does a Tornado Form?
Winds at different heights make an invisible rolling tube, and a storm tips it vertical.
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Step 1 of 5
Wind speed changes with height.
Near the ground the wind may be slow and southerly; higher up, fast and westerly. That difference is called wind shear.
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Step 2 of 5
Shear makes the air roll like a tube.
Fast air over slow air sets the layer between them rolling — an invisible horizontal tube of spinning air, lying flat in the sky.
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Step 3 of 5
A thunderstorm's updraught tips it upright.
A powerful storm sucks air upward hard enough to lift one end of the rolling tube and stand it on end. Now the storm itself is rotating.
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Step 4 of 5
Stretching makes it spin much faster.
As the updraught stretches the spinning column thinner, it must spin faster — the same reason a spinning skater speeds up by pulling their arms in.
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Step 5 of 5
It becomes visible when it reaches the ground.
You are seeing what the wind picked up, not the wind The funnel is not made of wind — it is cloud, formed as pressure drops, plus dust and debris sucked up from below.
Most rotating storms never make a tornado. Something about the final step goes wrong far more often than it goes right.
The short version
A tornado is sheared air rolling into a tube, tipped upright by a storm's updraught and stretched until it spins ferociously.
Try it yourself
Stir a bowl of water then pull the spoon up through the middle. The vortex you make tightens the same way a tornado does.