Nib asked
How does a tornado form step by step?
A tornado starts as a sideways tube of spinning air inside a thunderstorm; the storm's updraft tips it upright, and stretching it narrow makes it spin faster until it touches the ground.
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the monster
The strongest tornadoes rate EF5 on the Enhanced Fujita scale, with winds above 200 miles an hour (320 km/h). The strongest almost always begin inside one special kind of thunderstorm, a rotating storm called a supercell.
Most tornadoes are born inside rotating thunderstorms, and the strongest have winds above 200 mph.
2what you see
A tornado is a column of rapidly rotating air in contact with both the ground and the cloud above. The funnel you see is water vapor condensing in the low pressure at its core. A tornado can be on the ground, throwing up debris, before the funnel reaches down, and a funnel that does not touch the ground is not yet a tornado.
The visible funnel is condensed water; the tornado is the spinning air, shown by debris at the ground.
a common mix-upThe funnel is not the tornado: the tornado is the rotating air, which can reach the ground before the funnel does.
3unstable air
Tornado storms need warm, moist air near the ground with colder, drier air above it. That layering is unstable: a bubble of warm moist air is lighter than what is above, so once it gets going it keeps rising, fast, and builds a towering thundercloud.
Warm moist air under cold dry air is unstable, so it rises hard and builds a storm.
4wind shear
Wind shear means the wind changes with height, in speed and direction. Fast wind on top and slow wind below drag against each other and roll the air in between into a long horizontal tube, a bit like rolling a pencil between your palms. This tube is invisible, and it lies on its side.
Wind that speeds up with height rolls the air into a sideways spinning tube.
Why does a tornado storm need winds that change with height?
Show the answer
Fast wind above and slow wind below roll the air between them into a spinning horizontal tube.
5tipped upright
The thunderstorm's updraft is strong enough to lift the sideways tube and tip one end upward until it stands on end. The result is a rotating updraft several kilometres wide inside the cloud, called a mesocyclone. This rotating storm is the supercell. It is the parent of the tornado, and most such storms never make one.
The updraft tips the rolling tube upright into a rotating column, the mesocyclone.
6touchdown
Part of the rotating updraft is stretched downward and squeezed into a thinner column. Spin has to be conserved, so a thinner column spins faster, the way a figure skater spins faster by pulling in her arms. When this tight, fast-spinning column reaches the ground it throws up debris and is called a tornado.
Stretching the column thin makes it spin faster, and when it reaches the ground it is a tornado.
Why does a tornado spin faster as it narrows?
Show the answer
Spin is conserved, so squeezing the rotating air into a thinner column makes it turn faster.
7the whole idea
The four steps together: unstable warm, moist air rises as an updraft; wind shear rolls a horizontal tube of spinning air; the updraft tips the tube upright into a mesocyclone; and stretching narrows it so it spins faster and touches down. Many supercells never produce a tornado, which is why forecasters can warn of rotation long before they can say a tornado will form.
Rising warm air, shear, tipping and stretching: four steps make a tornado, and many rotating storms stop short.
A tornado is spin that a storm tips upright and then squeezes tight, so it only needs the right winds to wake it.