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Explainer: Why a tornado forms

Tornadoes start with a thunderstorm. But they also require other ingredients, such as instability.

A tornado is a violently rotating column of air extending from the ground to a thunderstorm above. Tornadoes can leave a path of damage more than 1.6 kilometers (1 mile) wide. They can travel more than 160 kilometers (100 miles) over land. And while some touch down briefly, others can last for more than an hour.

Tornadoes start with a thunderstorm. But they also require other ingredients, such as instability. Air is unstable when it is warmer closer to the ground than it is higher up. That warm air will rise, just as a hot-air balloon does.

If that air contains water vapor, the vapor may condense, creating water droplets at cooler temperatures higher up. The droplets can fall as rain or hail. The conversion of water from a gas to a liquid also releases heat. That heat creates strong upward currents of air. They’re known as updrafts.

Tornadoes also need wind shear. Wind shear occurs when winds at varying distances above the ground blow in different directions or at different speeds. As the winds blow, a horizontal, invisible tube of rotating air begins to form in the atmosphere. That tube rotates parallel to the ground — picture a giant spinning football or rolling pin.

A strong updraft can eventually lift that rotating tube of air, until it is perpendicular to the ground. Now it resembles a rolling pin spinning on end. Soon, the whole updraft starts to rotate. This creates a special type of thunderstorm known as a supercell. If its rotation tightens, it can morph into a tornado.

(Non-supercell tornadoes also form, although differently. They can evolve when ground-level winds blowing from different directions set a vertical tube of air spinning. An updraft then stretches that tube, creating a smaller and less violent tornado. When this occurs over water, it is called a waterspout.)

Power Words

tornado A violently rotating column of air extending from the ground to a thunderstorm above.

meteorology The study of weather and climate events.

supercell A rotating thunderstorm that can produce a violent tornado.

water vapor Water in its gas phase.

waterspout A tornado that forms over water.

updraft An upward current of air. A downdraft is a downward current of air.

wind shear The effect of winds at different levels above the ground blowing in different directions or at different speeds.

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