Oklahoma City tornado leaves at least 91 dead

A two-mile-wide tornado tore through the Oklahoma City suburb of Moore on Monday afternoon, claiming the lives of at least 91 people.

Tornadoes form when cool, dry air high above the ground collides with low-to-the-ground moist, warm air, converging in a zone in the central United States known as "Tornado Alley".

The difference in speed between winds in the upper atmosphere and those on the surface causes an atmospheric imbalance forming horizontal spinning columns of air.

These columns are bent upwards, turning vertically when caught in the updraft of thunderstorms.

The continuous upward and downward motion within the thunderstorm causes the rotating column of air to drift below the cloud base, creating a tornado at the surface.

Residents of Moore are no strangers to tornadoes, but a whirlwind the size of Monday's giant tornado is a rare occurrence.

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