How a Calm Afternoon Turns Into a Storm

A severe storm can build in less than an hour. It often starts on a warm, humid afternoon that feels completely normal.

The sun heats the ground, and the ground warms the air just above it. Warm, moist air is lighter than the air around it, so it starts to rise.

As that air rises, it cools. The moisture inside it turns into tiny water droplets, and a cloud begins to form.

If the air keeps rising fast enough, the cloud keeps growing. A fully grown thunderstorm cloud, called a cumulonimbus, can tower more than 6 miles into the sky.

Inside, powerful currents of air race up and down at the same time. That constant movement powers the lightning, wind, hail and heavy rain that follow.

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