The eyewall is a ring of extremely powerful thunderstorms that surrounds the calm center, or eye, of a hurricane. It's where the hurricane's strongest winds and heaviest rain occur. If you could see a hurricane from above, the eyewall would look like a circular wall of dark, towering clouds spinning around the eye.

Inside the eye itself, conditions are surprisingly calm: the wind dies down, the rain stops, and you might even see blue sky. But step into the eyewall and you're immediately hit with the storm's full fury. Wind speeds in the eyewall can exceed 150 miles per hour in major hurricanes, strong enough to tear roofs off buildings and snap thick tree trunks.

When a hurricane passes over land, people experience the storm in distinct stages. First come the outer rain bands, then the increasingly violent winds of the eyewall. If the eye passes directly overhead, there's a brief period of eerie calm that can last 20 to 40 minutes. Then the back side of the eyewall strikes with renewed force, often catching off guard people who think the storm has ended.

Meteorologists pay close attention to the eyewall because changes in its structure can signal whether a hurricane is strengthening or weakening. A tight, well-defined eyewall usually means an intense, dangerous storm.