Rainbands are the long, curved bands of heavy rain and thunderstorms that spiral outward from the center of a tropical cyclone. Often stretching hundreds of miles, these bands are one of the most visible and dramatic features of hurricanes and tropical storms when viewed from satellite imagery. They help give these storms their iconic pinwheel shape.

Within rainbands, weather conditions can vary dramatically. One moment might bring intense rainfall, gusty winds and even tornadoes; the next might offer a brief lull or lighter rain.

As these bands rotate around the storm’s eye, they can sweep over the same location repeatedly, leading to prolonged periods of rainfall and extreme flooding. Outer rainbands have been known to linger for hours or even days, bringing some of the worst damage to locations far from the center of the storm.

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While many tropical systems bring intense rainfall and flooding, Hurricane Harvey is one of the most extreme examples of what can happen when slow-moving rainbands set up over an area. The storm stalled over southeast Texas in 2017, unleashing relentless rain and flooding. Some areas received roughly 40 inches of rain in less than 48 hours.

Rainbands also play a crucial role in the structure and strength of a hurricane. The most intense storms tend to have well-organized, tightly packed rainbands, while looser or more fragmented bands can signal a weakening system. For those on the ground, the arrival of outer rainbands is often the first sign that a hurricane is approaching, even if the storm’s center is still far away.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.