Hurricanes are made up of organized bands of thunderstorms and heavy rain that spiral inward toward a calm center. Each successive band closer to that center — the hurricane's "eye" — has progressively stronger winds than the next band out. The final tightly spiraling band around the center forms a complete ring around the eye in well-developed hurricane.This ring of thunderstorms is the hurricane's eyewall. It contains a hurricane's strongest winds.

For reasons scientists are still trying to understand, most intense hurricanes — those with steady winds blowing faster than 110 mph — eventually form concentric eyewalls. That is, a second eyewall will form outside the original one, surrounding it.

Christopher Landsea of the National Oceanic and Atmospheric Administration's Hurricane Research Division says this occurs when spiraling bands outside the eyewall join together to create the second eyewall.

An eyewall replacement cycle occurs when the outer eyewall weakens the inner eyewall by robbing it of "needed moisture and momentum," he writes in the National Hurricane Center's Frequently Asked Questions about hurricanes guide. Without these necessary ingredients to grow, the inner eyewall will collapse. The outer eyewall then becomes the dominant eyewall, even though it's larger, and the replacement cycle is nearly complete.

The final phase of the cycle is for the new, larger eyewall to begin to contract. As this happens, any weakening that had occurred when the inner eyewall was replaced is almost always regained as the new eyewall contracts.

In some cases, Landsea writes, a hurricane coming out of an eyewall replacement cycle can be even stronger. This is what happened with Hurricane Andrew as it was nearing the Florida coast. It was at its strongest at landfall, right at the end of an eyewall replacement cycle.

Landsea notes that the discovery of eyewall replacement cycles in hurricanes contributed to ending Project Stormfury - the 1961 to 1983 government experiment to try to weaken hurricanes.

He says that the project's goal of "seeding" an outer ring of thunderstorms within a hurricane to cause that ring to grow at the expense of the thunderstorms in the eyewall is exactly "what was happening frequently as a natural part of hurricane dynamics."

You can read Landsey's complete answer on eyewall replacement cycles in NHC's Hurricane FAQ.