Sure, most of us have heard about the "eye of a hurricane," that calm, mostly sunny center of the storm where you can sometimes even see the sky if you are right in the center. All around is a raging storm with dangerous wind, lightning and even tornadoes.

But did you know some stronger, major hurricanes can have two eyes, seemingly at once?

Concentric eyewalls can form and actually weaken the storm. Most of the time, it's just temporary.

But understanding "eyewall replacement" cycles is very important in understanding the strengthening and weakening in major hurricanes, Category 3 or higher storms with wind speeds over 110 mph.

For instance, before Hurricane Andrew made landfall in South Florida in 1992, it had just come out of one of those replacement cycles and grew into a Category 5 storm with winds over 155 mph swirling near its core.

When a hurricane grows into a major hurricane with an eye at its center, it will often show signs of another eyewall forming concentrically around the first. When this happens, the inner eyewall will weaken and absorb its moisture and momentum, causing the inner eye's demise and temporarily stunting the hurricane's growth, or even weakening it.


The eyewall is where the strongest winds can be found within a typical hurricane, right around the center, or eye. (Kelvinsong/Wikimedia Commons)

Once the inner eyewall disappears, the new, now larger eyewall becomes the permanent replacement and contracts. As this happens, the storm can grow and strengthen even more, perhaps becoming a Cat–4 or Cat–5 hurricane.

Eyewall replacement can take up to several hours or even days before completion. The eyewall replacement cycle is just that, a cycle that can happen once or multiple times within the same storm.

So, if a hurricane is coming out of one of these cycles and contracts right before making landfall, watch out. Wind speeds can increase around the center and become more devastating than before. If the hurricane is just beginning a replacement cycle, it might bode well, as the storm could weaken some before the eyewall moves ashore.