Eyewall
The wall of clouds surrounding the clear eye in Hurricane Katrina
The dense wall of thunderstorms surrounding the eye-the eyewall, or donut-shaped ring of thunderstorms around the calm eye-has the strongest winds within the storm. Changes in the structure of the eye and eyewall can cause changes in the wind speed, which is an indicator of the storm’s intensity. The eye can grow or shrink in size, and double (concentric) eyewalls can form, as shown in the animation below.
Concentric eyewalls typically occur in major hurricanes, where an outer eyewall forms and surrounds the inner eyewall, as shown in this animation of Hurricane Ike (2008). The outer eyewall usually kills off and replaces the inner eyewall and then contracts. While the hurricane is going through this eyewall cycle, the storm tends to weaken and then can restrengthen again once a single eyewall is again in place. These eyewall cycles can take as little as 12 hours or as much as a couple days to complete.