Not all hurricanes are the same —Sandy, for example, is more than a thousand miles in diameter— but they share a common structure. Surrounding the calm eye of the storm, a hurricane’s eyewall harbors the most violent winds and intense precipitation. Eyewalls of long-lived storms can contract and a new eyewall may form. Hurricane Andrew ratcheted up to Category 5 as it hit land in 1992 and built a new eyewall.