Seen from space, a hurricane appears as a vast spiral of clouds stretching across hundreds of miles. Near the center of strong hurricanes is the eye, a roughly circular region of comparatively calm weather, light winds, and low pressure.
The eye is one of the storm’s most recognizable features, but it is not the most dangerous part. That distinction belongs to the eyewall, the ring of towering thunderstorms that surrounds the eye and contains the hurricane’s strongest winds and most intense weather.
The lowest barometric pressure in a hurricane is typically found inside the eye, where pressure can be much lower than in the surrounding storm. Just beyond the calm center, however, conditions change sharply. Across the eyewall, pressure rises quickly and winds accelerate, creating the steep pressure gradient that helps produce a hurricane’s powerful circulation.
The contrast between the eye and eyewall comes from the way air moves through the storm. Warm, moist air spirals inward near the surface and rises rapidly in the eyewall, fueling deep thunderstorms. Inside the eye, by contrast, air sinks gradually from above. As this sinking air descends, it warms and becomes more stable, which helps suppress cloud formation. That is why the eye of a strong hurricane can be relatively warm, calm and nearly cloud-free, aside from occasional low clouds or thin cirrus overhead.
A hurricane does not necessarily have an eye throughout its entire life cycle. Eyes usually form only after a tropical cyclone becomes well organized and sufficiently strong. Even then, the eye can change size, become less distinct, or disappear for a time, especially during eyewall replacement cycles, when a new eyewall forms around the old one. These changes are important because they can signal shifts in the storm’s structure and intensity.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
