Eyewall

The eye is a region of mostly calm weather at the center of strong tropical cyclones. The eye of a storm is a roughly circular area, typically 30–65 km (20–40 miles) in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather occurs. The cyclone's lowest barometric pressure occurs in the eye and can be as much as 15 percent lower than the pressure outside the storm.

In strong tropical cyclones, the eye is characterized by light winds and clear skies, surrounded on all sides by a towering, symmetric eyewall. In weaker tropical cyclones, the eye is less well defined and can be covered by the central dense overcast, an area of high, thick clouds that show up brightly on satellite imagery. Weaker or disorganized storms may also feature an eyewall that does not completely encircle the eye or have an eye that features heavy rain. In all storms, however, the eye is the location of the storm's minimum barometric pressure - where the atmospheric pressure at sea level is the lowest.

A typical tropical cyclone will have an eye of approximately 30–65 km (20–40 mi) across, usually situated at the geometric center of the storm. The eye may be clear or have spotty low clouds (a clear eye), it may be filled with low- and mid-level clouds (a filled eye), or it may be obscured by the central dense overcast. There is, however, very little wind and rain, especially near the center. This is in stark contrast to conditions in the eyewall, which contains the storm's strongest winds. Due to the mechanics of a tropical cyclone, the eye and the air directly above it are warmer than their surroundings.

While normally quite symmetric, eyes can be oblong and irregular, especially in weakening storms. A large ragged eye is a non-circular eye which appears fragmented, and is an indicator of a weak or weakening tropical cyclone. An open eye is an eye which can be circular, but the eyewall does not completely encircle the eye, also indicating a weakening, moisture-deprived cyclone. Both of these observations are used to estimate the intensity of tropical cyclones via Dvorak analysis. Eyewalls are typically circular; however, distinctly polygonal shapes ranging from triangles to hexagons occasionally occur.

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