In this remarkable photo we see the eyewall of Hurricane Katrina as seen from an NOAA Lockheed WP-3D Orion “hurricane hunter” aircraft.

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 and highest winds occur. The cyclone’s lowest barometric pressure occurs in the eye and can be as much as 15% lower than the pressure outside the storm. [source]