The eye is a region of mostly calm weather at the centre 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 eye wall, 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.