<P> Small / minuscule eyes--those less than 10 nmi (19 km, 12 mi) across--often trigger eyewall replacement cycles, where a new eyewall begins to form outside the original eyewall . This can take place anywhere from fifteen to hundreds of kilometers (ten to a few hundred miles) outside the inner eye . The storm then develops two concentric eyewalls, or an "eye within an eye". In most cases, the outer eyewall begins to contract soon after its formation, which chokes off the inner eye and leaves a much larger but more stable eye . While the replacement cycle tends to weaken storms as it occurs, the new eyewall can contract fairly quickly after the old eyewall dissipates, allowing the storm to re-strengthen . This may trigger another re-strengthen cycle of eyewall replacement . </P> <P> Eyes can range in size from 370 km (230 mi) (Typhoon Carmen) to a mere 3.7 km (2.3 mi) (Hurricane Wilma) across . While it is uncommon for storms with large eyes to become very intense, it does occur, especially in annular hurricanes . Hurricane Isabel was the eleventh most powerful North Atlantic hurricane in recorded history, and sustained a large, 65--80 km (40--50 mi) - wide eye for a period of several days . </P> <P> Tropical cyclones typically form from large, disorganized areas of disturbed weather in tropical regions . As more thunderstorms form and gather, the storm develops rainbands which start rotating around a common center . As the storm gains strength, a ring of stronger convection forms at a certain distance from the rotational center of the developing storm . Since stronger thunderstorms and heavier rain mark areas of stronger updrafts, the barometric pressure at the surface begins to drop, and air begins to build up in the upper levels of the cyclone . This results in the formation of an upper level anticyclone, or an area of high atmospheric pressure above the central dense overcast . Consequently, most of this built up air flows outward anticyclonically above the tropical cyclone . Outside the forming eye, the anticyclone at the upper levels of the atmosphere enhances the flow towards the center of the cyclone, pushing air towards the eyewall and causing a positive feedback loop . </P> <P> However, a small portion of the built - up air, instead of flowing outward, flows inward towards the center of the storm . This causes air pressure to build even further, to the point where the weight of the air counteracts the strength of the updrafts in the center of the storm . Air begins to descend in the center of the storm, creating a mostly rain - free area--a newly formed eye . </P>

Why does the eye form in a hurricane