<P> Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 km / h (115 mph), or major hurricanes (Category 3 or higher on the Saffir--Simpson hurricane scale). When tropical cyclones reach this intensity, and the eyewall contracts or is already sufficiently small (see above), some of the outer rainbands may strengthen and organize into a ring of thunderstorms--an outer eyewall--that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum . Since the strongest winds are located in a cyclone's eyewall, the tropical cyclone usually weakens during this phase, as the inner wall is "choked" by the outer wall . Eventually the outer eyewall replaces the inner one completely, and the storm can re-intensify . </P> <P> The discovery of this process was partially responsible for the end of the U.S. government's hurricane modification experiment Project Stormfury . This project set out to seed clouds outside the eyewall, causing a new eyewall to form and weakening the storm . When it was discovered that this was a natural process due to hurricane dynamics, the project was quickly abandoned . </P> <P> Almost every intense hurricane undergoes at least one of these cycles during its existence . Hurricane Allen in 1980 went through repeated eyewall replacement cycles, fluctuating between Category 5 and Category 3 status on the Saffir - Simpson scale several times . Hurricane Juliette was a rare documented case of triple eyewalls . </P> <P> A moat in a tropical cyclone is a clear ring outside the eyewall, or between concentric eyewalls, characterized by subsidence--slowly sinking air--and little or no precipitation . The air flow in the moat is dominated by the cumulative effects of stretching and shearing . The moat between eyewalls is an area in the storm where the rotational speed of the air changes greatly in proportion to the distance from the storm's center; these areas are also known as rapid filamentation zones . Such areas can potentially be found near any vortex of sufficient strength, but are most pronounced in strong tropical cyclones . </P>

Is there oxygen in the eye of a tornado