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Hurricanes at least consist of an eye with a ring of intense thuderstorms around them known as the eyewall. The storms in the eyewall are sloped gently outward with large anvil clouds at their tops. The eyewall storms form where air from the outside of the storm and air moving out from the eye collide and are forced upward. Here, in the most intense convection is where most of the hurricane's energy is derived. These storms produce the enormous anvils that obscure the lower clouds in satelite images and form the often breathtaking fan of clouds spiraling outwards at high levels. The winds at high levels are organized into a series of jets in intensifying storms and often loop back around in non-strengthening storms. The eye itself is normally about 10-30 miles across, in it winds are near calm and the sky is full of low stratus and puffy cumulus or near clear in more intense storms. The winds are calm in the eye because the pressure gradient in very weak; just outside the eye the pressure gradient and wind speed maximize below the eyewall before gradually weakening outward.
Beyond the eyewall is a large region of general altocumulus and stratus clouds that form the large stratiform region of the hurricane. Isolated thunderstorms build up in this region. The majority of clouds that form here derive their moisture from ice crystalls falling from the anvils high above and from mid-level outflow from the eye-wall thunderstorms. This area is called by some the moat, as it is like a broad moat protecting the inner thunderstorms.
Imbedded in or sometimes just outside the moat another ring of convection forms in stronger hurricanes and a secondary eyewall forms. Outflow at the top of this outer eyewall flows outward as in the first eyewall, but air flowing inward converges over the eye and eyewall and sinks, suppressing convection in the eyewall. Gradually the inner eyewall dies as its updrafts are subsumed by the downwelling of exhaust air from the secondary eyewall, the eye fills in and the storm weakens. The second eyewall slowly moves inward in this process and eventually replaces the eyewall it subsumed. The hurricane then begins to restrengthen and indeed often intensifies more. This process is known as an eyewall replacement cycle and although once deemed rare is now thought to be a common phenomenon in intense storms. The entire cycle takes around a few hours to a day and may cause the central pressure to fluctuate as much as 30 mb.
Why does the outer eyewall win?
Anyone who has seen a hurricane has seen the large bands of thunderstorms that spiral outward from the central mass of thunderstorms up to 1000 miles. These bands mark the areas where moist air is converging as it spirals into the storm. Individual thunderstorms form on the inner edge of the bands and then slowly move across and along the bands as the bands spiral in to the hurricane. Thunderstorms in the bands weaken and die on the outer edge of the bands as they move into the subsidence regions in between the bands. Some bands rotate around the hurricane, others remain fixed in position. Often in hurricanes one large fixed band is seen and called the primary rain band. This is believed to be something like a bow wave produced by ships. As the hurricane plows through the environmental air it forces air to pile up on its leading edge and this in turn starts the convection of the primary rain band.
Lastly while a hurricane starts out as a cyclone at the surface, it turns into an anti-cyclone aloft. As air spirals in at low levels it spins cyclonically, but as it exits the storms it is initially cyclonic but then suddenly turns anticyclonic at about 200 to 300 km from the hurricane's eye. Farther out this anticylonic layer deepens due to subsidence.
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