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Eye (cyclone)

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Eye of Category 4 Hurricane Isabel seen from the International Space
   Station on September 15, 2003
   Enlarge
   Eye of Category 4 Hurricane Isabel seen from the International Space
   Station on September 15, 2003

   The eye is a region of mostly calm weather found at the centre of
   strong tropical cyclones. The eye of a storm is usually circular and
   typically 25–40  miles (40–65 km) in diameter. It is surrounded by the
   eyewall, where the most severe weather of a cyclone occurs. The
   cyclone's lowest barometric pressure occurs in the eye, and can be as
   much as 15% lower than the atmospheric pressure outside of the storm.

Basic definitions

   A view from the interior of Hurricane Betsy's eye. In this photograph,
   taken by Hurricane Hunters on an aircraft in the middle of the eye, low
   clouds are seen covering the ocean in the foreground, with the eyewall
   looming miles high in the background.
   Enlarge
   A view from the interior of Hurricane Betsy's eye. In this photograph,
   taken by Hurricane Hunters on an aircraft in the middle of the eye, low
   clouds are seen covering the ocean in the foreground, with the eyewall
   looming miles high in the background.

   The eye is possibly the most recognizable feature of tropical cyclones.
   Surrounded by a vertical wall of thunderstorms (the eyewall), the eye
   is a roughly-circular area at the cyclone's centre of circulation. In
   strong tropical cyclones, the eye is characterised by light winds and
   clear skies, surrounded on all sides by a towering, symmetric eyewall.
   In weaker tropical cyclones, the eye is less well-defined, and can be
   covered by the central dense overcast, which is an area of high, thick
   clouds which show up brightly on satellite pictures. Weaker or
   disorganized storms may also feature an eyewall which does not
   completely encircle the eye, or have an eye which features heavy rain.
   In all storms, however, the eye is the location of the storm's minimum
   barometric pressure: the area where the atmospheric pressure at sea
   level is the lowest.
   A cross section diagram of a mature tropical cyclone, with arrows
   indicating air flow in and around the eye.
   Enlarge
   A cross section diagram of a mature tropical cyclone, with arrows
   indicating air flow in and around the eye.

Structure

   A typical tropical cyclone will have an eye approximately 25 mi (40 km)
   across, usually situated at the geometric centre of the storm. The eye
   may be clear or have spotty low clouds (a clear eye), it may be filled
   with low- and mid-level clouds (a filled eye), or it may be obscured by
   the central dense overcast. There is, however, very little wind and
   rain, especially near the centre. This is in stark contrast to
   conditions in the eyewall, which contains the storm's strongest winds.

   While normally quite symmetric, eyes can be oblong and irregular,
   especially in weakening storms. A large ragged eye is a non-circular
   eye which appears fragmented, and is an indicator of a weak or
   weakening tropical cyclone. An open eye is an eye which can be
   circular, but the eyewall does not completely encircle the eye, also
   indicating a weakening, moisture-deprived cyclone.
   Hurricane Nate, as seen in this picture on September 6, 2005, presents
   a cloud-filled eye.
   Enlarge
   Hurricane Nate, as seen in this picture on September 6, 2005, presents
   a cloud-filled eye.

   While typical mature storms have eyes that are a few dozen miles
   across, rapidly intensifying storms can develop an extremely small,
   clear, and circular eye, referred to as a pinhole eye. Storms with
   pinhole eyes are prone to large fluctuations in intensity, and provide
   difficulties and frustrations for forecasters.

   Small eyes (less than 10 nmi across) often trigger eyewall replacement
   cycles, where a new eyewall begins to form outside the original
   eyewall. This can take place anywhere from ten to a few hundred miles
   (fifteen to hundreds of kilometers) outside of the inner eye. This
   results in the storm having two concentric eyewalls, or an "eye within
   an eye". In most cases, the outer eyewall contracts soon after its
   formation, choking off the inner eye, and creating a much larger, but
   stable eye. While this process tends to weaken storms as it occurs, the
   new eyewall can contract fairly quickly after the old eyewall
   dissipates, causing the storm to re-strengthen and the process to
   repeat. The contracted new eyewall may trigger another cycle of eyewall
   replacement.

   Eyes can range in size from 200 miles (320 km) ( Typhoon Carmen) to a
   mere two miles (3  km) ( Hurricane Wilma) across. While it is very
   uncommon for storms with large eyes to become very intense, it does
   occur, especially in annular hurricanes. Hurricane Isabel was the
   eleventh most powerful Atlantic hurricane of all time, and sustained a
   large, 40–50 mile (65–80 km)-wide eye for a period of several days.

Formation

   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 centre. As the storm gains strength, a ring of stronger
   convection forms at a certain distance from the rotational centre 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. Consequentially, most of this built up air flows outward
   anticyclonically above the tropical cyclone.
   Ozone measurements collected over Hurricane Erin on September 12, 2001.
   In the eyewall (ring of blues and violets), air is rising rapidly from
   the earth's surface, where almost no ozone is present. In the eye
   (circle of green and yellow), air is sinking from the ozone-rich
   stratosphere, so more ozone is present.
   Enlarge
   Ozone measurements collected over Hurricane Erin on September 12, 2001.
   In the eyewall (ring of blues and violets), air is rising rapidly from
   the earth's surface, where almost no ozone is present. In the eye
   (circle of green and yellow), air is sinking from the ozone-rich
   stratosphere, so more ozone is present.

   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 centre of the storm, creating a
   mostly rain-free area; a newly-formed eye.

   There are many aspects of this process which remain a mystery.
   Scientists do not know why a ring of convection forms around the centre
   of circulation instead of on top of it, or why the upper-level
   anticyclone only ejects a portion of the excess air above the storm.
   Hundreds of theories exist as to the exact process by which the eye
   forms: all that is known for sure is that the eye is necessary for
   tropical cyclones to achieve high wind speeds.

Detection

   The formation of an eye is almost always an indicator of increasing
   tropical cyclone organisation and strength. Because of this,
   forecasters watch developing storms closely for signs of eye formation.

   For storms with a clear eye, detection of the eye is as simple as
   looking at pictures from a weather satellite. However, for storms with
   a filled eye, or an eye completely covered by the central dense
   overcast, other detection methods must be used. Observations from ships
   and Hurricane Hunters can pinpoint an eye visually, by looking for a
   drop in wind speed or lack of rainfall in the storm's centre. In the
   United States, a network of NEXRAD Doppler radar stations can detect
   eyes near the coast. Weather satellites also carry equipment for
   measuring atmospheric water vapor and cloud temperatures, which can be
   used to spot a forming eye. In addition, scientists have recently
   discovered that the amount of ozone in the eye is much higher than the
   amount in the eyewall, due to air sinking from the ozone-rich
   stratosphere.

Associated phenomena

Eyewall replacement cycles

   Eyewall replacement cycles, also called concentric eyewall cycles,
   naturally occur in intense tropical cyclones, generally with winds
   greater than 115 mph (185 km/h), or major hurricanes (cat 3 or above).
   When tropical cyclones reach this threshold of 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.

   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 of 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.

   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
   (2001) was a rare documented case of triple eyewalls.

Moats

   A moat in a tropical cyclone is a clear ring outside the eyewall, or
   between concentric eyewalls, characterized by slowly sinking air,
   little or no precipitation, and strain-dominated flow . The moat
   between eyewalls is just one example of a rapid filamentation zone, or
   an area in the storm where the rotational speed of the air changes
   greatly in proportion to the distance from the storm's centre. Such
   strain-dominated regions can potentially be found near any vortex of
   sufficient strength, but are most pronounced in strong tropical
   cyclones.

Eyewall mesovortices

   A picture of Hurricane Wilma's eye taken at 8:22 a.m. CDT October 19,
   2005, by the crew aboard NASA's international space station, 222 miles
   above earth. At the time, Wilma was the strongest Atlantic hurricane in
   history, with winds near 185 mph (280 km/h) and a minimum central
   pressure of only 882 mbar. Not only is this a classic example of a
   pinhole eye (the smallest ever observed—only 2 miles (3 km) across),
   but also of the stadium effect, where the eyewall slopes out and up.
   Enlarge
   A picture of Hurricane Wilma's eye taken at 8:22 a.m. CDT October 19,
   2005, by the crew aboard NASA's international space station, 222 miles
   above earth. At the time, Wilma was the strongest Atlantic hurricane in
   history, with winds near 185 mph (280 km/h) and a minimum central
   pressure of only 882  mbar. Not only is this a classic example of a
   pinhole eye (the smallest ever observed—only 2 miles (3 km) across),
   but also of the stadium effect, where the eyewall slopes out and up.

   Eyewall mesovortices are small scale rotational features found in the
   eyewalls of intense tropical cyclones. They are similar, in principle,
   to small "suction vortices" often observed in multiple-vortex
   tornadoes. In these vortices, wind speed can be up to 10% higher than
   in the rest of the eyewall. Eyewall mesovortices are most common during
   periods of intensification in tropical cyclones.

   Eyewall mesovortices often exhibit unusual behavior in tropical
   cyclones. They usually rotate around the low pressure centre, but
   sometimes they remain stationary. Eyewall mesovortices have even been
   documented to cross the eye of a storm. These phenomena have been
   documented observationally, experimentally, and theoretically.

   Eyewall mesovortices are a significant factor in the formation of
   tornadoes after tropical cyclone landfall. Mesovortices can spawn
   rotation in individual thunderstorms (a mesocyclone), which leads to
   tornadic activity. At landfall, friction is generated between the
   circulation of the tropical cyclone and land. This can allow the
   mesovortices to descend to the surface, causing large outbreaks of
   tornadoes.

Stadium effect

   The stadium effect is a phenomenon occasionally observed in strong
   tropical cyclones. It is a fairly common event, where the clouds of the
   eyewall curve outward from the surface with height. This gives the eye
   an appearance resembling an open dome from the air, akin to a sports
   stadium. An eye is always larger at the top of the storm, and smallest
   at the bottom of the storm because the rising air in the eyewall
   follows isolines of equal angular momentum, which also slope outward
   with height. This phenomenon refers to the characteristics of tropical
   cyclones with very small eyes, where the sloping phenomenon is much
   more pronounced.

Hazards

   Though the eye is by far the calmest part of the storm, with no wind at
   the center and typically clear skies, over the ocean it is possibly the
   most hazardous area. In the eyewall, wind-driven waves are all
   traveling in the same direction. In the centre of the eye, however,
   waves from all directions converge, creating erratic crests which can
   build on each other, creating rogue waves. The maximum height of
   hurricane waves is unknown, but new research indicates that typical
   hurricanes may have wave heights approaching 100 feet (33 m). This is
   in addition to any storm surge which may occur, as storm surges often
   extend into the eye.

   A common mistake, especially in areas where hurricanes are uncommon, is
   for residents to wander outside to inspect the damage while the eye
   passes over, thinking the storm is over. They are then caught
   completely by surprise by the violent winds in the opposite eyewall.
   The National Weather Service strongly discourages leaving shelter while
   the eye passes over.

Other storms

   Though only tropical cyclones have structures which are officially
   called "eyes", there are other storms which can exhibit eye-like
   structures:
   The North American blizzard of 2006, an extratropical storm, showed an
   eye-like structure at its peak intensity (here seen just to the east of
   the Delmarva Peninsula).
   Enlarge
   The North American blizzard of 2006, an extratropical storm, showed an
   eye-like structure at its peak intensity (here seen just to the east of
   the Delmarva Peninsula).

Polar lows

   Polar lows are mesoscale weather systems (typically smaller than
   600 miles or 1000 km across) found near the poles. Like tropical
   cyclones, they form over relatively warm water, can feature deep
   convection (thunderstorms), and feature winds of gale force or greater
   (> 31 mph, 50 km/h). Unlike storms of tropical nature, however, they
   thrive in much colder temperatures and at much higher latitudes. They
   are also smaller and last for shorter durations (few last longer than a
   day or so). Despite these differences, they can be very similar in
   structure to tropical cyclones, featuring a clear eye surrounded by an
   eyewall and rain/snow bands.

Extratropical storms

   Extratropical storms are areas of low pressure which exist at the
   boundary of different air masses. Almost all storms found at
   mid-latitudes are extratropical in nature, including classic North
   American nor'easters and European windstorms. The most severe of these
   can have a clear "eye" at the site of lowest barometric pressure,
   though it is usually surrounded by lower, non-convective clouds and is
   found near the back end of the storm.

Subtropical storms

   Subtropical storms are cyclones which have some extratropical
   characteristics and some tropical characteristics. As such, they may
   have an eye, but are not true tropical storms. Subtropical storms can
   be very hazardous, with high winds and seas, and often evolve into true
   tropical storms. As such, the National Hurricane Centre began including
   subtropical storms in their naming scheme in 2002.

Tornadoes

   Tornadoes are destructive, small-scale storms, which produce the
   fastest winds on earth. There are two main types—single-vortex
   tornadoes, which consist of a single spinning column of air, and
   multiple-vortex tornadoes, which consist of small suction vortices,
   resembling mini-tornadoes themselves, all rotating around a common
   centre. Both of these types of tornadoes are theorized to have calm
   centers, referred to by some meteorologists as "eyes". These theories
   are supported by doppler radar observations and eyewitness accounts.

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