User:Runningonbrains/eye



The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is usually circular and typically 25-40 miles 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
The eye is possibly the most recognizable feature of tropical cyclones. In strong tropical cyclones, the eye is a circular area of light winds and clear skies, surrounded on all sides by a towering vertical wall of thunderstorms, known as the eyewall. In weaker tropical cyclones, the eye is less defined, and can be covered by the central dense overcast, which is the area of high, thick clouds which show up brightly on satellite pictures. Rain may even fall heavily in the eye of a disorganized storm. 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.

Structure
A typical tropical cyclone will have an eye around 25 miles across, situated at the geometric center of the storm. The eye may be clear or have spotty low clouds (a "clear" eye), or it may be covered by the central dense overcast (a "filled" eye), and on occasion light rain will fall. There is, however, very little wind, especially near the center. This is in stark contrast to conditions in the eyewall, which contains the storm's strongest winds, heaviest waves, and highest seas.

While normally quite symmetrical, 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" can be circular, but the eyewall does not completely encircle the eye, also indicating a weakening, moisture-deprived cyclone.

While normal storms have eyes a few dozen miles across, rapidly intensifying storms can develop an extremely small, clear, and circular eye, known to meteorologists as a "pinhole eye". Storms with pinhole eyes are subject to large fluctuations in intensity, and are dreaded by forcasters.

Pinhole eyes often trigger eyewall replacement cycles, where a new eyewall begins to form outside the original eyewall. This can take place from ten to a few hundred miles outside of the inner eye. This results in the storm having two sets of 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, the new eyewall can contract fairly quickly, re-strengthening the storm and causing the process to repeat.

Because of cycles such as these, 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. Hurricane Isabel was the eleventh most powerful Atlantic Hurricane of all time, and sustained a large, 40-50 mile wide eye for a period of several days.

Eyewall Replacement Cycles
Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, i.e. major hurricanes (winds > 50 m/s, 100 kt, 115 mph) or Categories 3, 4, and 5 on the Saffir-Simpson Hurricane Scale. As tropical cyclones reach this threshold of intensity, they often develop. At this point, some of the outer rainbands may organize into a ring of thunderstorms&mdash;an outer eyewall&mdash;that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone usually weakens. Eventually the outer eyewall replaces the inner one completely, and the storm will most likely re-intensify. Hurricane Allen in 1980 went through repeated eyewall replacement cycles – fluctuating between Category 5 and Category 3 status several times.

It was the discovery of concentric eyewall cycles that was partially responsible for the end of the U.S. Governments's hurricane modification experiment Project Stormfury, since what the scientists had hoped to produce through cloud-seeding was happening frequently as a natural part of hurricane dynamics.

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 center of the eye, however, waves from all directions converge, creating erratic waves that can build on each other. The maximum height of typical hurricane waves is unknown, but new research indicates that typical hurricanes may have wave heights approaching 100 feet (33 meters). This is in addition to any storm surge which may occur, as storm surges often extend into the eye.

Other Storms
Though only tropical cyclones have structures that are officially called "eyes", there are other storms which can exhibit eye-like structures:

Polar Lows
Polar lows are mesoscale weather systems found near the poles. Like tropical cyclones, they form over water, however, they thrive in much colder temperatures and at much higher latitudes. 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 noreasters 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 low 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 not be true tropical storms.