The Eye of a Hurricane

A hurricane is a powerful storm, which originates above the warm waters near the equator and moves towards the poles. Hurricanes are known by different names like typhoons and cyclones, according to the area of their origin. These violent storms have a very calm center, which is called the eye of the hurricane. The eye of the hurricane is often circular in shape (when viewed from above) with a diameter of about 30 to 65 kilometers and runs through the center of the storm. It is encircled by the eyewall, which is a ring of dense thunderstorms. This is the most vehement part of a hurricane. Eyewalls too sport a circular shape, but sometimes can be triangular or polygonal. The lowest atmospheric pressure inside the hurricane is felt in the eye and sometimes, it can be 15% lower than the pressure outside the storm.

Eye Structure and Hurricane Intensity

If the hurricane is very strong and violent, its eye is usually well-defined with a light winds and clear skies. In case of weak hurricanes, the eye is covered by dense clouds. Such weak hurricanes may also have an incomplete eyewall or an eye characterized by heavy rain. An irregular-shaped eye may be an indication of a weak or a weakening hurricanes. It is believed that hurricanes with very small eyes are more severe in nature than those with large eyes. But there is a exception to this rule - annular hurricanes. One such example is Hurricane Isabel, which was a very powerful Atlantic hurricane, and the diameter of its eye was around 65 to 80 kilometers.

Eyewall Replacement

Normally, a typical mature hurricane has an eye with a diameter of 30 to 65 kilometers, but in severe cases, the violent storms can form a very small, clear eye, which is circular in shape. Such a hurricane eye is called a 'pinhole eye', which makes it difficult for the forecasters to predict the nature of the storm. These types of hurricanes are highly intense. It has also been observed that if the diameter of the hurricane eye is less than 19 kilometers, an additional eyewall is formed at a distance of 15 to 100 kilometers from the eye. Generally, this new eyewall contracts soon after its formation, weakening the hurricane. In some cases, the old eyewall breaks up and the new one contracts, strengthening the storm in the process. This can result in another cycle of eyewall replacement.

Eye Formation

Hurricanes are formed from large areas of disturbed weather in the tropical region. With the onset of heavy thunderstorms, rainbands (cloud and precipitation structure associated with an area of rainfall) form, which rotate around a common center. As the storm becomes stronger, a ring of convection (movement of molecules within fluids) form around its rotational center. The heavy rains and thunderstorms result in a strong upward vertical movement of air (updrafts), which leads to a drop in the barometric pressure in the upper levels of the hurricane. This causes an accumulation of air with high atmospheric pressure, above the densely clouded central area. A major part of this air flows outwards, but the remaining air flows inward, towards the center of the storm. This results in higher air pressure, which opposes the updrafts. More and more air descends through the center of the hurricane, creating a calm area, which is mostly free of rain. This cylindrical area, which runs through the center of the hurricane is called the eye.

Many of the processes which combine to form a hurricane are still unexplained. The formation of the eye is very important for a hurricane to achieve high speed winds and is a major indicator of the hurricane's strength.