Eye (cyclone)

The eye of a tropical cyclone is a central region of mostly calm weather. It is a roughly circular area located at the geometric center of the storm.

While the rest of the cyclone may be violent, the eye is characterized by light winds and often clear skies. This calm center is surrounded by the eyewall. The eyewall is a ring of towering thunderstorms. It is where the most severe weather and highest winds of the cyclone occur.

The eye is also the place where the cyclone's lowest barometric pressure is found. This pressure can be as much as 15 percent lower than the pressure outside the storm.

To understand how an eye forms, we must look at the mechanics of a developing cyclone. These storms often begin as disorganized areas of disturbed weather over warm ocean waters. As thunderstorms gather, they develop rainbands that rotate around a common center. As the storm gains strength, a ring of intense convection forms. This convection creates strong updrafts that cause the barometric pressure at the surface to drop.

High pressure, known as an upper-level anticyclone, builds up above the storm. Most of this air flows outward, but a small portion flows inward toward the center. This inward flow causes the air pressure to drop even further. Eventually, the weight of the air counteracts the updrafts, causing air to descend in the center. This descending air creates the rain-free area we call the eye.

Eyes come in many different shapes and sizes. A typical mature tropical cyclone has an eye approximately 30 to 65 kilometers across.

Some eyes are clear, while others are "filled" with low- and mid-level clouds. In weaker storms, the eye might be obscured by the central dense overcast. This is an area of high, thick clouds visible on satellite imagery. Some eyes are even irregular or oblong, which often indicates a weakening storm. A large, ragged, non-circular eye is a sign that the cyclone is losing strength. Conversely, rapidly intensifying storms can develop a "pinhole eye." These are extremely small, clear, and circular eyes that are less than 19 kilometers across.

Intense tropical cyclones often undergo a process called an eyewall replacement cycle. This occurs in major hurricanes with winds greater than 185 kilometers per hour. During this cycle, a new outer eyewall forms from outer rainbands. This new ring moves inward and robs the inner eyewall of moisture and angular momentum. As the outer wall chokes the inner wall, the storm usually weakens. Eventually, the outer eyewall replaces the inner one, and the storm can re-intensify.

Research shows that 53 percent of intense hurricanes undergo at least one of these cycles. This natural process was once misunderstood during the U.S. government's Project Stormfury, which tried to modify storms by seeding clouds.

Scientists use several methods to detect and study these eyes. For storms with clear eyes, weather satellites provide easy identification. However, for filled eyes, researchers must use other tools. Ships and "hurricane hunters" can visually pinpoint an eye by observing drops in wind speed or rainfall. In the United States and South Korea, NEXRAD Doppler weather radar stations can detect eyes near the coast.

Scientists also use satellite equipment to measure atmospheric water vapor and cloud temperatures. Interestingly, the amount of ozone in the eye is much higher than in the eyewall. This is because air from the ozone-rich stratosphere sinks into the center of the storm.

There are several unique phenomena associated with the eye and eyewall. In very strong storms, the clouds of the eyewall may curve outward as they rise. This is called the "stadium effect," which makes the eye look like a sports stadium from above.

Between concentric eyewalls, a clear ring called a "moat" may exist. A moat is characterized by sinking air and very little rain. Inside the eyewall itself, small rotational features called eyewall mesovortices can form.

These mesovortices can reach very high wind speeds. They are significant because they can help spawn tornadoes when a cyclone makes landfall and encounters friction from the ground.

Understanding the eye is vital for forecasting the intensity of tropical cyclones. Observations of eye shape and size are used in Dvorak analysis to estimate storm strength. Because pinhole eyes cause large fluctuations in intensity, they can be difficult for forecasters to predict. By studying the relationship between the calm eye and the violent eyewall, meteorologists gain a better understanding of the entire cyclone system. This knowledge helps protect people living in the paths of these powerful natural events.