Tropical Cyclone Structure

The main parts of a tropical cyclone are the rainbands, the eye, and the eyewall. Air spirals in toward the center in a counter-clockwise pattern in the northern hemisphere (clockwise in the southern hemisphere), and out the top in the opposite direction.

In the very center of the storm, air sinks, forming an "eye" that is mostly cloud-free.

The Eye

The hurricane's center is a relatively calm, generally clear area of sinking air and light winds that usually do not exceed 15 mph (24 km/h) and is typically 20-40 miles (32-64 km) across. An eye will usually develop when the maximum sustained wind speeds go above 74 mph (119 km/h) and is the calmest part of the storm. 

But why does an eye form? The cause of eye formation is still not fully understood. It probably has to do with the combination of "the conservation of angular momentum" and centrifugal force. The conservation of angular momentum means is objects will spin faster as they move toward the center of circulation. So, air increases it speed as it heads toward the center of the tropical cyclone.

One way of looking at this is watching figure skaters spin. The closer they hold their hands to the body, the faster they spin. Conversely, the farther the hands are from the body the slower they spin. In tropical cyclone, as the air moves toward the center, the speed must increase.

However, as the speed increases, an outward-directed force, called the centrifugal force, occurs because the wind's momentum wants to carry the wind in a straight line. Since the wind is turning about the center of the tropical cyclone, there is a pull outward. The sharper the curvature, and/or the faster the rotation, the stronger is the centrifugal force.

Around 74 mph (119 km/h) the strong rotation of air around the cyclone balances inflow to the center, causing air to ascend about 10-20 miles (16-32 km) from the center forming the eyewall. This strong rotation also creates a vacuum of air at the center, causing some of the air flowing out the top of the eyewall to turn inward and sink to replace the loss of air mass near the center.

This sinking air suppresses cloud formation, creating a pocket of generally clear air in the center. People experiencing an eye passage at night often see stars.

Trapped birds are sometimes seen circling in the eye, and ships trapped in a hurricane report hundreds of exhausted birds resting on their decks. The landfall of Hurricane Gloria (1985) on southern New England was accompanied by thousands of birds in the eye.

The sudden change of very strong winds to a near calm state is a dangerous situation for people ignorant about a hurricane's structure.

Some people experiencing light wind and fair weather of an eye may think the hurricane has passed. Actually, the storm is only half over with dangerous eyewall winds returning shortly, this time from the opposite direction.

The Eyewall

Where the strong wind gets as close as it can is the eyewall. The eyewall consists of a ring of tall thunderstorms that produce heavy rains and usually the strongest winds. Changes in the structure of the eye and eyewall can cause changes in the wind speed, which is an indicator of the storm's intensity. The eye can grow or shrink in size, and double (concentric) eyewalls can form.

In intense tropical cyclones, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening.

Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger.

Rainbands

Curved bands of clouds and thunderstorms that trail away from the eye wall in a spiral fashion. These bands are capable of producing heavy bursts of rain and wind, as well as tornadoes. There are sometimes gaps in between spiral rain bands where no rain or wind is found.

In fact, if one were to travel between the outer edge of a hurricane to its center, one would normally progress from light rain and wind, to dry and weak breeze, then back to increasingly heavier rainfall and stronger wind, over and over again with each period of rainfall and wind being more intense and lasting longer.

Tropical Cyclone Size

Typical hurricane strength tropical cyclones are about 300 miles (483 km) wide although they can vary considerably.

Size is not necessarily an indication of hurricane intensity. Hurricane Andrew (1992), the second most devastating hurricane to hit the United States, next to Katrina in 2005, was a relatively small hurricane.

On record, Typhoon Tip (1979) was the largest storms with gale force winds (39 mph/63 km/h) that extended out for 675 miles (1087 km) in radius in the Northwest Pacific on 12 October, 1979. The smallest storm was Tropical Storm Marco with gale force winds that only extended 11.5 miles (18.5 km) radius when it struck Misantla, Mexico, on October 7, 2008.

However, the hurricane's destructive winds and rains cover a wide swath. Hurricane-force winds can extend outward more than 150 miles (242 km) for a large one. The area over which tropical storm-force winds occur is even greater, ranging as far out as almost 300 miles (483 km) from the eye of a large hurricane.

The strongest hurricane on record for the Atlantic Basin is Hurricane Wilma (pdf) (2005). With a central pressure of 882 mb (26.05") Wilma produced sustained winds of 184 mph (160 kt / 280 km/h). Air Force reconnaissance observations indicated that the eye of the hurricane had contracted to as small as 2 n mi (3.7 km) in diameter.

With an estimated sustained wind speed of 213 mph (185 kt / 325 km/h), the strongest hurricane in the Western Hemisphere was Hurricane Patricia (pdf) (2015). However, Patricia's hurricane force winds only extended out 20-25 miles (32-40 kilometers) from the compact, 7-mile (11 kilometer) diameter eye.