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Hurricanes
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Exploring Hurricanes
The main parts of a hurricane are the rainbands, the eye, and the eyewall. Air spirals in toward the center in a
counter-clockwise pattern in the norther 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 main parts of a hurricane are the rainbands, the eye, and the eyewall. Air spirals in toward the center in a
counter-clockwise pattern in the norther 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 doesn't exceed
15 mph (24 km/h) and is typically 20-40 (32-64 km) miles
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 hurricane.
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.
The hurricane's center is a relatively calm, generally clear
area of sinking air and light winds that usually doesn't exceed
15 mph (24 km/h) and is typically 20-40 (32-64 km) miles
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 hurricane.
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 hurricane 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 the
light wind and fair weather of an eye may think the hurricane has passed, when in fact the storm is only half over with
dangerous eyewall winds returning, this time from the opposite direction within a few minutes.
The Eyewall
Where the strong wind gets as close to the center as possible 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.
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.
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 the
light wind and fair weather of an eye may think the hurricane has passed, when in fact the storm is only half over with
dangerous eyewall winds returning, this time from the opposite direction within a few minutes.
The Eyewall
Where the strong wind gets as close to the center as possible 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.
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.
NOAA Graphic
Latest Official Hurricane Information
Click here for the latest official hurricane information
from the National Hurricane Center.
Click here for the latest official hurricane information
from the National Hurricane Center.