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Hurricanes

Hurricanes are tropical cyclones that have an organized circulation, with sustained winds exceeding 74 mph. Hurricanes develop over tropical waters. Tropical cyclones forming in the Atlantic and Eastern Pacific are called hurricanes, while in the Western Pacific they are called typhoons, and in the Indian Ocean they are called cyclones.


Tropical Cyclone Classification

An organized system of clouds and thunderstorms in the tropics with a defined circulation, and maximum sustained winds of 38 mph or less is called a "tropical depression". Once a tropical depression has sustained winds of at least 39 mph, it is called a "tropical storm." This is when a tropical cyclone is assigned a name. A tropical storm becomes a hurricane when it reaches maximum sustained winds of 74 mph. Hurricanes are classified by their wind speeds using the Saffir-Simpson Scale.

Saffir-Simpson Scale
Category Wind Speed Damage
1 74-95 mph Damage mainly to unanchored mobile homes, shrubbery, and trees.
2 96-110 mph Some damage to roofs of buildings, considerable damage to shrubbery and trees, with some trees blown down and major damage to mobile homes.
3 111-130 mph Some structural damage to small residences, mobile homes destroyed, foliage blown off trees and large trees blown down.
4 131-155 mph Extensive damage to doors, windows and roofs, shrubs, trees and all signs blown down, and complete destruction of mobile homes.
5 >155 mph Severe window and door damage, extensive roof damage to residences and industrial buildings, some complete building failures with small buildings blown over or away.


Category 1 Category 2 Category 3 Category 4 Category 5



Tropical Cyclone Structure

The main parts of a tropical cyclone are the eye, the eyewall, and the rainbands. Air near the surface spirals in towards the center and rotates counterclockwise around the storm center in the northern hemisphere (clockwise in the southern hemisphere). The air rises in the eyewall, and in the spiral rainbands. The air then moves out at the top of the cyclone in the opposite direction.





The Eye

The eye is located in the center of the storm (see satellite image to the right) and is a region of generally clear skies and light winds. The size of the eye is typically 20-40 miles across, but can be larger or smaller depending on the storm. The skies are generally clear in the eye because the air is sinking in this region of the hurricane. This sinking air actually suppresses cloud formation. At the ground, the transition from the very strong winds under the eyewall to the near calm conditions in the eye can be deceiving. Some people think the storm is over when the eye is passing over, when in fact it is only half over and the dangerous winds on the other side of the eye are still to come.  


The Eyewall

The eyewall is a wall of deep clouds (see photo to the right) that produce the torrential rainfall that surrounds the eye of hurricanes. The strongest winds are found under the eyewall. The eyewall goes through periods where it will shrink in size and sometimes a double (concentric) eyewall will form. When the eyewall and/or the eye are going through changes in their structure, there will be associated changes in the surface wind speed. Therefore, these structural changes are a good indication of changes in the storm's intensity.  


Rainbands

The clouds and thunderstorms that swirl in toward the storm's center are called spiral rainbands (see radar image to the right). Spiral rainbands can produce heavy downpours and wind, as well as tornadoes.  


Tropical Cyclone Environments

Since hurricanes need warm waters for development, they only form over warm, tropical oceans. They rarely form within 5° latitude of the equator, because the Coriolis Force is weak near the equator and the thunderstorm clusters will not rotate. (The Coriolis Force is zero at the equator and increases towards the poles.) There are seven regions around the world where tropical cyclones form:

  • Atlantic Basin (light green)
  • Northeast Pacific Basin (yellow)
  • Northwest Pacific Basin (orange)
  • North Indian Basin (pink)
  • Southwest Indian Basin (purple)
  • Southeast Indian/Australian Basin (blue)
  • Australian/Southwest Pacific Basin (green)



The following environmental conditions must be present for a tropical cyclone to develop:
  • The ocean waters must be warm (at least 80°F / 27°C) to a depth of approximately 150 ft.
  • Relatively moist air must be present throughout most of the lower troposphere.
  • The storm must form at least 5° latitude north or south of the equator.
  • Winds must not change signficantly between the lower and upper troposphere (low values of vertical wind shear).


Tropical Cyclone Developmental Process

  When a cluster of thunderstorms develops or moves into environment described above, the disturbance can become more organized, which leads to the formation of a tropical depression. The warm water is one of the most important contributors to tropical cyclone formation because it acts as the "fuel" for the storm. As water vapor rises, it cools and once saturation is reached, the water vapor condenses into liquid water that we see as clouds. During the process of condensation, heat is released. This warms the atmosphere, making the air lighter and causing it to rise further. As this occurs, more air must move in near the surface to take its place. This inflowing air will begin to rotate under the influence of the Coriolis Force. As the pressure drops in the center of the storm, signifying strengthening, the pressure gradient becomes stronger. The pressure gradient is directly related to wind speed and the stronger the pressure gradient, the faster the wind speed.

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