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About Hurricanes

By Wikipedia

In meteorology, a tropical cyclone (or tropical disturbance, tropical depression, tropical storm, typhoon, or hurricane, depending on strength and location) is a type of low pressure system which generally forms in the tropics. While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.

Atlantic hurricane seasons
1970 1971 1972 1973 1974
1975 1976 1977 1978 1979

Terms for tropical cyclones

The term used to describe tropical cyclones with maximum sustained winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h) depends on the region:

  • hurricane in the North Atlantic Ocean, North Pacific Ocean east of the dateline, and the South Pacific Ocean east of 160°E
  • typhoon in the Northwest Pacific Ocean west of the dateline
  • severe tropical cyclone in the Southwest Pacific Ocean west of 160°E or Southeast Indian Ocean east of 90°E
  • severe cyclonic storm in the North Indian Ocean
  • tropical cyclone in the Southwest Indian Ocean
Hurricanes form when the energy released by the condensation of moisture in rising air causes a chain reaction. The air heats up, rising further, which leads to more condensation. The air flowing out of the top of this “chimney” drops towards the ground, forming powerful winds.

Definition: a heat engine

Structurally, a tropical cyclone is a large, rotating system of clouds, wind and thunderstorm activity. The primary energy source of a tropical cyclone is the release of the heat of condensation from water vapor condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.

The ingredients for a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.

This use of condensation as a driving force is the primary difference which distinguishes tropical cyclones from other meteorological phenomena. Mid-latitude cyclones, for example, draw their energy mostly from pre-existing temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength will diminish rapidly.

Waves in the trade winds in the Atlantic Ocean—areas of converging winds that move along the same track as the prevailing wind—create instabilities in the atmosphere that may lead to the formation of hurricanes.


The formation of tropical cyclones is still the topic of extensive research, and is still not fully understood. Five factors are necessary to make tropical cyclone formation possible:

  1. Sea surface temperatures above 26.5 degrees Celsius to at least a depth of 50 meters. Warm waters are the energy source for tropical cyclones. When these storms move over land or cooler areas of water they weaken rapidly.
  2. Upper level conditions must be conducive to thunderstorm formation. Temperatures in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist.
  3. A pre-existing weather disturbance. This is most frequently provided by tropical waves—non-rotating areas of thunderstorms that move through the world's tropical oceans.
  4. A distance of approximately 10 degrees or more from the equator, so that the Coriolis effect is strong enough to initiate the cyclone's rotation. (2004's Hurricane Ivan, the strongest storm to be so close to the equator, started its formation at 9.7 degrees north.)
  5. Lack of vertical wind shear (change in wind velocity over height). High levels of wind shear can break apart the vertical structure of a tropical cyclone.

Tropical cyclones can occasionally form despite not meeting these conditions. A combination of a pre-existing disturbance, upper level divergence and a monsoon-related cold spell led to the creation of Typhoon Vamei at only 1.5 degrees north of the equator in 2001. It is estimated that the factors leading to the formation of this typhoon occur only once every 400 years.

When do tropical cyclones form?

Tropical storms and hurricanes by month, for the period 1944-2004
(North Atlantic region)
Month Total Average
January–April 4 0.1
May 8 0.1
June 33 0.5
July 53 0.9
August 168 2.8
September 219 3.6
October 108 1.8
November 30 0.5
December 6 0.1
Source: NOAA + additions for 2001-04

Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest. However, each particular basin has its own seasonal patterns.

In the North Atlantic, a distinct hurricane season occurs from June 1 to November 30, sharply peaking from late August through September. The statistical peak of the North Atlantic hurricane season is September 10. The Northeast Pacific has a broader period of activity, but in a similar timeframe to the Atlantic. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November.

In the Southern Hemisphere, tropical cyclone activity begins in late October and ends in May. Southern Hemisphere activity peaks in mid-February to early March.

Where do tropical cyclones form?

Nearly all tropical cyclones form within 30 degrees of the equator and 87% form within 20 degrees of it. However, because the Coriolis effect initiates and maintains tropical cyclone rotation, such cyclones almost never form or move within about 10 degrees of the equator (where the Coriolis effect is weakest). However, it is possible for tropical cyclones to form within this boundary if another source of initial rotation is provided. These conditions are extremely rare, and such storms are believed to form at a rate of less than one a century.

Most tropical cyclones form in a worldwide band of thunderstorm activity known as the Intertropical convergence zone (ITCZ).

Worldwide, an average of 80 tropical cyclones form each year.

Structure of a hurricane.

Structure and classification

A strong tropical cyclone consists of the following components.

  • Surface low: All tropical cyclones rotate around an area of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.
  • Warm core: Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as moist air is carried upwards and its water vapor condenses. This heat is distributed vertically, around the center of the storm. Thus, at any given altitude (except close to the surface where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
  • Central Dense Overcast (CDO): The Central Dense Overcast is a dense shield of rain bands and thunderstorm activity surrounding the central low. Tropical cyclones with symmetrical CDO tend to be strong and well developed.
  • Eye: A strong tropical cyclone will harbor an area of sinking air at the center of circulation. Weather in the eye is normally calm and free of clouds (however, the sea may be extremely violent). Eyes are home to the coldest temperatures of the storm at the surface, and the warmest temperatures at the upper levels. The eye is normally circular in shape, and may range in size from 8 km to 200 km (5 miles to 125 miles) in diameter. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.
  • Eyewall: The eyewall is a circular band of intense convection and winds immediately surrounding the eye. It has the most severe conditions in a tropical cyclone. Intense cyclones show eye-wall replacement cycles, in which outer eye walls form to replace inner ones. The mechanisms that make this occur are still not fully understood.
  • Outflow: The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to the warm core at the center of the storm.

Categories and ranking

Hurricanes are ranked according to their maximum winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest maximum winds, a Category 5 hurricane has the highest. The rankings are not absolute in terms of effects. Lower-category storms can inflict greater damage than higher-category storms, depending on factors such as local terrain and total rainfall. In fact, tropical systems of less than hurricane strength can produce significant damage and human casualties, especially from flooding and landslides.

The U.S. National Hurricane Center classifies hurricanes of Category 3 and above as Major Hurricanes. The Joint Typhoon Warning Center classifies typhoons with wind speeds of at least 150 mi/h (67 m/s or 241 km/h, equivalent to a strong Category 4 storm) as Super Typhoons.



Hurricane Ivan viewed from the International Space Station, September 2004.

NASA photo by Edward Fincke.

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It uses material from this Wikipedia article, which is probably more up to date than ours (retrieved August 12, 2005).

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