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
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:
- 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.
- 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.
- 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.
- 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.)
- 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 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
(North Atlantic region)
|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
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
- 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.