Worldwide, tropical cyclone activity peaks
in late summer when the difference between temperatures aloft and at sea surface
are the greatest. However, each particular water basin has its own seasonal
patterns. On a worldwide scale, May is the least active month, while September
is the most active.
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 time frame 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.
Tropical cyclone is the meteorological term
for a type of storm system characterized by a low pressure center and
thunderstorms, producing strong wind and flooding rain. A tropical cyclone feeds
on the heat released when moist air rises and the water vapor condenses. The
adjective "tropical" refers to both the geographic origin of these systems,
which form almost exclusively in tropical regions of the globe, and their
formation in tropical, or more precisely, 'maritime tropical' air masses. The
term "Cyclone" refers to such storms' cyclonic nature, with counterclockwise
rotation in the Northern Hemisphere and clockwise rotation in the Southern
Hemisphere. Tropical cyclones are distinguished from other cyclonic windstorms
such as nor'easters, European windstorms, and polar lows by the heat mechanism
that fuels them, which makes them "warm core" storm systems. Depending on their
location and strength, there are various terms by which tropical cyclones are
known, such as hurricane, typhoon, tropical storm, cyclonic storm, and tropical
Tropical cyclones can produce extremely strong and powerful
winds, tornadoes, torrential rain, high waves, and storm surge. They are born
and sustained over large bodies of warm water, and lose their strength over
land. This is the reason coastal regions can receive significant damage from a
tropical cyclone, while inland regions are relatively safe from receiving strong
winds. Heavy rains, however, can produce significant flooding inland, and storm
surges can produce extensive coastal flooding up to 25 mi (40 km) inland.
Although their effects on human populations can be devastating, tropical
cyclones can also relieve drought conditions. They carry heat away from the
tropics, an important mechanism of the global atmospheric circulation that helps
maintain equilibrium in the Earth's troposphere.
Many tropical cyclones develop when the atmospheric
conditions around a weak disturbance in the atmosphere are favorable. Others
form when other types of cyclones acquire tropical characteristics. Tropical
systems are then moved by steering winds in the troposphere; if the conditions
remain favorable, the tropical disturbance intensifies, and can develop an eye.
On the other end of the spectrum, if the conditions around the system
deteriorate, or the tropical cyclone makes landfall, the system weakens and
dissipates. A tropical depression is an organized system of clouds and
thunderstorms with a defined surface circulation and maximum sustained winds of
less than 17 m/s (33 kt, 38 mph, or 62 km/h). It has no eye and does not
typically have the organization or the spiral shape of more powerful storms.
However, it is already a low-pressure system, hence the name "depression." The
practice of the Philippines is to name tropical depressions from their own
naming convention when the depressions are within the Philippines' area of
A tropical storm is an organized system of strong
thunderstorms with a defined surface circulation and maximum sustained winds
between 17 and 32 m/s (34?63 kt, 39?73 mph, or 62?117 km/h). At this point, the
distinctive cyclonic shape starts to develop, although an eye is not usually
present. Government weather services, other than the Philippines, first assign
names to systems that reach this intensity (thus the term named
A hurricane or typhoon (sometimes simply referred to as a
tropical cyclone, as opposed to a depression or storm) is a system with
sustained winds of at least 33 m/s (64 kt, 74 mph, or 118 km/h). A cyclone
of this intensity tends to develop an eye, an area of relative calm (and lowest
atmospheric pressure) at the center of circulation. The eye is often visible in
satellite images as a small, circular, cloud-free spot. Surrounding the eye is
the eyewall, an area about 16 to 80 km (10 to 50mi) wide in which the strongest
thunderstorms and winds circulate around the storm's center. Maximum sustained
winds in the strongest tropical cyclones have been estimated at about 85 m/s
(165 kt, 190 mph, 305 km/h).