Intensity
classifications
Tropical
waves, or
easterly waves, also known as African easterly waves in the Atlantic
region, are a type of atmospheric trough, an elongated area of
relatively low air pressure, oriented north to south, which move from
east to west across the tropics causing areas of cloudiness and
thunderstorms. West-moving waves can also form from the tail end of
frontal zones in the subtropics and tropics and may be referred to as
easterly waves, but these waves are not properly called tropical waves;
they are a form of inverted trough sharing many characteristics with
fully tropical waves. All tropical waves form in the easterly flow
along the southern side of the subtropical ridge or belt of high
pressure which lies north and south of the Intertropical Convergence
Zone (ITCZ). Tropical waves are generally carried westward by the
prevailing easterly winds along the tropics and subtropics near the
equator. They can lead to the formation of tropical cyclones in the
north Atlantic and northeast Pacific basins.
Three
tropical
cyclones at different stages of development. The youngest,
though
recently upgraded to a tropical storm, lacks the organization of the
other two.Tropical cyclones are classified into three main groups,
based on intensity: tropical depressions, tropical storms, and a third
group of more intense storms, whose name depends on the region.
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. It is already a low-pressure system, however,
hence the name "depression."
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,
though an eye is usually not present. Government weather services
assign first names to systems that reach this intensity (thus the term
named storm).
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 greater than 33
m/s (64 kt, 74 mph, or 118 km/h).[1] A tropical cyclone 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 10–50 mi (16–80
km) wide in which the strongest thunderstorms and winds circulate
around the storm's center.
2009 Hurricane Names
Ana
Bill
Claudette
Danny
Erika
Fred
Grace
Henri
Ida
Joaquin
Kate
Larry
Mindy
Nicholas
Odette
Peter
Rose
Sam
Teresa
Victor
Wanda
|
Dear
Miss
Mermaid (Live Reports)
Hurricane
Season Satellite in Motion