Please check this
page from the National Hurricane Center for
climatological information on the occurrence of hurricanes
in the United States.
Hurricanes (also known regionally as typhoons and cyclones)
are the most intense example of a whole range of tropical
weather systems. They form in specific oceanic
"basins" where conditions of conditions of warm ocean
temperatures, weak vertical wind shear, and incipient
disturbances supplying cyclonic vorticity (spin) are all
credit: NWS Jetstream
They begin as semi-organized areas of thunderstorms in
tropics called tropical disturbances
and progress through the following sequence
Tropical disturbance => tropical depression (<39mph
[34 kt]) => tropical storm (39-73 mph [34-63 kt]) =>
hurricane(74+ mph [64 kts])
This process is depicted in the Atlantic hurricane basin
below. The latter three types of systems have closed
(rotating) circulations, that is, there is a low pressure center,
and are classified generically as tropical cyclones.
Hurricanes are often classified by the Saffir-Simpson
scale defined by the speed of the maximum sustained
wind measured in the storm:
A Category 5 storm is much more damaging than a Cat. 4
because the kinetic energy, and therefore force, of the wind
increases with the square of the wind speed.
in the Atlantic hurricane basin
form from incipient disturbances in tropical weather that
create groups of thunderstorms. The thunderstorms release
latent heat which begins to build higher pressure aloft
(remember, isobaric surfaces are farther apart in warm air),
but low pressure at the surface. The high aloft
creates divergence aloft which leads to more lowering of
pressure at the surface.
There are several examples of incipient tropical
disturbances that may develop into hurricanes if conditions
are right (usually water temperature above 26oC,
and weak vertical wind shear):
1. Easterly Wave:
Also called tropical
wave, this is an inverted trough of low pressure
moving generally westward in the tropical easterlies. A
trough is defined as a region of relative low pressure. From
the National Hurricane Center: "While only about 60% of the
Atlantic tropical storms and minor hurricanes (
Saffir-Simpson Scale categories 1 and 2) originate from
easterly waves, nearly 85% of the intense (or major)
hurricanes have their origins as easterly waves (Landsea
Many begin over Africa and progress westward over the
In satellite imagery they appear as concentrated groups of
clouds mostly on the east side of the inverted trough, best
seen in IR because their cold tops allow them to be
distuinguished from lower level clouds:
IR image with inverted trough superimposed.
IR satellite animation of an easterly wave.
Visible image of an easterly wave.
Visible image of two easterly waves.
2. West African
Disturbance Line (WADL): This is a line of
convection (similar to a squall line) which forms over West
Africa and moves into the Atlantic Ocean. WADL's usually
move faster than tropical waves.
3. TUTT (Tropical Upper
Tropospheric Trough): A trough, or cold core
low in the upper atmosphere, which produces convection. On
occasion, one of these develops into a warm-core tropical
cyclone, but they can either help or
hinder tropical cyclone development depending on where the
surface low is developing in relation to the upper trough.
The shear zone on the south side of the trough can impede
TUTT on a satellite water vapor image.
4. Old Frontal Boundary:
Remnants of a polar front can become lines of convection and
occasionally generate a tropical cyclone. In the Atlantic
Ocean storms, this will occur early or late in the hurricane
season in the Gulf of Mexico or Caribbean Sea.
Tropical disturbance near the Bahamas at the tip of an old
Tropical depression on an old frontal boundary. In the
early stages, the convection is often sheared away from the
center of the surface circulation.
Structure of Hurricanes
Hurricanes are warm core
storms meaning they are composed of a single warm (maritime
tropical) air mass, have low pressure at the surface but
weaken and become an anticyclone aloft, and therefore
contain their storngest winds just above the surface
(friction slows down the surface winds somewhat). Thus
they are fundamentally different from mid-latitude lows
which have fronts, are cold-core systems, strengthen aloft
and tilt with height to the west.
Hurricanes have air spiraling inward at the
surface, sinking air in the eye, rising air and sometimes
thunderstorms in the eyewall and the rainbands, and sinking
air between the rainbands and outside the storm.
These features can be seen in satellite imagery:
The main "mass" of the storm is called the central
dense overcast (CDO--labeled here as the "dense cirrus
Many hurricanes contain an "outflow jet" where air
at the top of the troposphere can be seen in satellite
imagery spinning off anticyclonically (in the northern
hemisphere it happens mostly on the north side of the
storm). It is due to the strong pressure gradient
built by the latent heat release and warming which builds a
high pressure aloft, leading to a strong upper level
pressure gradient on the poleward side of the storm.
The outflow jet may look like a big spiral band but there
are also thin cirrus features and often transverse waves.
Enhanced IR of outflow jet on the west and north sides of
Bertha in 2008.
Outflow jet on the southern side of a southern hemisphere
Animation of 2004
Scan Animations of Hurricane Isabel
Mexico from Atlantic to Pacific
Satellite Method of Estimating Tropical Cyclone Intensity
Weaken Hurricanes--Does it Work?