Tropical cyclones are one
of the most dangerous natural hazards to people.
Every year, they cause considerable loss of
life and do immense damage to property.
However, tropical cyclones are essential features
of the Earth's atmosphere, as they transfer
heat and energy between the equator and the
cooler regions nearer the poles.
A tropical cyclone is a large storm rotating
around an area of very low pressure, with strong
winds blowing around the centre. The whole storm
system may be five to six miles high and 300 to
400 miles wide, although sometimes can be even
bigger. It moves forward like an immense spinning
top, at speeds of up to 30 m.p.h. At its very early
and weak stages it is called a Tropical
Depression. When the winds reach 39 m.p.h.
it is called a Tropical Storm.
If the wind should reach 74 m.p.h. or more the
tropical storm is called a Hurricane in
the Atlantic and the north-east Pacific or a Typhoon in
the north-west Pacific. In other parts of the world,
such as the Indian Ocean and South Pacific the
term Cyclone or Tropical
Cyclone is used.
In the tropics there is a broad zone of low pressure
which stretches either side of the equator. The winds
on the north side of this zone blow from the north-east
(the North-east Trades) and on the southern side blow
from the south-east (South-east Trades).
Within this area of low pressure the air is heated
over the warm tropical ocean. This air rises in discrete
bubbles, causing thundery showers to form. These showers
usually come and go, but from time to time, they group
together into large clusters of thunderstorms. This
creates a current of very warm, moist, rapidly rising
air, leading to the development of a centre of low
pressure, or depression, at the surface.
There are various trigger mechanisms required to transform
these cloud clusters into a tropical cyclone. These
trigger mechanisms depend on several conditions being
'right' at the same time. The most influential factors
are:
a source of very warm, moist air - derived from
tropical oceans with surface temperatures greater
than 26 °C;
low amounts of wind shear (i.e. small changes in
wind speed through the lower part of the atmosphere);
sufficient distance from the equator to provide
spin or twist.
The development of the surface depression causes an
increase in the strength of the Trade Winds. This imparts
the twisting force necessary to cause rotation of the
entire thunderstorm cluster. The spiralling winds accelerate
inwards and upwards, releasing heat and moisture as
they do so. The Coriolis force caused by the rotation
of the Earth helps the spin of this column of rising
air.
As the depression strengthens it becomes tropical
storm and then a hurricane or typhoon. A mature hurricane
or typhoon takes the form of a cylinder of deep thundercloud
around a centre that is relatively free from clouds.
There is a relatively small area of intense horizontal
winds at the surface, often well over 100 m.p.h., while
air rises strongly above, maintaining the deep cumulonimbus
clouds.
Further aloft at about six miles, the cloud tops are
carried outwards to give thick layer clouds due to
the outward-spiralling winds leaving the tropical cyclone
core. At the centre of the tropical cyclone, air is
subsiding, which makes it dry and often cloud free,
and there is little or no wind at the surface. This
is called the eye of the storm.
Fig 3: Structure of
a tropical cyclone
How does
the tropical cyclone obtain its energy?
Great amounts of energy are transferred when warm
water is evaporated from tropical seas. This energy
is stored within the water vapour contained in moist
air. As this air ascends, 90% of the stored energy
is released by condensation, giving rise to the towering
cumulus clouds and rain. The release of heat energy
warms the air locally, causing a further decrease in
pressure aloft. Consequently, air rises faster to fill
this area of low pressure, and more warm, moist air
is drawn off the sea, feeding further energy to the
system. Thus, a self-sustaining heat engine is created.
As little as 3% of the heat energy may be converted
into mechanical energy of the circulating winds. This
relatively small amount of mechanical energy equates
to a power supply of 360 billion kilowatt hours per
day - or six months' supply of electrical energy for
the whole of the USA!
Where and
when do tropical cyclones occur?
Tropical cyclones form between approximately 5°
and 30° latitude and initially move westward (owing
to easterly winds) and slightly towards the poles.
Many tropical cyclones eventually drift far enough
from the equator to move into areas dominated by westerly
winds (found in the middle latitudes). These winds
tend to reverse the direction of the tropical cyclone
to an eastward path. As the tropical cyclone moves
polewards it picks up forward speed and may reach 30
m.p.h. or more. An average tropical cyclone can travel
about 300 to 400 miles a day, or about 3,000 miles
before it dies out.
Fig 4: Areas
affected by tropical cyclones
In the northern hemisphere, tropical cyclones occur
mainly between June and November in the North Atlantic,
the North Pacific and the North Indian Ocean. In the
southern hemisphere, they occur mainly between November
and April in the South Indian Ocean, the South Pacific
Ocean and in the seas around northern Australia. One
has recently been observed in the South Atlantic, but
this is a very rare occurrence. Figure
4 shows the areas of the world affected by tropical
cyclones.
The 2005 Atlantic hurricane season saw 27 tropical
storms of which 15 became hurricanes; including seven
major hurricanes. There were so many tropical cyclones
in the Atlantic that the list of names was exhausted
and, for the first time, forecasters had to use names
from the Greek alphabet to name them. Figure
5 shows the observed tracks of hurricanes in
the North
Atlantic during the 2005 season. Figure
6 shows the same information for the north-west
Pacific.
Once a tropical depression is identified, it is allocated
a number. If it intensifies to an estimated maximum
sustained wind above 17 metres per second (34 knots,
39 m.p.h., 63 km/h), it is termed a Tropical Storm
and is given a name.
Once the maximum sustained wind is above 33 metres
per second (64 knots, 74 m.p.h. or 117 km/h), it becomes
a Hurricane, Typhoon or Cyclone.
Hurricane strength is measured on the Saffir-Simpson
scale
Category 1 sustained wind speeds of 74 to 95 m.p.h.
Category 2 sustained wind speeds of 96 to 110 m.p.h.
Category 3 sustained wind speeds of 111 to 130
m.p.h.
Category 4 sustained wind speeds of 131 to 155
m.p.h.
Category 5 sustained wind speeds greater than 155
m.p.h.
Categories 3-5 are classed as major or intense hurricanes.
Accompanying
phenomena
The most common phenomena associated with tropical
cyclones are strong winds. Other phenomena include:
high seas - large waves of up to 15 metres
high are caused by the strong winds and are
hazardous to shipping;
storm surge - a surge of water of up to several
metres can cause extensive flooding and damage
in coastal regions;
heavy rain - the tropical cyclone can pick
up two billion tons of moisture per day and release
it as rain. This also leads to extensive flooding
- often well inland from where the tropical cyclone
hit the coast;
tornadoes - tropical cyclones sometimes spawn
many tornadoes as they hit land which can cause
small areas of extreme wind damage
These phenomena can cause major destruction, especially
when the tropical cyclone's path takes it over land.
However, a path over land also causes the destruction
of the tropical cyclone itself. As it moves over land,
its energy source is depleted and friction across the
land surface distorts the air flow. This leads to the
eye filling with cloud and the tropical cyclone dies.
Tropical
cyclone prediction
Due to advances in numerical weather prediction computer
models, forecasting the initial development of tropical
cyclones has improved greatly in recent years.
There are certain areas over the oceans that are particularly
favourable for tropical cyclone development, but it
is usually certain characteristics in a cluster of
thunderclouds that leads forecasters to recognise them
as tropical depressions. This is done by people at
specialist tropical cyclone forecasting centres around
the globe such as the National Hurricane Center in
Miami, Florida who are constantly studying satellite
images, instruments and other weather data to detect
and track them through their life-cycle. Once detected,
their track is forecast using a combination of numerical
forecasting models, synoptic forecasting and statistical
methods, which have been developed from the study of
the behaviour of past storms.
Observations from ships at sea are always very useful,
although, once the existence of a storm and its forecast
track have been broadcast, ships tend to move out of
their way! Reinforced aircraft, fitted with instruments,
fly through and over tropical cyclones, and weather
radar can locate storms within 200 miles of the radar
station. In addition, tropical cyclones are tracked
by satellites, which provide very useful information
both to forecasters and numerical forecast models.
Tropical
cyclone warnings
America has the most sophisticated tropical cyclone
warning system. Once a tropical depression has been
identified, a series of forecast advisories, which
detail the expected track and likely strength of the
tropical cyclone, is broadcast. When there are definite
indications that a tropical cyclone is approaching
land, watches and warnings along coastal regions are
raised, which aim to give information to the local
authorities of places likely to be in the tropical
cyclone's path, so that they can make preparations
to protect public safety.
Specially designed shelters in which people can
ride out the worst of the weather may have been
built in advance. Coastal regions immediately in
a tropical cyclone's path are particularly dangerous
and people are usually advised to move away from
these. In the USA, this can involve the mass evacuation
of a million or more people. Warnings are broadcast
on TV and radio. NOAA Weather radio is available
on a variety of frequencies and gives up-to-date
information and advice. There is also a lot of
information on a variety of web sites, the most
authoritative being those maintained by NOAA and
the National Hurricane Center.
Katrina - 2005 - Katrina was the most costly hurricane
on record causing an estimated $75 billion in damage
in Louisiana and Mississippi
Andrew - 1992 - Andrew was a Category 5 hurricane
which hit south-east Florida and south-east Louisiana
causing $44,878 million of damage
The most deadly tropical cyclone ever recorded
hit Bangladesh in 1970 killing approximately 300,000
people as a result of the storm surge
Camille - 1969 - Camille was a Category 5 hurricane
with winds of 190 m.p.h. It hit Mississippi, south-east
Louisiana and Virginia, causing damage totalling
$14,870 million
Typhoon Tip in the north-west Pacific on 12 October
1979 was measured to have a central pressure of 870
mb and estimated sustained winds of 190 m.p.h., making
it the most intense tropical cyclone on record
Hurricane Wilma in 2005 was the most intense hurricane
recorded in the North Atlantic, with an estimated
central pressure of 882 mb.
Do tropical
cyclones occur in Britain?
Strictly speaking, tropical cyclones do not occur
over the British Isles. However, we are sometimes affected
by deep depressions that are the remnants of tropical
cyclones. Intense mid-latitude depressions,
even those which do not originate from a tropical cyclone
can produce winds of a strength equivalent to a tropical
cyclone.
The most widely publicised such depression occurred
on 16 October 1987, known as The Great Storm. Some
gusts were over 100 m.p.h., but this was not the strongest
wind recorded over the British Isles. That was on 13
February 1989 when a deep depression passing just to
the north of Scotland produced a gust of 142 m.p.h.
(123 knots) at Fraserburgh. The reason that The
Great Storm of October 1987 gained such publicity
was that it occurred in a densely populated area and
caused enormous damage to trees.
Further information
The following links provide further information on
tropical cyclones.
