Tropical cyclones
Definition of a Tropical Cyclone
A tropical cyclone is an intense low pressure systems in the tropics with a minimum
central pressure of 970hPa or less which occur in the summer season of the hemisphere
they form in. In the southern hemisphere they have well defined clockwise wind
circulations flowing inwards towards the eye. A region surrounding the eye with gale
force winds (sustained winds of 63km/h and gusts in excess of 90km/h) is called the eye
wall. If the sustained winds around the centre reach 119km/h with gusts in excess of
170km/h then the system is called a severe tropical cyclone.
Tropical cyclones follow unpredictable paths. Tropical cyclones are also known as
hurricanes or typhoons in other countries.
Characteristics of a Tropical Cyclone
The three distinctive parts of a tropical cyclone are:
 The circular eye
 The eye wall
 The Spiral Rain Bands
 The Circular eye
The circular eye or
centre of a tropical cyclone is characterized by light winds and often by clear
skies. The eye diameter of a tropical cyclone eye is usually around 40km but can
range from under 10km to over 100km. The eye of a tropical cyclone is
surrounded by the eye wall.
 The Eye Wall
The eye wall is the region around the eye. The eye wall marks the area of the
strongest winds and heaviest rainfall. It is a dense ring of cloud which is about
16km high.

The Spiral Rain Bands
Tropical cyclones are surrounded by spiral rain bands. These are bands of heavy
convective showers that spiral inwards towards the tropical cyclones centre.
Cumulus and Cumulonimbus clouds ascend and lightning develops in this area.
Tropical cyclones have clockwise wind circulations in the southern hemisphere
and anti-clockwise wind circulations in the northern hemisphere.
Causes and the formation of tropical cyclones
Tropical cyclones develop between 8 and 15 degrees north or south of the equator in the
cyclone season. The cyclone will not form within 8 degrees north and 8 degrees south of
the equator because the Coriolis force is too weak. They form in the summer season of
that hemisphere over the ocean. In order for tropical cyclones to form the following
environmental factors must be present.
1) The cyclone must originate over ocean water that is least 26.5 °C. Tropical
cyclones derive their energy from the warm water of the tropics and from latent
heat of condensation.
2) There must be an atmosphere that cools quickly with height so that it is
potentially unstable. If the air is unstable, it will continue rising and the
disturbance will grow. The disturbance will only grow when winds at all levels of
the atmosphere from the ocean up to 10km or higher are blowing at the same
speed and from the same direction
3) An upper atmosphere high pressure area above the growing storm should be
present. The air in such high pressure areas is flowing outward. This pushes away
the air that is rising in the storm, which encourages even more air to rise from the
low levels.
A tropical cyclone will not always form if these conditions are present but will only form
if they are present.
A tropical cyclone forms through three stages.
1) Tropical depression - An organized system of clouds and thunderstorms with a
well defined circulation and maximum sustained wind of 37 to 62 km/h
2) Tropical storm - An organized system of strong thunderstorms with a well defined
circulation and maximum sustained winds of 63 to 117km/h. It is at this point that
the storm is given a name.
3) Tropical Cyclone - An intense tropical weather system with a well defined
circulation and maximum sustained winds of 118km/h or higher. At this stage, the
storm has an eye.
Human Impact
Tropical cyclones can have several types of impacts on humans and human activities
The short term impacts include:
 Strong Winds
 Storm surge
 Heavy rainfall
Long term impacts include:
 Land Erosion
 Damage to crops
Short term effects
The high winds of a tropical cyclone are a hazard for shipping and boating
offshore. Near the eye the wind is going in all directions in a very small area
generating high waters that are moving in apparent random directions .In coastal
areas, the strong winds of tropical cyclones can cause significant damage. When a
tropical cyclone makes landfall or approaches a coast, winds, can destroy houses,
buildings, and other structures. Flying debris can also be a hazard. Storm surge is
the rapid rise in sea level that occurs as a storm approaches a coastline. The sea
level near the coast rises due to the heavy onshore winds that pile up the water
near the coast. Also when a tropical cyclone passes the water is being pulled
upward due to the extremely low pressure of the tropical cyclone. Storm surge in
combination with high tides can have extremely devastating consequences. Heavy
rainfall associated with tropical cyclones can produce flooding that towns are not
able to handle the increase of runoff because of gentle topography. In rough
topography events like mudslides may occur.
Long term effects
Land erosion can be caused by tropical cyclones by the heavy rainfall and
excessive winds especially around coastal areas. When tropical cyclone Vance
struck the North West cape in 1999 the sand dunes were heavily eroded by the
intense winds and rainfall. There are only few sand dunes left on the North West
cape. The heavy rainfall and high winds produced by tropical cyclones can
damage crops inland. The intense rainfall can produce flash flooding endangering
crops, property and life.
Human Responses
Humans have responded to tropical cyclones by preparing themselves. In 1999 cyclone
Vance struck Exmouth and the highest winds ever recorded on the Australian mainland
were recorded. However nobody died because they were prepared.
When warnings are issued humans are advised to prepare for a cyclone by:
 Preparing an emergency kit
 Tie down or fill with water all large relatively light objects such as boats and bins
 Put wooden or plastic outdoor furniture in your pool or inside with other loose
items
 Fill vehicles fuel tanks
 Close shutters or board up windows
These are just a few ways that humans prepare for cyclones. In some towns that
experience cyclones there are no letterboxes, the power poles are metal, and the houses
must conform to cyclone standards which raise the cost of building a house dramatically.
Also dune restoration programs are in use to restore dunes on the North West cape
Conclusion
Tropical cyclones are intense low pressure system storms that form between 8 and 15
degrees north and south of the equator. Cyclones are named alternatively between male
and female names. The first is named starting with an A the second with a B.
A tropical cyclone consists of inwards flowing clockwise winds in the southern
hemisphere. It has an eye, an eye wall and an intense rain band around it. Tropical
cyclones can be deadly and cause many problems for humans however humans have
learnt to prepare themselves for the worst when a cyclone warning is issued.