A Tropical cyclone is an intense tropical storm that
develops over warm oceans between 5 degrees
and 20 degrees north or south of the equator.
They have a circular wind pattern which rotates
clockwise in the southern hemisphere and
anti-clockwise in the northern hemisphere. A
cyclone has four stages in its life. These are
the formative stage, the immature cyclone,
the mature cyclone and the decay stage. Cyclones
occur all around the world. In the US they are
known as hurricanes and in Asia they are referred
to as typhoons. The major targets of cyclones in
Australia are the Northern Territory, the north
coast of Western Australia and Queensland.
Cyclones are categorized from 1 to 5, the most
severe being 5 as shown in the table below.
Category
Average Wind (km/h)
Strongest Gusts (km/h)
Typical Effects
1
63-89
Below 125
Negligible house damage. Damage to some crops, trees and caravans.
2
90-124
125-169
Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.
3
125-164
170-224
Some roof and structural damage. Some caravans destroyed. Power failures likely.
4
165-224
225-279
Significant structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.
5
Above 224
Above 279
Extremely dangerous with widespread destruction
Cyclones on their own can cause significant devastation but they often create other problems that increase the death toll and damage dramatically. When cyclones cross the coast, the combination of low pressure near the centre of the cyclone and the strong onshore winds can cause the sea level to rise and flood coastal regions. This is called a storm surge. Also cyclones are often accompanied by flood rains that can cause massive flooding as well as triggering landslides in mountainous areas. These other events are what cause a cyclone to go from a natural hazard to a natural disaster.
Storm Surge (Bureau of Meteorology , 2010)
Structure of a cyclone
A cyclone is made up of several different parts. These are the eye, the eye wall, the spiral bands and the cirrus canopy. Eye
The eye is the centre of a tropical cyclone and is the area of lowest pressure. It is characterised by little or no wind and often clear, cloudless skies. In severe cyclones the eye usually shows up as a circular hole in the central cloud mass. The eye’s diameter is usually about 40 km, but can vary between less than 10 km and more than 100 km. Eye Wall
The eye wall surrounds the eye. It is a wall of dense cloud about 15km above the atmosphere. It is also where the most violent winds and heaviest rainfall occur. Spiral Bands
Spiral bands are a distinctive pattern of cloud bands spiraling into the eye wall. These bands usually extend to about 1000km from the cyclone centre, and contain heavy rain and wind squalls. Cirrus Canopy
The cirrus canopy is the extremely powerful lift of moist air within the clouds of the eye wall and inner spiral bands. This produces a huge discharge of cirrus cloud in the upper atmosphere. This forms a huge canopy over the cyclone.
Structure of a cyclone (Bureau of Meteorology , 2010)
Structure of a cyclone (Northern Territory Government, 2007)
A Tropical cyclone is an intense tropical storm that
develops over warm oceans between 5 degrees
and 20 degrees north or south of the equator.
They have a circular wind pattern which rotates
clockwise in the southern hemisphere and
anti-clockwise in the northern hemisphere. A
cyclone has four stages in its life. These are
the formative stage, the immature cyclone,
the mature cyclone and the decay stage. Cyclones
occur all around the world. In the US they are
known as hurricanes and in Asia they are referred
to as typhoons. The major targets of cyclones in
Australia are the Northern Territory, the north
coast of Western Australia and Queensland.
Cyclones are categorized from 1 to 5, the most
severe being 5 as shown in the table below.
Cyclones on their own can cause significant devastation but they often create other problems that increase the death toll and damage dramatically. When cyclones cross the coast, the combination of low pressure near the centre of the cyclone and the strong onshore winds can cause the sea level to rise and flood coastal regions. This is called a storm surge. Also cyclones are often accompanied by flood rains that can cause massive flooding as well as triggering landslides in mountainous areas. These other events are what cause a cyclone to go from a natural hazard to a natural disaster.
Structure of a cyclone
A cyclone is made up of several different parts. These are the eye, the eye wall, the spiral bands and the cirrus canopy.
Eye
The eye is the centre of a tropical cyclone and is the area of lowest pressure. It is characterised by little or no wind and often clear, cloudless skies. In severe cyclones the eye usually shows up as a circular hole in the central cloud mass. The eye’s diameter is usually about 40 km, but can vary between less than 10 km and more than 100 km.
Eye Wall
The eye wall surrounds the eye. It is a wall of dense cloud about 15km above the atmosphere. It is also where the most violent winds and heaviest rainfall occur.
Spiral Bands
Spiral bands are a distinctive pattern of cloud bands spiraling into the eye wall. These bands usually extend to about 1000km from the cyclone centre, and contain heavy rain and wind squalls.
Cirrus Canopy
The cirrus canopy is the extremely powerful lift of moist air within the clouds of the eye wall and inner spiral bands. This produces a huge discharge of cirrus cloud in the upper atmosphere. This forms a huge canopy over the cyclone.