Cyclones
Introduction
A tropical cyclone is an intense low pressure system that occurs in the summer months near northern
Australia. They form over warm tropical waters. Tropical cyclones produce high winds and heavy rains.
They can be highly destructive. Tropical cyclones have other names in other parts of the world. In the
north Atlantic, they are called hurricanes, while in the north Pacific they are known as typhoons. Tropical
cyclones can last as long as two to three weeks but they can be as short as a few days.
Tropical cyclone formation
Cyclones that affect Australia usually form between November and April. Tropical cyclones form over
tropical waters with a surface temperature of 26 degrees Celsius or more. These conditions are mostly
found between the latitudes of 5 degrees and 30 degrees on both sides of the equator. Warm water also
warms the air, creating areas of intense low pressure. This also encourages water to evaporate. The
evaporated water molecules rise into the sky, eventually rising to a point where they condense into clouds.
When many clouds form in one place in a way that could lead to the formation of a cyclone, the storm
system is called a tropical depression.
As more clouds amass, the energy released by rising, cooling air and water causes the air pressure to drop
further. Due to cooler air pressing in from outside the storm system and the Coriolis effect, the cyclone
begins to spin. In the southern hemisphere, cyclones spin in a clockwise direction, while they spin in an anticlockwise direction in the northern hemisphere. As the cyclone spins, it generates high winds and heavy
rainfall. The storm is then pushed on by winds in a largely westward direction.
When a tropical cyclone is considered 'mature,' it has a calm eye at the centre with relatively few clouds
and calm conditions. This eye is surrounded by an enormous wall of clouds 16 kilometres (km) high. The eye
is usually between 10km and 48 km in diameter, while the entire storm is usually between 400 and 600 km
in diameter.
In order to aid people in identifying storms, tropical depressions and cyclones are given names to identify
them. Tropical storms were once always named with women's names, but this changed in the 1970s. Today,
tropical storms are given both male and female names. These names are picked from a list of names. Each
storm is given the next name in alphabetical order on the list.
Some scientists believe that global climate change is leading to an increase in tropical cyclones and other
tropical storms. Since water temperature is rising in many parts of the ocean, more and stronger storms
are forming. This poses a significant hazard to many coastal areas affected by tropical cyclones.
Tropical cyclone conditions
The conditions within a tropical cyclone are very harsh. Winds in severe tropical cyclones can reach and
exceed speeds of 120 kilometres per hour (km/h), with gusts exceeding 170 km/h. Winds at these speeds
can do great damage to property, including causing boats to slip from their moorings, downing trees and
even ripping roofs and walls from buildings. Power lines are often damaged in tropical cyclones, causing
power outages. High levels of rainfall cause flooding in some areas.
Another risk from tropical cyclones is the risk of a storm surge. Storm surges are a rise in sea level
caused by high winds pushing water towards shore and lower atmospheric pressure. Storm surges are
affected by wind speeds within the cyclone, the speed at which the cyclone is travelling towards the coast,
the angle at which the cyclone crosses the coast, the shape of the sea floor, and local topography. When
high tide comes to an area at the same time as a storm surge, a phenomenon known as a storm tide can
cause the sea level to be even higher. This can threaten areas that would usually be safe from a high tide,
causing flooding and sea damage. In low-lying areas, seawater flooding can extend for kilometres inland. In
some cases, storm surges can be deadlier than the storm itself. In 2005, for example, the storm surge
from Hurricane Katrina flooded and devastated the city of New Orleans in the south of the United States.
There are five categories of tropical cyclone:
1.
2.
3.
4.
5.
Little damage to houses and other buildings. Some damage to crops and other vegetation. Wind
speeds less than 125 km/h.
Minor damage to houses and other buildings. Damage to signs, trees and caravans. Heavy damage
to crops and other vegetation. Small boats break moorings. Risk of power failure. Wind speeds
between 125 km/h and 169 km/h.
Some damage to roofs and building structures. Some caravans destroyed. High risk of power
failure. Wind speeds between 170 km/h and 224 km/h.
Heavy damage to roofs and building structures. Caravans destroyed and blown away. Risk of
damage from flying debris. Widespread power failures. Wind speeds between 225km/h and 279
km/h.
Very heavy and widespread destruction. Wind speeds over 280 km/h.
Cyclone defences
The most important way that cities protect themselves from tropical cyclones is through detection
systems. In Australia, the Bureau of Meteorology identifies potentially dangerous cyclones and tracks
them using weather stations on small islands and information from satellite pictures. If a cyclone is
deemed especially dangerous, a cyclone warning is issued in areas that might be affected. These warnings
are issued from three Tropical Cyclone Warning Centres (TCWC) located in Brisbane, Darwin and Perth.
Cyclone warnings tell people how to prepare for the coming storm. This can range from securing loose
objects that might cause damage to evacuating the area until the storm has passed.
Cyclones can have very erratic paths, however, especially in Australia. People living in areas prone to
tropical cyclones have to take precautions at all times to protect themselves and their property from
severe damage. Citizens are encouraged to pack emergency kits and evacuation kits to keep in their houses
at all times, especially during the cyclone season. In addition, houses in at-risk areas are built to strict
building codes to ensure they are not destroyed in a cyclone.
A severe tropical cyclone: Cyclone Tracy
On 20 December 1974, meteorologists detected a large tropical depression in the Arafura Sea. The next
day, it was upgraded to a cyclone as wind speeds exceeded 63 km/h. The meteorologists gave it the next
name on the list - Tracy.
On Christmas Eve, Cyclone Tracy passed over the city of Darwin. Although Cyclone Tracy was only Category
4, it caused widespread damage, severely damaging or destroying the majority of Darwin's buildings. The
storm caused so much damage because the eye of the storm passed directly over the city. Winds over 217
km/h were detected, and torrential rain flooded the city. Although there had been tropical cyclone
warnings in effect, many people had not evacuated Darwin. This may have been the result of a cyclone
warning earlier in the year that ended up being a false alarm. After the storm had passed, 65 people had
been killed, 49 on land, 16 at sea. Widespread power failures affected the city, as did shortages of fresh
food and water.
After the storm, emergency services were mobilised from the southern States. Most of Darwin's
population of 48 000 was evacuated to safety. It took two years for Darwin to be rebuilt. When Darwin
was rebuilt, it was rebuilt with more cyclone-resistant buildings to prevent such a tragedy from happening
again. All told, the cost to rebuild Darwin was over $3.5 billion.