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Weather Hazards:
Tropical Cyclones
The cyclone has made landfall as a Category 4, and continues its path of
destruction across towns and communities along the east coast. Monster
waves caused by the storm surge are lashing the coast. Torrential rain is falling,
and destructive winds of 240 kilometres an hour have been recorded. Flying
debris is a threat to human life, and power lines are down. Hundreds of
thousands of homes are without electricity. There's still many hours to go, and
we urge people to remain inside their homes and stay sheltered. Seek higher
ground if flooding occurs.
Thirty percent of the world's population live in areas that are affected by
tropical cyclones. The better we understand them, the better we can take
action to protect people and property. So let's look at what qualifies as a
tropical cyclone, how they're formed, and the conditions required to bring
them about.
‘Tropical cyclones’ is the generic term used by meteorologists to describe a
rotating, organised system, strong winds, thick clouds, intense rain and
thunderstorms that form and develop as part of a low pressure system over
tropical or sub-tropical waters.
Once its winds sustain 118 kilometres an hour, it is officially classed as either a
cyclone, hurricane, or typhoon. The name differs according to the part of the
world in which they occur, but they all refer to tropical cyclones.
In the regions of the South Pacific and Indian Ocean, the term 'cyclone' or
'tropical cyclone' is used. In the Atlantic and northeast Pacific, they're called
'hurricanes', while in the northwest Pacific they're known as 'typhoons'. We'll
use the term 'tropical cyclone', the general, global term used for such storms.
Tropical cyclones occur every year, and many may develop in any one warm
season. Tropical cyclones typically form in late summer when the oceans are
warmed. This is mid-July to September in the northern hemisphere, and midJanuary to March in the southern hemisphere. To differentiate them, in most
regions they are given alternating male and female names which follow an
alphabetical list. This makes communication between meteorologists and the
public more straightforward when issuing forecasts, watches, and warnings.
The exception is the western north Pacific and north Indian Oceans, where
they are named after animals, flowers, trees, foods, or descriptive adjectives.
Tropical cyclones form in the world's warmest areas, the tropics, at latitudes
between about 5 and 30 degrees. It can take hours or even days for a
tropical cyclone to develop. Conditions have to be favourable. So what are
tropical cyclones and how do they form?
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Weather Hazards:
Tropical Cyclones
There are three basic stages in the life of a tropical cyclone: its origin or
source, the mature stage, and the dissipation stage where it dies out. These
occur in a continuous process, not as separate and distinct stages. Each
stage may occur more than once during the life cycle as the strength of the
cyclone rises and falls. It may reach land, weaken, then go back out to sea
where it strengthens once more.
The formation of a cyclone depends upon the following conditions
coinciding: a large, still, and warm ocean area with a surface temperature
that exceeds 26.5 degrees Celsius over an extended period. This allows a
body of warm air to develop above the ocean's surface.
Low altitude winds are also needed to form a tropical cyclone. As air warms
over the ocean it expands, becomes lighter, and rises. Other local winds blow
in to replace the air that has risen, then this air is also warmed and rises.
The rising air contains huge amounts of moisture evaporated from the ocean's
surface. As it rises it cools, condensing to form huge clouds about ten
kilometres up in the troposphere. More warm air rushes in and rises, drawn by
the draught above. The rising draughts of air carry moisture high into the
atmosphere so that these clouds eventually become very thick and heavy.
Condensation then releases the latent heat energy stored in the water
vapour, providing the cyclone with more power. This creates a self-sustaining
heat cycle.
Drawn further upwards by the new release of energy, the clouds can grow to
12 to 15 kilometres high. The force created by the Earth's rotation on a tilted
axis, the Coriolis effect, causes rising currents of air to spiral around the centre
of the tropical cyclone. It is at this stage that the cyclone matures and the eye
of the storm is created. As the air rises and cools, some of this dense air
descends to form a clear, still eye as the cyclone rages around it.
The eye wall, where the wind is strongest, behaves like a whirling cylinder.
Cyclones rotate clockwise in the southern hemisphere, anti-clockwise in the
The lowest air pressure in a tropical cyclone is always found at the centre, and
is typically 950 millibars or less. The average air pressure at the Earth's surface is
about 1,010 millibars. Tropical cyclones have significantly lower air pressure
than the air that surrounds them. The bigger the pressure difference, the
stronger the wind force. One of the lowest air pressures ever recorded was 877
millibars for Typhoon Ida which hit the Philippines in 1958, where winds
reached 300 kilometres an hour.
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Weather Hazards:
Tropical Cyclones
Once formed, the cyclone's movement, or track, follows a pathway away
from its source, driven by global wind circulation. As warm ocean waters feed
it heat and moisture, the cyclone continues to enlarge.
Tropical cyclones do not form near the equator where the Coriolis effect is
weak, but between 5 and 30 degrees north and south of the equator. Satellite
photography is used to identify where a cyclone originates, which then helps
weather forecasters to plot where it will go next.
Dissipation is the result of the tropical cyclone moving over land or cool water.
When it reaches land it loses its energy source, the warm ocean water. Air
pressure rises as the temperature falls, and winds drop as the land surface
interferes with the air flow. Rainfall decreases, and eventually it weakens to
become a tropical storm.
Tropical cyclones can be an incredibly destructive and deadly force of
nature. Understanding their life cycles is critical to minimising the loss of life
and property they cause, and for mounting effective disaster responses.
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