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TROPICAL
CYCLONES
•Characteristics
•Stages
•How managed
What are the causes and effects of hurricanes?
Hurricanes
Click here for a playlist on hurricanes
By the end of this section, you will:
Know how hurricanes are formed.
How Hurricanes are named.
Weather patterns of Hurricanes.
Factors necessary for their
development.
The stages of development.
Long and short term effects.
Characteristics of a Hurricane.
Case study Hurricane Katrina.
Scoop It
TROPICAL CYCLONE CHARACTERISTICS:
ATMOSPHERIC CIRCULATION
Tropical Cyclones Introduction
• Tropical cyclones are
intense, rotating, lowpressure storms
• Develop over warm,
tropical oceans
• Its size varies between
200 and 2 000 km
• Produce strong winds
and heavy rainfall
• Can lead to immense
flooding
Destructive Winds
Naming of Hurricanes
Locations of Tropical Cyclones
• Warm water generates formation of tropical cyclones
•
•
•
•
Ocean temperatures must be at least 27°C
Between 5° - 20° N and S of the equator
Coriolis effect is greatly reduced at equator
No tropical cyclones occur at equator
ITCZ
Tropical Cyclone
Appearances
TROPICAL CYCLONE: DEVELOPMENT
Formative
Stage
pressure
ABOVE
1000 hPa
MATURE
Stage
pressure
WELL
BELOW
1 000hPa
source: http://sageography.myschoolstuff.co.za
Developing
Stage
Pressure
BELOW
1 000 hPa
Degenration/
Dissipation
Stage
pressure
RISES
STAGES IN THE DEVELOPMENT OF
TROPICAL CYCLONES
INITIAL
pressure
ABOVE
1000 hPa
 Surface
convergence and
heating cause warm
moist air to rise
 Identified by low
pressure on the
surface
 Growth of tall vertical
clouds
 Clockwise
movement around
the centre of uplift
(SH)
2
MATURE
pressure
WELL
BELOW
1 000hPa
Convergence and vortex –
• Warm moist air converges at
the surface
• Uplift, condensation and the
release of latent heat occurs
Two important
mechanisms
operating in the
mature tropical
cyclone:
• Where air is rising and is
warmer than its surroundings
– unstable
• Air flows around the centre of
low and a vortex exists
Most dangerous part of the Hurricane
3
• Decaying stage
• Cyclone begins to cool down
as it reaches cooler latitudes
• Less warm moist air is fed
into the system
• Dissipation also occurs as
cyclone strikes land
• Friction reduces the system
and no more moisture is fed
into the storm
DISSIPATION
pressure
RISES
Divergence
• In the upper part of the storm, air diverges
• As more air is pumped out more air can move in at the
surface
• Some air sinks in to the centre of the storm. Where air is
sinking, stable conditions exist. This results in clear and
calm conditions in the storm centre – called the eye of the
storm
• As the air sinks it is compressed and heated. It adds further
energy to the storm
The Eye - the clear, calm center of storm
 Cool air descends into this center (downdrafts)
 Eye wall – ring of cumulonimbus clouds up to 15 km high
 Clouds extended away up to 500 km either side of the eye
 Pressure drops to below 1 000 hPa.
 Wind speeds - hurricane strength about 117 km/ hour
 Mature stage can last up to 7 days
 Heavy rainfall

Eye of a Hurricane
Development of Tropical Cyclones
3) As air rises, moisture
condenses out, warming air
and allowing it to continue
rising
1) Warm, moist
air spirals
towards center
of Hurricane
5) As pressures decrease,
winds intensify and even
more warm moist air is
forced to converge and
rise
2) Convergence of air
forces air to rise
L
4) As air rises, it causes even
lower pressures at the surface
How are hurricanes classified?
Characteristics of a Tropical Cyclone
Equator/Ewenaar
From EAST to WEST
EYE
No wind, rain
& Clouds
DESTRUCTION
•Storm winds
•Torrential rain
•Flooding
Turns EAST at 30°
source: http://sageography.myschoolstuff.co.za
DISSIPATES
•No Moisture
•No warm air
•Friction
WEATHER ASSOCIATED WITH A
TROPICAL CYCLONE
TORRENTIAL RAIN
• up to 3 000 mm
• Soils become saturated
resulting in increased runoff
into rivers, wetlands and dams
• Risk of flooding
Storm Surges
• Occur when tropical cyclones reach land
• Storm surge – large wave
• Around 6 meters higher than normal tides
• Created by heavy winds and cause flooding
TROPICALCYCLONE - MANAGEMENT
source: http://sageography.myschoolstuff.co.za
Preparing for the worst!
What precautions should be taken when there is a Hurricane warning!
Avoid crossing strongly-flowing rivers at causeways /drifts.
Dwellings very close to rivers / floodplains should be evacuated.
Farmers should move pumps away from rivers.
Small / medium sized fishing boats should return to port.
Small fishing boats should be moved well above the high water mark.
Listen to the radio / television for weather reports.
Listen to the advice of the local disaster risk management officials.
TROPICAL CYCLONES AND
SOUTH AFRICA
Occurs during the summer – peak period is during January and
February
Only cyclones that move along the Mozambique channel are likely
to influence SA’s weather patterns
Many reach Madagascar first where their intensity is reduced
Many dissipate before reaching the coast of SA, because the
conditions necessary to sustain a cyclone like warm ocean surface,
are largely absent
IMPACT OF TROPICAL CYCLONES
Threat to human life, property and economic activities
Hurricane Katrina
Hurricane Katrina was the most
powerful hurricane to hit the USA
in known history.
It hit New Orleans on the morning
of the 29th August 2005 bringing
with it terrible destruction.
New Orleans
Winds of over 250
kilometres per hour were
recorded as Katrina hit the
coast, causing a storm
surge 8.5 metres high.
Devastation
At first New Orleans seemed
to have weathered the worst
of the hurricane, but later
storm surges breached the
city’s protective levees.
People that had not left their
properties were stranded
and had to wait to be
rescued or wade through
polluted floodwater.
It was a hugely devastating natural disaster.
The aftermath
How badly did Hurricane Katrina affect New Orleans?
Flood levels were over six metres high.
80% of the city was flooded.
Over a million homes were left without electricity.
There were 700 deaths in New Orleans.
Over a million people had to leave their homes.
Damaged oil refineries spilt 24 million litres of crude oil.
Over $81 billion worth of damage was done.
TROPICAL CYCLONES
NAME
Alphabet
Andrew
Bonny
Carl
Debora
IN EYE
No :
Wind
Clouds
Rain
ORIGEN
In tropics
OCEANS
Not nearer
than 5° from
equator
WEATHER
Before eye
During eye
After eye
MOVEMENT
From E to W
Away from
Equator
Turns east
at 30°
DESTRUCTION
Storm winds
Very hard
rain
Flooding
DISSIPATION
Over land
No Moisture
No Warm air
Friction
MANAGED
Sand bags
Early warning
Track storm
Services
Inform people
Evacuation