Global distribution of areas most at risk from large scale tropical
atmospheric disturbances and small-scale atmospheric disturbances
hazardous tropical cyclones are very large low-pressure systems with wind speeds
above 119km/h and deep low pressure (880mb), stronger winds than tropical
storms. called hurricanes in the Caribbean Sea, Gulf of Mexico and west coast of
Mexico. called cyclones in Indian ocean Bay of Bengal and north Australia.
typhoons in south china sea and west Pacific Ocean. apart from hurricanes on west
coast of Mexico, tropical cyclones affect east coast of continents and most
hazardous to Caribbean and densely populated low-lying Bangladesh coasts, Bay of
Bengal and south east USA.
tornadoes occur on every continent but Antarctica. most frequent in great plains
and east USA (belt called tornado alley). occur at any time but frequent in USA
between 4-9pm in May/June after a long period of heating the land.
most disastrous tornado, Bangladesh 1989, 1000 died. deaths double those in rest
of world combined.
Formation of tropical cyclones
Warm tropical waters. 28 degrees or higher.
Warm most air above these waters starts to rise. This leaves behind areas of low
air pressure.
Air from the surrounding area is sucked in to this low air pressure gap, it is heated
by the warm waters and rises. This process is repeated and adds to the cycle.
Thick cumulonimbus clouds start to form from the warm moist air.
As more warm moist air rises the storm grows and stretches in size. Rotating
faster and faster.
Large down drafts of air start to fall within the centre of the storm forming the eye.
Low pressure, cloud free area.
Beside/behind the eye is the eye wall. Fastest winds, highest temperatures and
most violent part of the storm.
As air is sucked into the eye wall rain bands are formed adding to the viciousness
of the storm.
The hurricane is pushed by the north easterly trade winds (winds that converge at
the equator) from east to west.
Once the storm hits land – energy is lost and the storm dies out.
The Coriolis Effect
“It is the apparent curving of wind caused by the rotation of the earth on its axis”.
The coriolis effect aids the movement of hurricanes in a westerly direction.
(Basically, it pushes the hurricane west, allowing it to spin faster)
hazards from large scale atmospheric disturbances
degree of hazard from tropical cyclones depends on how frequently they occur in
the same area, their magnitude and location of area affected. most powerful aren’t
the most hazardous.
high winds: Saffir-Simpson scale (revised 2012) indicates the potential damage to
structures by sustained wind speeds in tropical storms of cyclone strength. Dvorak
technique has been developed to estimated cyclone intensity on a scale from 1
(least) to 8. based on satellite images of patterns in weather systems, especially
difference between temp within eye and coldness of surrounding thunderstorm
clouds. the greater, the stronger.
storm surges and coastal flooding: storm surges (abnormal rises of seawater) are
primary hazards of tropical cyclones. strong winds drive the surge which rises in
shallow water and pushes inland, causing flooding. highest part of surge is where
the strongest winds are. the lower the atmospheric pressure, the higher the surge
as air rises in low pressure systems, taking weight off the sea surface, allowing it to
rise. high surge when; storm has string onshore winds, approaches coast at right
angles, coastline has bays and inlets to funnel the water, sea floor is gently sloping
with a wide continental shelf, few obstructions, such as spits and islands to slow
the flow of water, storm coincides with high tide. greatest risk from cyclones is at
the coast, as they lose power as they pass over land. slower the cyclone moves,
the greater the damage. on northern hemisphere the greatest risk is for cities on
right of the eyes path as there the travel speed is added to the speed of the winds
that are rotating anticlockwise.
intense rainfall: places crossed by eye of a cyclone experience two bands of
intense rainfall from the cumulonimbus clouds around the eye. causes severe river
flooding and mass movement, especially if the system is slow moving. river
flooding in the case study of cyclone Yasi.
mass movement: super typhoon durian, category 5 storm, affected Philippines in
November 2006. nearly all the 1200 deaths occurred when torrential rains
saturated volcanic deposits on slopes of mayo volcano, causing mudflows that
raced down the steep slopes onto the towns beneath. people had no time to
escape as entire villages were buried under volcanic debris.
case study: a comparison of intense tropical storms and super-typhoon
Haiyan, Philippines (an LIC) and Cyclone Yasi, Australia (an HIC)
super typhoon Haiyan cyclone Yasi
date 7 November 2013 3 February 2011
width 600km 1450km
category 5 5
Dvorak intensity 8.1
central atmospheric
pressure
930mb 930mb
maximum sustained wind
speed
314km/h 285km/h
highest storm surge 7m 7m
deaths 6000 1
homeless 2 million few thousand
damage to structures 90% of some towns
destroyed
4000 buildings severely
damaged
total number of people
affected
11 million 400,000
cost $US 2.9b $US 3.6b
case study: super typhoon Haiyan (local name Yolanda), Philippines, 2013
super typhoon is a storm in the north-west pacific with sustained wind speeds of
242km/h or more.
Development: grew from tropical storm as it moved westwards, stirring up 15m
high waves. reached its maximum intensity as it made landfall at Guiuan on east
coast of central Philippines as the 25th typhoon of the season. so intense that it
was off the Dvorak scale at 8.1
prediction monitoring and perception of risk: alerted by satellite images and track
of the typhoon, public storm warnings were issued and shelters opened but they
weren’t enough for everyone and some were damaged. many were basic and not
well stocked with essentials. only half a million could use them. government
advised evacuation but didn’t make it mandatory so many stayed as they were
afraid of looting. many lives saved by major of Guiuan who urged evacuation.
army, air force, navy and hospitals were on standby and disaster response teams
sent to areas expected to be affected.
primary and secondary impacts on lives and property: system moved so quickly
across the country that rainfall totals didn’t produce the floods and landslides that
cause the most damage. mudflows were less severe, coastal areas like mud
wastelands, mudflows/debris slides destroyed many buildings in Guiuan. nearly all
damage caused by storm surge when Haiyan moved on to make landfall on islands
of Samar and Leyte. made more dangerous because Tacloban (city in Leyte on land
less than 5m above sea level) is on a bay where water funnelled, greater surge,
like a tsunami. water wall over 7m high, caused devastation, especially in shanty
areas. Tacloban city convention centre 1st floor submerged, drowned evacuees
there. 90% of wooden buildings in poor Tacloban destroyed. well-built concrete
structures had damaged roofs and windows. many drowned in streets and injuries
as roads became rivers. floods swept away bridges and deposited large ships on
top of the devastation. few buildings in worst hit rural places survived. fallen trees,
roofs and tossed up cars blocked roads. power lines destroyed, darkness, 4
provinces had no power. rotting corpses contaminated water supplies and spread
disease. survivors without shelter food water sanitation or fuel. winds broke phone
lines so no contact with outside world. satellite images invaluable for showing
which areas need assistances. hard to get aid to islands. emergency supplies
decimated month before typhoon with a 7.2 EQ on Bohol island, 5000 people still
living in tents. with no relief, desperate people looted for food/water to survive.
airport was damaged by storm surge and lost electricity supply, only 20 daylight
flights could bring in relief supplies and evacuate. distributing aid was hard and
slow as blocked roads. danger of attack from people to intercept aid caused delay
on going in until security improved. some islands only accessible by ship and
plane, planes couldn’t fly until runway cleared of debris. aid reached Tacloban 7
days after cyclone but 9 days before food distributed. Guiuan waited longer.
disaster relief system failed as of power/communication breakdown
long term effects: mangrove forests replanted along coast, replacing destroyed or
previously felled ones as they protect against storm surges and strong winds.
disaster scale greater as Tacloban’s population x3 in recent years despite east
coast known to be most hazardous for typhoon damage. Tacloban and Leyte
people moved to less affected areas. 20,000 went to manila and Catbalogan
population doubled with refugees. agriculture, main economy, badly affected and
temporarily slowed economy growth.
case study: cyclone Yasi, Australia 2011
prediction and monitoring: 4th cyclone of season to hit. began as tropical lowpressure system grew near Fiji. grew in intensity as moved west before turning
WSW over warm south Pacific Ocean. expected to make landfall somewhere in
Queensland along 300km coast stretch, cairns and Townsville here, Brisbane also
told to prepare for flash floods. heavy summer rain caused by la Nina conditions
had fallen for previous 3 months, damaged buildings and 35 river flood deaths in
Brisbane. more rain on saturated ground is dangerous. 3 days before Yasi’s
landfall, cyclone Anthony has uprooted trees and damaged Queensland power.
perception of risk: good building standards enforced after cyclone in Darwin in
1974 killed 74 people. public were informed through media to prepare for the
worst. storm surge forecasts given for a high tide situation. low-lying areas along
coast from cairns to Townsville told to evacuate and 29,000 did, hospital patients
moved by military. schools, unis, airports and ports closed.
primary impacts on lives and property: category 5, destroyed anemometer, radar
equipment and communications with world on Willis island. moved onto mainland,
affected 540km of coast with 400,000 inhabitants, missed main cities. made
landfall at night at small settlements of mission beach, cardwell and Tully. 100’s of
homes/business damaged. strong winds and 7m storm surge caused damage
300km inland. roofs ripped off. Tully high school destroyed. 226,000 homes without
power and water in Townsville failed. boats and yachts destroyed. people
evacuated so few injuries and 1 death. less severe impact because; Yasi turns
south before landfall into low population density area, missing cairns, moved at
rapid speed, 35km/h, limited rainfall and length of time structures were battered
by winds, as storm surge didn’t coincide with high tide, 2m lower than would have
been. maintained cyclone strength until more than 700km from coast, downgraded
to tropical storm near mount isa. costliest cyclone as banana (75% of crop) and
sugar cane fields damaged, states main export lost. agriculture, mining and local
government losses; $2b, $1b cost to tourist industry. record breaking rainfall in
Queensland. floods were extensive and many roads cut. highest totals fell from
cloud band south of the eye. mission beach recorded the highest daily total of
471mm and most places had more than 200mm. little need for urgent relief as
good authority planning and sensible response of residents. problem of delivery
emergency supplies to communities isolated by flooding. power supplies restored
fast and military helped clear up. 3000km road repaired and railway rebuilt.
secondary impacts resulting from primary impacts: parks and forests damaged,
cassowary (endangered bird) lost habitat and food sources. strict building regs in
place to make new coastal properties storm surge proof. heavy rain dragged great
barrier reef; polluted run-off caused increased population of crown of thorns
starfish that eat coral. Queensland people resilient. infrastructure repaired fast. 1
year after farmers were growing banana and sugar again. bank loans and
insurance allowed homes to be re built and businesses re started. less resilience of
New Orleans after hurricane Katrina as many people too poor to insure. population
halved after disaster.